Add Go module

Use *lexer.describe in parse error message

CNAME

@@ -1 +1 @@
-www.gopl.io
+gopl.io

README.md

@@ -1,4 +1,21 @@
+# The Go Programming Language
 
-This branch holds the source for the http://www.gopl.io website.
+This repository provides the downloadable example programs
+for the book, "The Go Programming Language"; see http://www.gopl.io.
+
+These example programs are licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-nc-sa/4.0/">Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License</a>.<br/>
+<a rel="license" href="http://creativecommons.org/licenses/by-nc-sa/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by-nc-sa/4.0/88x31.png"/></a>
+
+You can download, build, and run the programs with the following commands:
+
+	$ export GOPATH=$HOME/gobook            # choose workspace directory
+	$ go get gopl.io/ch1/helloworld         # fetch, build, install
+	$ $GOPATH/bin/helloworld                # run
+	Hello, 世界
+
+Many of the programs contain comments of the form `//!+` and `//!-`.
+These comments bracket the parts of the programs that are excerpted in the
+book; you can safely ignore them.  In a few cases, programs
+have been reformatted in an unnatural way so that they can be presented
+in stages in the book.
 
-This entire repo is public, so don't check in anything sensitive.

ch1/dup1/main.go

@@ -0,0 +1,31 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 8.
+//!+
+
+// Dup1 prints the text of each line that appears more than
+// once in the standard input, preceded by its count.
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+)
+
+func main() {
+	counts := make(map[string]int)
+	input := bufio.NewScanner(os.Stdin)
+	for input.Scan() {
+		counts[input.Text()]++
+	}
+	// NOTE: ignoring potential errors from input.Err()
+	for line, n := range counts {
+		if n > 1 {
+			fmt.Printf("%d\t%s\n", n, line)
+		}
+	}
+}
+
+//!-

ch1/dup2/main.go

@@ -0,0 +1,48 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 10.
+//!+
+
+// Dup2 prints the count and text of lines that appear more than once
+// in the input.  It reads from stdin or from a list of named files.
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+)
+
+func main() {
+	counts := make(map[string]int)
+	files := os.Args[1:]
+	if len(files) == 0 {
+		countLines(os.Stdin, counts)
+	} else {
+		for _, arg := range files {
+			f, err := os.Open(arg)
+			if err != nil {
+				fmt.Fprintf(os.Stderr, "dup2: %v\n", err)
+				continue
+			}
+			countLines(f, counts)
+			f.Close()
+		}
+	}
+	for line, n := range counts {
+		if n > 1 {
+			fmt.Printf("%d\t%s\n", n, line)
+		}
+	}
+}
+
+func countLines(f *os.File, counts map[string]int) {
+	input := bufio.NewScanner(f)
+	for input.Scan() {
+		counts[input.Text()]++
+	}
+	// NOTE: ignoring potential errors from input.Err()
+}
+
+//!-

ch1/dup3/main.go

@@ -0,0 +1,38 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 12.
+
+//!+
+
+// Dup3 prints the count and text of lines that
+// appear more than once in the named input files.
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"strings"
+)
+
+func main() {
+	counts := make(map[string]int)
+	for _, filename := range os.Args[1:] {
+		data, err := ioutil.ReadFile(filename)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "dup3: %v\n", err)
+			continue
+		}
+		for _, line := range strings.Split(string(data), "\n") {
+			counts[line]++
+		}
+	}
+	for line, n := range counts {
+		if n > 1 {
+			fmt.Printf("%d\t%s\n", n, line)
+		}
+	}
+}
+
+//!-

ch1/echo1/main.go

@@ -0,0 +1,24 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 4.
+//!+
+
+// Echo1 prints its command-line arguments.
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	var s, sep string
+	for i := 1; i < len(os.Args); i++ {
+		s += sep + os.Args[i]
+		sep = " "
+	}
+	fmt.Println(s)
+}
+
+//!-

ch1/echo2/main.go

@@ -0,0 +1,24 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 6.
+//!+
+
+// Echo2 prints its command-line arguments.
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	s, sep := "", ""
+	for _, arg := range os.Args[1:] {
+		s += sep + arg
+		sep = " "
+	}
+	fmt.Println(s)
+}
+
+//!-

ch1/echo3/main.go

@@ -0,0 +1,20 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 8.
+
+// Echo3 prints its command-line arguments.
+package main
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+//!+
+func main() {
+	fmt.Println(strings.Join(os.Args[1:], " "))
+}
+
+//!-

ch1/fetch/main.go

@@ -0,0 +1,34 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 16.
+//!+
+
+// Fetch prints the content found at each specified URL.
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"os"
+)
+
+func main() {
+	for _, url := range os.Args[1:] {
+		resp, err := http.Get(url)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "fetch: %v\n", err)
+			os.Exit(1)
+		}
+		b, err := ioutil.ReadAll(resp.Body)
+		resp.Body.Close()
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "fetch: reading %s: %v\n", url, err)
+			os.Exit(1)
+		}
+		fmt.Printf("%s", b)
+	}
+}
+
+//!-

ch1/fetchall/main.go

@@ -0,0 +1,49 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 17.
+//!+
+
+// Fetchall fetches URLs in parallel and reports their times and sizes.
+package main
+
+import (
+	"fmt"
+	"io"
+	"io/ioutil"
+	"net/http"
+	"os"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	ch := make(chan string)
+	for _, url := range os.Args[1:] {
+		go fetch(url, ch) // start a goroutine
+	}
+	for range os.Args[1:] {
+		fmt.Println(<-ch) // receive from channel ch
+	}
+	fmt.Printf("%.2fs elapsed\n", time.Since(start).Seconds())
+}
+
+func fetch(url string, ch chan<- string) {
+	start := time.Now()
+	resp, err := http.Get(url)
+	if err != nil {
+		ch <- fmt.Sprint(err) // send to channel ch
+		return
+	}
+
+	nbytes, err := io.Copy(ioutil.Discard, resp.Body)
+	resp.Body.Close() // don't leak resources
+	if err != nil {
+		ch <- fmt.Sprintf("while reading %s: %v", url, err)
+		return
+	}
+	secs := time.Since(start).Seconds()
+	ch <- fmt.Sprintf("%.2fs  %7d  %s", secs, nbytes, url)
+}
+
+//!-

ch1/helloworld/main.go

@@ -0,0 +1,16 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 1.
+
+// Helloworld is our first Go program.
+//!+
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("Hello, 世界")
+}
+
+//!-

ch1/lissajous/main.go

@@ -0,0 +1,86 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// Run with "web" command-line argument for web server.
+// See page 13.
+//!+main
+
+// Lissajous generates GIF animations of random Lissajous figures.
+package main
+
+import (
+	"image"
+	"image/color"
+	"image/gif"
+	"io"
+	"math"
+	"math/rand"
+	"os"
+)
+
+//!-main
+// Packages not needed by version in book.
+import (
+	"log"
+	"net/http"
+	"time"
+)
+
+//!+main
+
+var palette = []color.Color{color.White, color.Black}
+
+const (
+	whiteIndex = 0 // first color in palette
+	blackIndex = 1 // next color in palette
+)
+
+func main() {
+	//!-main
+	// The sequence of images is deterministic unless we seed
+	// the pseudo-random number generator using the current time.
+	// Thanks to Randall McPherson for pointing out the omission.
+	rand.Seed(time.Now().UTC().UnixNano())
+
+	if len(os.Args) > 1 && os.Args[1] == "web" {
+		//!+http
+		handler := func(w http.ResponseWriter, r *http.Request) {
+			lissajous(w)
+		}
+		http.HandleFunc("/", handler)
+		//!-http
+		log.Fatal(http.ListenAndServe("localhost:8000", nil))
+		return
+	}
+	//!+main
+	lissajous(os.Stdout)
+}
+
+func lissajous(out io.Writer) {
+	const (
+		cycles  = 5     // number of complete x oscillator revolutions
+		res     = 0.001 // angular resolution
+		size    = 100   // image canvas covers [-size..+size]
+		nframes = 64    // number of animation frames
+		delay   = 8     // delay between frames in 10ms units
+	)
+	freq := rand.Float64() * 3.0 // relative frequency of y oscillator
+	anim := gif.GIF{LoopCount: nframes}
+	phase := 0.0 // phase difference
+	for i := 0; i < nframes; i++ {
+		rect := image.Rect(0, 0, 2*size+1, 2*size+1)
+		img := image.NewPaletted(rect, palette)
+		for t := 0.0; t < cycles*2*math.Pi; t += res {
+			x := math.Sin(t)
+			y := math.Sin(t*freq + phase)
+			img.SetColorIndex(size+int(x*size+0.5), size+int(y*size+0.5),
+				blackIndex)
+		}
+		phase += 0.1
+		anim.Delay = append(anim.Delay, delay)
+		anim.Image = append(anim.Image, img)
+	}
+	gif.EncodeAll(out, &anim) // NOTE: ignoring encoding errors
+}
+
+//!-main

ch1/server1/main.go

@@ -0,0 +1,26 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 19.
+//!+
+
+// Server1 is a minimal "echo" server.
+package main
+
+import (
+	"fmt"
+	"log"
+	"net/http"
+)
+
+func main() {
+	http.HandleFunc("/", handler) // each request calls handler
+	log.Fatal(http.ListenAndServe("localhost:8000", nil))
+}
+
+// handler echoes the Path component of the requested URL.
+func handler(w http.ResponseWriter, r *http.Request) {
+	fmt.Fprintf(w, "URL.Path = %q\n", r.URL.Path)
+}
+
+//!-

ch1/server2/main.go

@@ -0,0 +1,41 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 20.
+//!+
+
+// Server2 is a minimal "echo" and counter server.
+package main
+
+import (
+	"fmt"
+	"log"
+	"net/http"
+	"sync"
+)
+
+var mu sync.Mutex
+var count int
+
+func main() {
+	http.HandleFunc("/", handler)
+	http.HandleFunc("/count", counter)
+	log.Fatal(http.ListenAndServe("localhost:8000", nil))
+}
+
+// handler echoes the Path component of the requested URL.
+func handler(w http.ResponseWriter, r *http.Request) {
+	mu.Lock()
+	count++
+	mu.Unlock()
+	fmt.Fprintf(w, "URL.Path = %q\n", r.URL.Path)
+}
+
+// counter echoes the number of calls so far.
+func counter(w http.ResponseWriter, r *http.Request) {
+	mu.Lock()
+	fmt.Fprintf(w, "Count %d\n", count)
+	mu.Unlock()
+}
+
+//!-

ch1/server3/main.go

@@ -0,0 +1,37 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 21.
+
+// Server3 is an "echo" server that displays request parameters.
+package main
+
+import (
+	"fmt"
+	"log"
+	"net/http"
+)
+
+func main() {
+	http.HandleFunc("/", handler)
+	log.Fatal(http.ListenAndServe("localhost:8000", nil))
+}
+
+//!+handler
+// handler echoes the HTTP request.
+func handler(w http.ResponseWriter, r *http.Request) {
+	fmt.Fprintf(w, "%s %s %s\n", r.Method, r.URL, r.Proto)
+	for k, v := range r.Header {
+		fmt.Fprintf(w, "Header[%q] = %q\n", k, v)
+	}
+	fmt.Fprintf(w, "Host = %q\n", r.Host)
+	fmt.Fprintf(w, "RemoteAddr = %q\n", r.RemoteAddr)
+	if err := r.ParseForm(); err != nil {
+		log.Print(err)
+	}
+	for k, v := range r.Form {
+		fmt.Fprintf(w, "Form[%q] = %q\n", k, v)
+	}
+}
+
+//!-handler

ch10/cross/main.go

@@ -0,0 +1,19 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 295.
+
+// The cross command prints the values of GOOS and GOARCH for this target.
+package main
+
+import (
+	"fmt"
+	"runtime"
+)
+
+//!+
+func main() {
+	fmt.Println(runtime.GOOS, runtime.GOARCH)
+}
+
+//!-

ch10/jpeg/main.go

@@ -0,0 +1,52 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 287.
+
+//!+main
+
+// The jpeg command reads a PNG image from the standard input
+// and writes it as a JPEG image to the standard output.
+package main
+
+import (
+	"fmt"
+	"image"
+	"image/jpeg"
+	_ "image/png" // register PNG decoder
+	"io"
+	"os"
+)
+
+func main() {
+	if err := toJPEG(os.Stdin, os.Stdout); err != nil {
+		fmt.Fprintf(os.Stderr, "jpeg: %v\n", err)
+		os.Exit(1)
+	}
+}
+
+func toJPEG(in io.Reader, out io.Writer) error {
+	img, kind, err := image.Decode(in)
+	if err != nil {
+		return err
+	}
+	fmt.Fprintln(os.Stderr, "Input format =", kind)
+	return jpeg.Encode(out, img, &jpeg.Options{Quality: 95})
+}
+
+//!-main
+
+/*
+//!+with
+$ go build gopl.io/ch3/mandelbrot
+$ go build gopl.io/ch10/jpeg
+$ ./mandelbrot | ./jpeg >mandelbrot.jpg
+Input format = png
+//!-with
+
+//!+without
+$ go build gopl.io/ch10/jpeg
+$ ./mandelbrot | ./jpeg >mandelbrot.jpg
+jpeg: image: unknown format
+//!-without
+*/

ch11/echo/echo.go

@@ -0,0 +1,41 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 308.
+//!+
+
+// Echo prints its command-line arguments.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+)
+
+var (
+	n = flag.Bool("n", false, "omit trailing newline")
+	s = flag.String("s", " ", "separator")
+)
+
+var out io.Writer = os.Stdout // modified during testing
+
+func main() {
+	flag.Parse()
+	if err := echo(!*n, *s, flag.Args()); err != nil {
+		fmt.Fprintf(os.Stderr, "echo: %v\n", err)
+		os.Exit(1)
+	}
+}
+
+func echo(newline bool, sep string, args []string) error {
+	fmt.Fprint(out, strings.Join(args, sep))
+	if newline {
+		fmt.Fprintln(out)
+	}
+	return nil
+}
+
+//!-

ch11/echo/echo_test.go

@@ -0,0 +1,45 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// Test of echo command.  Run with: go test gopl.io/ch11/echo
+
+//!+
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"testing"
+)
+
+func TestEcho(t *testing.T) {
+	var tests = []struct {
+		newline bool
+		sep     string
+		args    []string
+		want    string
+	}{
+		{true, "", []string{}, "\n"},
+		{false, "", []string{}, ""},
+		{true, "\t", []string{"one", "two", "three"}, "one\ttwo\tthree\n"},
+		{true, ",", []string{"a", "b", "c"}, "a,b,c\n"},
+		{false, ":", []string{"1", "2", "3"}, "1:2:3"},
+	}
+
+	for _, test := range tests {
+		descr := fmt.Sprintf("echo(%v, %q, %q)",
+			test.newline, test.sep, test.args)
+
+		out = new(bytes.Buffer) // captured output
+		if err := echo(test.newline, test.sep, test.args); err != nil {
+			t.Errorf("%s failed: %v", descr, err)
+			continue
+		}
+		got := out.(*bytes.Buffer).String()
+		if got != test.want {
+			t.Errorf("%s = %q, want %q", descr, got, test.want)
+		}
+	}
+}
+
+//!-

ch11/storage1/storage.go

@@ -0,0 +1,45 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 311.
+
+// Package storage is part of a hypothetical cloud storage server.
+//!+main
+package storage
+
+import (
+	"fmt"
+	"log"
+	"net/smtp"
+)
+
+var usage = make(map[string]int64)
+
+func bytesInUse(username string) int64 { return usage[username] }
+
+// Email sender configuration.
+// NOTE: never put passwords in source code!
+const sender = "notifications@example.com"
+const password = "correcthorsebatterystaple"
+const hostname = "smtp.example.com"
+
+const template = `Warning: you are using %d bytes of storage,
+%d%% of your quota.`
+
+func CheckQuota(username string) {
+	used := bytesInUse(username)
+	const quota = 1000000000 // 1GB
+	percent := 100 * used / quota
+	if percent < 90 {
+		return // OK
+	}
+	msg := fmt.Sprintf(template, used, percent)
+	auth := smtp.PlainAuth("", sender, password, hostname)
+	err := smtp.SendMail(hostname+":587", auth, sender,
+		[]string{username}, []byte(msg))
+	if err != nil {
+		log.Printf("smtp.SendMail(%s) failed: %s", username, err)
+	}
+}
+
+//!-main

ch11/storage2/quota_test.go

@@ -0,0 +1,53 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+//!+test
+package storage
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestCheckQuotaNotifiesUser(t *testing.T) {
+	var notifiedUser, notifiedMsg string
+	notifyUser = func(user, msg string) {
+		notifiedUser, notifiedMsg = user, msg
+	}
+
+	const user = "joe@example.org"
+	usage[user] = 980000000 // simulate a 980MB-used condition
+
+	CheckQuota(user)
+	if notifiedUser == "" && notifiedMsg == "" {
+		t.Fatalf("notifyUser not called")
+	}
+	if notifiedUser != user {
+		t.Errorf("wrong user (%s) notified, want %s",
+			notifiedUser, user)
+	}
+	const wantSubstring = "98% of your quota"
+	if !strings.Contains(notifiedMsg, wantSubstring) {
+		t.Errorf("unexpected notification message <<%s>>, "+
+			"want substring %q", notifiedMsg, wantSubstring)
+	}
+}
+
+//!-test
+
+/*
+//!+defer
+func TestCheckQuotaNotifiesUser(t *testing.T) {
+	// Save and restore original notifyUser.
+	saved := notifyUser
+	defer func() { notifyUser = saved }()
+
+	// Install the test's fake notifyUser.
+	var notifiedUser, notifiedMsg string
+	notifyUser = func(user, msg string) {
+		notifiedUser, notifiedMsg = user, msg
+	}
+	// ...rest of test...
+}
+//!-defer
+*/

ch11/storage2/storage.go

@@ -0,0 +1,49 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 312.
+
+// Package storage is part of a hypothetical cloud storage server.
+package storage
+
+import (
+	"fmt"
+	"log"
+	"net/smtp"
+)
+
+var usage = make(map[string]int64)
+
+func bytesInUse(username string) int64 { return usage[username] }
+
+// E-mail sender configuration.
+// NOTE: never put passwords in source code!
+const sender = "notifications@example.com"
+const password = "correcthorsebatterystaple"
+const hostname = "smtp.example.com"
+
+const template = `Warning: you are using %d bytes of storage,
+%d%% of your quota.`
+
+//!+factored
+var notifyUser = func(username, msg string) {
+	auth := smtp.PlainAuth("", sender, password, hostname)
+	err := smtp.SendMail(hostname+":587", auth, sender,
+		[]string{username}, []byte(msg))
+	if err != nil {
+		log.Printf("smtp.SendMail(%s) failed: %s", username, err)
+	}
+}
+
+func CheckQuota(username string) {
+	used := bytesInUse(username)
+	const quota = 1000000000 // 1GB
+	percent := 100 * used / quota
+	if percent < 90 {
+		return // OK
+	}
+	msg := fmt.Sprintf(template, used, percent)
+	notifyUser(username, msg)
+}
+
+//!-factored

ch11/word1/word.go

@@ -0,0 +1,21 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 303.
+//!+
+
+// Package word provides utilities for word games.
+package word
+
+// IsPalindrome reports whether s reads the same forward and backward.
+// (Our first attempt.)
+func IsPalindrome(s string) bool {
+	for i := range s {
+		if s[i] != s[len(s)-1-i] {
+			return false
+		}
+	}
+	return true
+}
+
+//!-

ch11/word1/word_test.go

@@ -0,0 +1,43 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+//!+test
+package word
+
+import "testing"
+
+func TestPalindrome(t *testing.T) {
+	if !IsPalindrome("detartrated") {
+		t.Error(`IsPalindrome("detartrated") = false`)
+	}
+	if !IsPalindrome("kayak") {
+		t.Error(`IsPalindrome("kayak") = false`)
+	}
+}
+
+func TestNonPalindrome(t *testing.T) {
+	if IsPalindrome("palindrome") {
+		t.Error(`IsPalindrome("palindrome") = true`)
+	}
+}
+
+//!-test
+
+// The tests below are expected to fail.
+// See package gopl.io/ch11/word2 for the fix.
+
+//!+more
+func TestFrenchPalindrome(t *testing.T) {
+	if !IsPalindrome("été") {
+		t.Error(`IsPalindrome("été") = false`)
+	}
+}
+
+func TestCanalPalindrome(t *testing.T) {
+	input := "A man, a plan, a canal: Panama"
+	if !IsPalindrome(input) {
+		t.Errorf(`IsPalindrome(%q) = false`, input)
+	}
+}
+
+//!-more

ch11/word2/word.go

@@ -0,0 +1,29 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 305.
+//!+
+
+// Package word provides utilities for word games.
+package word
+
+import "unicode"
+
+// IsPalindrome reports whether s reads the same forward and backward.
+// Letter case is ignored, as are non-letters.
+func IsPalindrome(s string) bool {
+	var letters []rune
+	for _, r := range s {
+		if unicode.IsLetter(r) {
+			letters = append(letters, unicode.ToLower(r))
+		}
+	}
+	for i := range letters {
+		if letters[i] != letters[len(letters)-1-i] {
+			return false
+		}
+	}
+	return true
+}
+
+//!-

ch11/word2/word_test.go

@@ -0,0 +1,148 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package word
+
+import (
+	"fmt"
+	"math/rand"
+	"time"
+)
+
+//!+bench
+
+import "testing"
+
+//!-bench
+
+//!+test
+func TestIsPalindrome(t *testing.T) {
+	var tests = []struct {
+		input string
+		want  bool
+	}{
+		{"", true},
+		{"a", true},
+		{"aa", true},
+		{"ab", false},
+		{"kayak", true},
+		{"detartrated", true},
+		{"A man, a plan, a canal: Panama", true},
+		{"Evil I did dwell; lewd did I live.", true},
+		{"Able was I ere I saw Elba", true},
+		{"été", true},
+		{"Et se resservir, ivresse reste.", true},
+		{"palindrome", false}, // non-palindrome
+		{"desserts", false},   // semi-palindrome
+	}
+	for _, test := range tests {
+		if got := IsPalindrome(test.input); got != test.want {
+			t.Errorf("IsPalindrome(%q) = %v", test.input, got)
+		}
+	}
+}
+
+//!-test
+
+//!+bench
+func BenchmarkIsPalindrome(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		IsPalindrome("A man, a plan, a canal: Panama")
+	}
+}
+
+//!-bench
+
+//!+example
+
+func ExampleIsPalindrome() {
+	fmt.Println(IsPalindrome("A man, a plan, a canal: Panama"))
+	fmt.Println(IsPalindrome("palindrome"))
+	// Output:
+	// true
+	// false
+}
+
+//!-example
+
+/*
+//!+random
+import "math/rand"
+
+//!-random
+*/
+
+//!+random
+// randomPalindrome returns a palindrome whose length and contents
+// are derived from the pseudo-random number generator rng.
+func randomPalindrome(rng *rand.Rand) string {
+	n := rng.Intn(25) // random length up to 24
+	runes := make([]rune, n)
+	for i := 0; i < (n+1)/2; i++ {
+		r := rune(rng.Intn(0x1000)) // random rune up to '\u0999'
+		runes[i] = r
+		runes[n-1-i] = r
+	}
+	return string(runes)
+}
+
+func TestRandomPalindromes(t *testing.T) {
+	// Initialize a pseudo-random number generator.
+	seed := time.Now().UTC().UnixNano()
+	t.Logf("Random seed: %d", seed)
+	rng := rand.New(rand.NewSource(seed))
+
+	for i := 0; i < 1000; i++ {
+		p := randomPalindrome(rng)
+		if !IsPalindrome(p) {
+			t.Errorf("IsPalindrome(%q) = false", p)
+		}
+	}
+}
+
+//!-random
+
+/*
+// Answer for Exercicse 11.1: Modify randomPalindrome to exercise
+// IsPalindrome's handling of punctuation and spaces.
+
+// WARNING: the conversion r -> upper -> lower doesn't preserve
+// the value of r in some cases, e.g., µ Μ, ſ S, ı I
+
+// randomPalindrome returns a palindrome whose length and contents
+// are derived from the pseudo-random number generator rng.
+func randomNoisyPalindrome(rng *rand.Rand) string {
+	n := rng.Intn(25) // random length up to 24
+	runes := make([]rune, n)
+	for i := 0; i < (n+1)/2; i++ {
+		r := rune(rng.Intn(0x200)) // random rune up to \u99
+		runes[i] = r
+		r1 := r
+		if unicode.IsLetter(r) && unicode.IsLower(r) {
+			r = unicode.ToUpper(r)
+			if unicode.ToLower(r) != r1 {
+				fmt.Printf("cap? %c %c\n", r1, r)
+			}
+		}
+		runes[n-1-i] = r
+	}
+	return "?" + string(runes) + "!"
+}
+
+func TestRandomNoisyPalindromes(t *testing.T) {
+	// Initialize a pseudo-random number generator.
+	seed := time.Now().UTC().UnixNano()
+	t.Logf("Random seed: %d", seed)
+	rng := rand.New(rand.NewSource(seed))
+
+	n := 0
+	for i := 0; i < 1000; i++ {
+		p := randomNoisyPalindrome(rng)
+		if !IsPalindrome(p) {
+			t.Errorf("IsNoisyPalindrome(%q) = false", p)
+			n++
+		}
+	}
+	fmt.Fprintf(os.Stderr, "fail = %d\n", n)
+}
+*/

ch12/display/display.go

@@ -0,0 +1,90 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 333.
+
+// Package display provides a means to display structured data.
+package display
+
+import (
+	"fmt"
+	"reflect"
+	"strconv"
+)
+
+//!+Display
+
+func Display(name string, x interface{}) {
+	fmt.Printf("Display %s (%T):\n", name, x)
+	display(name, reflect.ValueOf(x))
+}
+
+//!-Display
+
+// formatAtom formats a value without inspecting its internal structure.
+// It is a copy of the the function in gopl.io/ch11/format.
+func formatAtom(v reflect.Value) string {
+	switch v.Kind() {
+	case reflect.Invalid:
+		return "invalid"
+	case reflect.Int, reflect.Int8, reflect.Int16,
+		reflect.Int32, reflect.Int64:
+		return strconv.FormatInt(v.Int(), 10)
+	case reflect.Uint, reflect.Uint8, reflect.Uint16,
+		reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		return strconv.FormatUint(v.Uint(), 10)
+	// ...floating-point and complex cases omitted for brevity...
+	case reflect.Bool:
+		if v.Bool() {
+			return "true"
+		}
+		return "false"
+	case reflect.String:
+		return strconv.Quote(v.String())
+	case reflect.Chan, reflect.Func, reflect.Ptr,
+		reflect.Slice, reflect.Map:
+		return v.Type().String() + " 0x" +
+			strconv.FormatUint(uint64(v.Pointer()), 16)
+	default: // reflect.Array, reflect.Struct, reflect.Interface
+		return v.Type().String() + " value"
+	}
+}
+
+//!+display
+func display(path string, v reflect.Value) {
+	switch v.Kind() {
+	case reflect.Invalid:
+		fmt.Printf("%s = invalid\n", path)
+	case reflect.Slice, reflect.Array:
+		for i := 0; i < v.Len(); i++ {
+			display(fmt.Sprintf("%s[%d]", path, i), v.Index(i))
+		}
+	case reflect.Struct:
+		for i := 0; i < v.NumField(); i++ {
+			fieldPath := fmt.Sprintf("%s.%s", path, v.Type().Field(i).Name)
+			display(fieldPath, v.Field(i))
+		}
+	case reflect.Map:
+		for _, key := range v.MapKeys() {
+			display(fmt.Sprintf("%s[%s]", path,
+				formatAtom(key)), v.MapIndex(key))
+		}
+	case reflect.Ptr:
+		if v.IsNil() {
+			fmt.Printf("%s = nil\n", path)
+		} else {
+			display(fmt.Sprintf("(*%s)", path), v.Elem())
+		}
+	case reflect.Interface:
+		if v.IsNil() {
+			fmt.Printf("%s = nil\n", path)
+		} else {
+			fmt.Printf("%s.type = %s\n", path, v.Elem().Type())
+			display(path+".value", v.Elem())
+		}
+	default: // basic types, channels, funcs
+		fmt.Printf("%s = %s\n", path, formatAtom(v))
+	}
+}
+
+//!-display

ch12/display/display_test.go

@@ -0,0 +1,259 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package display
+
+import (
+	"io"
+	"net"
+	"os"
+	"reflect"
+	"sync"
+	"testing"
+
+	"gopl.io/ch7/eval"
+)
+
+// NOTE: we can't use !+..!- comments to excerpt these tests
+// into the book because it defeats the Example mechanism,
+// which requires the // Output comment to be at the end
+// of the function.
+
+func Example_expr() {
+	e, _ := eval.Parse("sqrt(A / pi)")
+	Display("e", e)
+	// Output:
+	// Display e (eval.call):
+	// e.fn = "sqrt"
+	// e.args[0].type = eval.binary
+	// e.args[0].value.op = 47
+	// e.args[0].value.x.type = eval.Var
+	// e.args[0].value.x.value = "A"
+	// e.args[0].value.y.type = eval.Var
+	// e.args[0].value.y.value = "pi"
+}
+
+func Example_slice() {
+	Display("slice", []*int{new(int), nil})
+	// Output:
+	// Display slice ([]*int):
+	// (*slice[0]) = 0
+	// slice[1] = nil
+}
+
+func Example_nilInterface() {
+	var w io.Writer
+	Display("w", w)
+	// Output:
+	// Display w (<nil>):
+	// w = invalid
+}
+
+func Example_ptrToInterface() {
+	var w io.Writer
+	Display("&w", &w)
+	// Output:
+	// Display &w (*io.Writer):
+	// (*&w) = nil
+}
+
+func Example_struct() {
+	Display("x", struct{ x interface{} }{3})
+	// Output:
+	// Display x (struct { x interface {} }):
+	// x.x.type = int
+	// x.x.value = 3
+}
+
+func Example_interface() {
+	var i interface{} = 3
+	Display("i", i)
+	// Output:
+	// Display i (int):
+	// i = 3
+}
+
+func Example_ptrToInterface2() {
+	var i interface{} = 3
+	Display("&i", &i)
+	// Output:
+	// Display &i (*interface {}):
+	// (*&i).type = int
+	// (*&i).value = 3
+}
+
+func Example_array() {
+	Display("x", [1]interface{}{3})
+	// Output:
+	// Display x ([1]interface {}):
+	// x[0].type = int
+	// x[0].value = 3
+}
+
+func Example_movie() {
+	//!+movie
+	type Movie struct {
+		Title, Subtitle string
+		Year            int
+		Color           bool
+		Actor           map[string]string
+		Oscars          []string
+		Sequel          *string
+	}
+	//!-movie
+	//!+strangelove
+	strangelove := Movie{
+		Title:    "Dr. Strangelove",
+		Subtitle: "How I Learned to Stop Worrying and Love the Bomb",
+		Year:     1964,
+		Color:    false,
+		Actor: map[string]string{
+			"Dr. Strangelove":            "Peter Sellers",
+			"Grp. Capt. Lionel Mandrake": "Peter Sellers",
+			"Pres. Merkin Muffley":       "Peter Sellers",
+			"Gen. Buck Turgidson":        "George C. Scott",
+			"Brig. Gen. Jack D. Ripper":  "Sterling Hayden",
+			`Maj. T.J. "King" Kong`:      "Slim Pickens",
+		},
+
+		Oscars: []string{
+			"Best Actor (Nomin.)",
+			"Best Adapted Screenplay (Nomin.)",
+			"Best Director (Nomin.)",
+			"Best Picture (Nomin.)",
+		},
+	}
+	//!-strangelove
+	Display("strangelove", strangelove)
+
+	// We don't use an Output: comment since displaying
+	// a map is nondeterministic.
+	/*
+		//!+output
+		Display strangelove (display.Movie):
+		strangelove.Title = "Dr. Strangelove"
+		strangelove.Subtitle = "How I Learned to Stop Worrying and Love the Bomb"
+		strangelove.Year = 1964
+		strangelove.Color = false
+		strangelove.Actor["Gen. Buck Turgidson"] = "George C. Scott"
+		strangelove.Actor["Brig. Gen. Jack D. Ripper"] = "Sterling Hayden"
+		strangelove.Actor["Maj. T.J. \"King\" Kong"] = "Slim Pickens"
+		strangelove.Actor["Dr. Strangelove"] = "Peter Sellers"
+		strangelove.Actor["Grp. Capt. Lionel Mandrake"] = "Peter Sellers"
+		strangelove.Actor["Pres. Merkin Muffley"] = "Peter Sellers"
+		strangelove.Oscars[0] = "Best Actor (Nomin.)"
+		strangelove.Oscars[1] = "Best Adapted Screenplay (Nomin.)"
+		strangelove.Oscars[2] = "Best Director (Nomin.)"
+		strangelove.Oscars[3] = "Best Picture (Nomin.)"
+		strangelove.Sequel = nil
+		//!-output
+	*/
+}
+
+// This test ensures that the program terminates without crashing.
+func Test(t *testing.T) {
+	// Some other values (YMMV)
+	Display("os.Stderr", os.Stderr)
+	// Output:
+	// Display os.Stderr (*os.File):
+	// (*(*os.Stderr).file).fd = 2
+	// (*(*os.Stderr).file).name = "/dev/stderr"
+	// (*(*os.Stderr).file).nepipe = 0
+
+	var w io.Writer = os.Stderr
+	Display("&w", &w)
+	// Output:
+	// Display &w (*io.Writer):
+	// (*&w).type = *os.File
+	// (*(*(*&w).value).file).fd = 2
+	// (*(*(*&w).value).file).name = "/dev/stderr"
+	// (*(*(*&w).value).file).nepipe = 0
+
+	var locker sync.Locker = new(sync.Mutex)
+	Display("(&locker)", &locker)
+	// Output:
+	// Display (&locker) (*sync.Locker):
+	// (*(&locker)).type = *sync.Mutex
+	// (*(*(&locker)).value).state = 0
+	// (*(*(&locker)).value).sema = 0
+
+	Display("locker", locker)
+	// Output:
+	// Display locker (*sync.Mutex):
+	// (*locker).state = 0
+	// (*locker).sema = 0
+	// (*(&locker)) = nil
+
+	locker = nil
+	Display("(&locker)", &locker)
+	// Output:
+	// Display (&locker) (*sync.Locker):
+	// (*(&locker)) = nil
+
+	ips, _ := net.LookupHost("golang.org")
+	Display("ips", ips)
+	// Output:
+	// Display ips ([]string):
+	// ips[0] = "173.194.68.141"
+	// ips[1] = "2607:f8b0:400d:c06::8d"
+
+	// Even metarecursion!  (YMMV)
+	Display("rV", reflect.ValueOf(os.Stderr))
+	// Output:
+	// Display rV (reflect.Value):
+	// (*rV.typ).size = 8
+	// (*rV.typ).ptrdata = 8
+	// (*rV.typ).hash = 871609668
+	// (*rV.typ)._ = 0
+	// ...
+
+	// a pointer that points to itself
+	type P *P
+	var p P
+	p = &p
+	if false {
+		Display("p", p)
+		// Output:
+		// Display p (display.P):
+		// ...stuck, no output...
+	}
+
+	// a map that contains itself
+	type M map[string]M
+	m := make(M)
+	m[""] = m
+	if false {
+		Display("m", m)
+		// Output:
+		// Display m (display.M):
+		// ...stuck, no output...
+	}
+
+	// a slice that contains itself
+	type S []S
+	s := make(S, 1)
+	s[0] = s
+	if false {
+		Display("s", s)
+		// Output:
+		// Display s (display.S):
+		// ...stuck, no output...
+	}
+
+	// a linked list that eats its own tail
+	type Cycle struct {
+		Value int
+		Tail  *Cycle
+	}
+	var c Cycle
+	c = Cycle{42, &c}
+	if false {
+		Display("c", c)
+		// Output:
+		// Display c (display.Cycle):
+		// c.Value = 42
+		// (*c.Tail).Value = 42
+		// (*(*c.Tail).Tail).Value = 42
+		// ...ad infinitum...
+	}
+}

ch12/format/format.go

@@ -0,0 +1,44 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 332.
+
+// Package format provides an Any function that can format any value.
+//!+
+package format
+
+import (
+	"reflect"
+	"strconv"
+)
+
+// Any formats any value as a string.
+func Any(value interface{}) string {
+	return formatAtom(reflect.ValueOf(value))
+}
+
+// formatAtom formats a value without inspecting its internal structure.
+func formatAtom(v reflect.Value) string {
+	switch v.Kind() {
+	case reflect.Invalid:
+		return "invalid"
+	case reflect.Int, reflect.Int8, reflect.Int16,
+		reflect.Int32, reflect.Int64:
+		return strconv.FormatInt(v.Int(), 10)
+	case reflect.Uint, reflect.Uint8, reflect.Uint16,
+		reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		return strconv.FormatUint(v.Uint(), 10)
+	// ...floating-point and complex cases omitted for brevity...
+	case reflect.Bool:
+		return strconv.FormatBool(v.Bool())
+	case reflect.String:
+		return strconv.Quote(v.String())
+	case reflect.Chan, reflect.Func, reflect.Ptr, reflect.Slice, reflect.Map:
+		return v.Type().String() + " 0x" +
+			strconv.FormatUint(uint64(v.Pointer()), 16)
+	default: // reflect.Array, reflect.Struct, reflect.Interface
+		return v.Type().String() + " value"
+	}
+}
+
+//!-

ch12/format/format_test.go

@@ -0,0 +1,24 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package format_test
+
+import (
+	"fmt"
+	"testing"
+	"time"
+
+	"gopl.io/ch12/format"
+)
+
+func Test(t *testing.T) {
+	// The pointer values are just examples, and may vary from run to run.
+	//!+time
+	var x int64 = 1
+	var d time.Duration = 1 * time.Nanosecond
+	fmt.Println(format.Any(x))                  // "1"
+	fmt.Println(format.Any(d))                  // "1"
+	fmt.Println(format.Any([]int64{x}))         // "[]int64 0x8202b87b0"
+	fmt.Println(format.Any([]time.Duration{d})) // "[]time.Duration 0x8202b87e0"
+	//!-time
+}

ch12/methods/methods.go

@@ -0,0 +1,29 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 351.
+
+// Package methods provides a function to print the methods of any value.
+package methods
+
+import (
+	"fmt"
+	"reflect"
+	"strings"
+)
+
+//!+print
+// Print prints the method set of the value x.
+func Print(x interface{}) {
+	v := reflect.ValueOf(x)
+	t := v.Type()
+	fmt.Printf("type %s\n", t)
+
+	for i := 0; i < v.NumMethod(); i++ {
+		methType := v.Method(i).Type()
+		fmt.Printf("func (%s) %s%s\n", t, t.Method(i).Name,
+			strings.TrimPrefix(methType.String(), "func"))
+	}
+}
+
+//!-print

ch12/methods/methods_test.go

@@ -0,0 +1,49 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package methods_test
+
+import (
+	"strings"
+	"time"
+
+	"gopl.io/ch12/methods"
+)
+
+func ExamplePrintDuration() {
+	methods.Print(time.Hour)
+	// Output:
+	// type time.Duration
+	// func (time.Duration) Hours() float64
+	// func (time.Duration) Minutes() float64
+	// func (time.Duration) Nanoseconds() int64
+	// func (time.Duration) Seconds() float64
+	// func (time.Duration) String() string
+}
+
+func ExamplePrintReplacer() {
+	methods.Print(new(strings.Replacer))
+	// Output:
+	// type *strings.Replacer
+	// func (*strings.Replacer) Replace(string) string
+	// func (*strings.Replacer) WriteString(io.Writer, string) (int, error)
+}
+
+/*
+//!+output
+methods.Print(time.Hour)
+// Output:
+// type time.Duration
+// func (time.Duration) Hours() float64
+// func (time.Duration) Minutes() float64
+// func (time.Duration) Nanoseconds() int64
+// func (time.Duration) Seconds() float64
+// func (time.Duration) String() string
+
+methods.Print(new(strings.Replacer))
+// Output:
+// type *strings.Replacer
+// func (*strings.Replacer) Replace(string) string
+// func (*strings.Replacer) WriteString(io.Writer, string) (int, error)
+//!-output
+*/

ch12/params/params.go

@@ -0,0 +1,90 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 349.
+
+// Package params provides a reflection-based parser for URL parameters.
+package params
+
+import (
+	"fmt"
+	"net/http"
+	"reflect"
+	"strconv"
+	"strings"
+)
+
+//!+Unpack
+
+// Unpack populates the fields of the struct pointed to by ptr
+// from the HTTP request parameters in req.
+func Unpack(req *http.Request, ptr interface{}) error {
+	if err := req.ParseForm(); err != nil {
+		return err
+	}
+
+	// Build map of fields keyed by effective name.
+	fields := make(map[string]reflect.Value)
+	v := reflect.ValueOf(ptr).Elem() // the struct variable
+	for i := 0; i < v.NumField(); i++ {
+		fieldInfo := v.Type().Field(i) // a reflect.StructField
+		tag := fieldInfo.Tag           // a reflect.StructTag
+		name := tag.Get("http")
+		if name == "" {
+			name = strings.ToLower(fieldInfo.Name)
+		}
+		fields[name] = v.Field(i)
+	}
+
+	// Update struct field for each parameter in the request.
+	for name, values := range req.Form {
+		f := fields[name]
+		if !f.IsValid() {
+			continue // ignore unrecognized HTTP parameters
+		}
+		for _, value := range values {
+			if f.Kind() == reflect.Slice {
+				elem := reflect.New(f.Type().Elem()).Elem()
+				if err := populate(elem, value); err != nil {
+					return fmt.Errorf("%s: %v", name, err)
+				}
+				f.Set(reflect.Append(f, elem))
+			} else {
+				if err := populate(f, value); err != nil {
+					return fmt.Errorf("%s: %v", name, err)
+				}
+			}
+		}
+	}
+	return nil
+}
+
+//!-Unpack
+
+//!+populate
+func populate(v reflect.Value, value string) error {
+	switch v.Kind() {
+	case reflect.String:
+		v.SetString(value)
+
+	case reflect.Int:
+		i, err := strconv.ParseInt(value, 10, 64)
+		if err != nil {
+			return err
+		}
+		v.SetInt(i)
+
+	case reflect.Bool:
+		b, err := strconv.ParseBool(value)
+		if err != nil {
+			return err
+		}
+		v.SetBool(b)
+
+	default:
+		return fmt.Errorf("unsupported kind %s", v.Type())
+	}
+	return nil
+}
+
+//!-populate

ch12/search/main.go

@@ -0,0 +1,60 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 348.
+
+// Search is a demo of the params.Unpack function.
+package main
+
+import (
+	"fmt"
+	"log"
+	"net/http"
+)
+
+//!+
+
+import "gopl.io/ch12/params"
+
+// search implements the /search URL endpoint.
+func search(resp http.ResponseWriter, req *http.Request) {
+	var data struct {
+		Labels     []string `http:"l"`
+		MaxResults int      `http:"max"`
+		Exact      bool     `http:"x"`
+	}
+	data.MaxResults = 10 // set default
+	if err := params.Unpack(req, &data); err != nil {
+		http.Error(resp, err.Error(), http.StatusBadRequest) // 400
+		return
+	}
+
+	// ...rest of handler...
+	fmt.Fprintf(resp, "Search: %+v\n", data)
+}
+
+//!-
+
+func main() {
+	http.HandleFunc("/search", search)
+	log.Fatal(http.ListenAndServe(":12345", nil))
+}
+
+/*
+//!+output
+$ go build gopl.io/ch12/search
+$ ./search &
+$ ./fetch 'http://localhost:12345/search'
+Search: {Labels:[] MaxResults:10 Exact:false}
+$ ./fetch 'http://localhost:12345/search?l=golang&l=programming'
+Search: {Labels:[golang programming] MaxResults:10 Exact:false}
+$ ./fetch 'http://localhost:12345/search?l=golang&l=programming&max=100'
+Search: {Labels:[golang programming] MaxResults:100 Exact:false}
+$ ./fetch 'http://localhost:12345/search?x=true&l=golang&l=programming'
+Search: {Labels:[golang programming] MaxResults:10 Exact:true}
+$ ./fetch 'http://localhost:12345/search?q=hello&x=123'
+x: strconv.ParseBool: parsing "123": invalid syntax
+$ ./fetch 'http://localhost:12345/search?q=hello&max=lots'
+max: strconv.ParseInt: parsing "lots": invalid syntax
+//!-output
+*/

ch12/sexpr/decode.go

@@ -0,0 +1,162 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 344.
+
+// Package sexpr provides a means for converting Go objects to and
+// from S-expressions.
+package sexpr
+
+import (
+	"bytes"
+	"fmt"
+	"reflect"
+	"strconv"
+	"text/scanner"
+)
+
+//!+Unmarshal
+// Unmarshal parses S-expression data and populates the variable
+// whose address is in the non-nil pointer out.
+func Unmarshal(data []byte, out interface{}) (err error) {
+	lex := &lexer{scan: scanner.Scanner{Mode: scanner.GoTokens}}
+	lex.scan.Init(bytes.NewReader(data))
+	lex.next() // get the first token
+	defer func() {
+		// NOTE: this is not an example of ideal error handling.
+		if x := recover(); x != nil {
+			err = fmt.Errorf("error at %s: %v", lex.scan.Position, x)
+		}
+	}()
+	read(lex, reflect.ValueOf(out).Elem())
+	return nil
+}
+
+//!-Unmarshal
+
+//!+lexer
+type lexer struct {
+	scan  scanner.Scanner
+	token rune // the current token
+}
+
+func (lex *lexer) next()        { lex.token = lex.scan.Scan() }
+func (lex *lexer) text() string { return lex.scan.TokenText() }
+
+func (lex *lexer) consume(want rune) {
+	if lex.token != want { // NOTE: Not an example of good error handling.
+		panic(fmt.Sprintf("got %q, want %q", lex.text(), want))
+	}
+	lex.next()
+}
+
+//!-lexer
+
+// The read function is a decoder for a small subset of well-formed
+// S-expressions.  For brevity of our example, it takes many dubious
+// shortcuts.
+//
+// The parser assumes
+// - that the S-expression input is well-formed; it does no error checking.
+// - that the S-expression input corresponds to the type of the variable.
+// - that all numbers in the input are non-negative decimal integers.
+// - that all keys in ((key value) ...) struct syntax are unquoted symbols.
+// - that the input does not contain dotted lists such as (1 2 . 3).
+// - that the input does not contain Lisp reader macros such 'x and #'x.
+//
+// The reflection logic assumes
+// - that v is always a variable of the appropriate type for the
+//   S-expression value.  For example, v must not be a boolean,
+//   interface, channel, or function, and if v is an array, the input
+//   must have the correct number of elements.
+// - that v in the top-level call to read has the zero value of its
+//   type and doesn't need clearing.
+// - that if v is a numeric variable, it is a signed integer.
+
+//!+read
+func read(lex *lexer, v reflect.Value) {
+	switch lex.token {
+	case scanner.Ident:
+		// The only valid identifiers are
+		// "nil" and struct field names.
+		if lex.text() == "nil" {
+			v.Set(reflect.Zero(v.Type()))
+			lex.next()
+			return
+		}
+	case scanner.String:
+		s, _ := strconv.Unquote(lex.text()) // NOTE: ignoring errors
+		v.SetString(s)
+		lex.next()
+		return
+	case scanner.Int:
+		i, _ := strconv.Atoi(lex.text()) // NOTE: ignoring errors
+		v.SetInt(int64(i))
+		lex.next()
+		return
+	case '(':
+		lex.next()
+		readList(lex, v)
+		lex.next() // consume ')'
+		return
+	}
+	panic(fmt.Sprintf("unexpected token %q", lex.text()))
+}
+
+//!-read
+
+//!+readlist
+func readList(lex *lexer, v reflect.Value) {
+	switch v.Kind() {
+	case reflect.Array: // (item ...)
+		for i := 0; !endList(lex); i++ {
+			read(lex, v.Index(i))
+		}
+
+	case reflect.Slice: // (item ...)
+		for !endList(lex) {
+			item := reflect.New(v.Type().Elem()).Elem()
+			read(lex, item)
+			v.Set(reflect.Append(v, item))
+		}
+
+	case reflect.Struct: // ((name value) ...)
+		for !endList(lex) {
+			lex.consume('(')
+			if lex.token != scanner.Ident {
+				panic(fmt.Sprintf("got token %q, want field name", lex.text()))
+			}
+			name := lex.text()
+			lex.next()
+			read(lex, v.FieldByName(name))
+			lex.consume(')')
+		}
+
+	case reflect.Map: // ((key value) ...)
+		v.Set(reflect.MakeMap(v.Type()))
+		for !endList(lex) {
+			lex.consume('(')
+			key := reflect.New(v.Type().Key()).Elem()
+			read(lex, key)
+			value := reflect.New(v.Type().Elem()).Elem()
+			read(lex, value)
+			v.SetMapIndex(key, value)
+			lex.consume(')')
+		}
+
+	default:
+		panic(fmt.Sprintf("cannot decode list into %v", v.Type()))
+	}
+}
+
+func endList(lex *lexer) bool {
+	switch lex.token {
+	case scanner.EOF:
+		panic("end of file")
+	case ')':
+		return true
+	}
+	return false
+}
+
+//!-readlist

ch12/sexpr/encode.go

@@ -0,0 +1,97 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 339.
+
+package sexpr
+
+import (
+	"bytes"
+	"fmt"
+	"reflect"
+)
+
+//!+Marshal
+// Marshal encodes a Go value in S-expression form.
+func Marshal(v interface{}) ([]byte, error) {
+	var buf bytes.Buffer
+	if err := encode(&buf, reflect.ValueOf(v)); err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}
+
+//!-Marshal
+
+// encode writes to buf an S-expression representation of v.
+//!+encode
+func encode(buf *bytes.Buffer, v reflect.Value) error {
+	switch v.Kind() {
+	case reflect.Invalid:
+		buf.WriteString("nil")
+
+	case reflect.Int, reflect.Int8, reflect.Int16,
+		reflect.Int32, reflect.Int64:
+		fmt.Fprintf(buf, "%d", v.Int())
+
+	case reflect.Uint, reflect.Uint8, reflect.Uint16,
+		reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		fmt.Fprintf(buf, "%d", v.Uint())
+
+	case reflect.String:
+		fmt.Fprintf(buf, "%q", v.String())
+
+	case reflect.Ptr:
+		return encode(buf, v.Elem())
+
+	case reflect.Array, reflect.Slice: // (value ...)
+		buf.WriteByte('(')
+		for i := 0; i < v.Len(); i++ {
+			if i > 0 {
+				buf.WriteByte(' ')
+			}
+			if err := encode(buf, v.Index(i)); err != nil {
+				return err
+			}
+		}
+		buf.WriteByte(')')
+
+	case reflect.Struct: // ((name value) ...)
+		buf.WriteByte('(')
+		for i := 0; i < v.NumField(); i++ {
+			if i > 0 {
+				buf.WriteByte(' ')
+			}
+			fmt.Fprintf(buf, "(%s ", v.Type().Field(i).Name)
+			if err := encode(buf, v.Field(i)); err != nil {
+				return err
+			}
+			buf.WriteByte(')')
+		}
+		buf.WriteByte(')')
+
+	case reflect.Map: // ((key value) ...)
+		buf.WriteByte('(')
+		for i, key := range v.MapKeys() {
+			if i > 0 {
+				buf.WriteByte(' ')
+			}
+			buf.WriteByte('(')
+			if err := encode(buf, key); err != nil {
+				return err
+			}
+			buf.WriteByte(' ')
+			if err := encode(buf, v.MapIndex(key)); err != nil {
+				return err
+			}
+			buf.WriteByte(')')
+		}
+		buf.WriteByte(')')
+
+	default: // float, complex, bool, chan, func, interface
+		return fmt.Errorf("unsupported type: %s", v.Type())
+	}
+	return nil
+}
+
+//!-encode

ch12/sexpr/pretty.go

@@ -0,0 +1,183 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package sexpr
+
+// This file implements the algorithm described in Derek C. Oppen's
+// 1979 Stanford technical report, "Pretty Printing".
+
+import (
+	"bytes"
+	"fmt"
+	"reflect"
+)
+
+func MarshalIndent(v interface{}) ([]byte, error) {
+	p := printer{width: margin}
+	if err := pretty(&p, reflect.ValueOf(v)); err != nil {
+		return nil, err
+	}
+	return p.Bytes(), nil
+}
+
+const margin = 80
+
+type token struct {
+	kind rune // one of "s ()" (string, blank, start, end)
+	str  string
+	size int
+}
+
+type printer struct {
+	tokens []*token // FIFO buffer
+	stack  []*token // stack of open ' ' and '(' tokens
+	rtotal int      // total number of spaces needed to print stream
+
+	bytes.Buffer
+	indents []int
+	width   int // remaining space
+}
+
+func (p *printer) string(str string) {
+	tok := &token{kind: 's', str: str, size: len(str)}
+	if len(p.stack) == 0 {
+		p.print(tok)
+	} else {
+		p.tokens = append(p.tokens, tok)
+		p.rtotal += len(str)
+	}
+}
+func (p *printer) pop() (top *token) {
+	last := len(p.stack) - 1
+	top, p.stack = p.stack[last], p.stack[:last]
+	return
+}
+func (p *printer) begin() {
+	if len(p.stack) == 0 {
+		p.rtotal = 1
+	}
+	t := &token{kind: '(', size: -p.rtotal}
+	p.tokens = append(p.tokens, t)
+	p.stack = append(p.stack, t) // push
+	p.string("(")
+}
+func (p *printer) end() {
+	p.string(")")
+	p.tokens = append(p.tokens, &token{kind: ')'})
+	x := p.pop()
+	x.size += p.rtotal
+	if x.kind == ' ' {
+		p.pop().size += p.rtotal
+	}
+	if len(p.stack) == 0 {
+		for _, tok := range p.tokens {
+			p.print(tok)
+		}
+		p.tokens = nil
+	}
+}
+func (p *printer) space() {
+	last := len(p.stack) - 1
+	x := p.stack[last]
+	if x.kind == ' ' {
+		x.size += p.rtotal
+		p.stack = p.stack[:last] // pop
+	}
+	t := &token{kind: ' ', size: -p.rtotal}
+	p.tokens = append(p.tokens, t)
+	p.stack = append(p.stack, t)
+	p.rtotal++
+}
+func (p *printer) print(t *token) {
+	switch t.kind {
+	case 's':
+		p.WriteString(t.str)
+		p.width -= len(t.str)
+	case '(':
+		p.indents = append(p.indents, p.width)
+	case ')':
+		p.indents = p.indents[:len(p.indents)-1] // pop
+	case ' ':
+		if t.size > p.width {
+			p.width = p.indents[len(p.indents)-1] - 1
+			fmt.Fprintf(&p.Buffer, "\n%*s", margin-p.width, "")
+		} else {
+			p.WriteByte(' ')
+			p.width--
+		}
+	}
+}
+func (p *printer) stringf(format string, args ...interface{}) {
+	p.string(fmt.Sprintf(format, args...))
+}
+
+func pretty(p *printer, v reflect.Value) error {
+	switch v.Kind() {
+	case reflect.Invalid:
+		p.string("nil")
+
+	case reflect.Int, reflect.Int8, reflect.Int16,
+		reflect.Int32, reflect.Int64:
+		p.stringf("%d", v.Int())
+
+	case reflect.Uint, reflect.Uint8, reflect.Uint16,
+		reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		p.stringf("%d", v.Uint())
+
+	case reflect.String:
+		p.stringf("%q", v.String())
+
+	case reflect.Array, reflect.Slice: // (value ...)
+		p.begin()
+		for i := 0; i < v.Len(); i++ {
+			if i > 0 {
+				p.space()
+			}
+			if err := pretty(p, v.Index(i)); err != nil {
+				return err
+			}
+		}
+		p.end()
+
+	case reflect.Struct: // ((name value ...)
+		p.begin()
+		for i := 0; i < v.NumField(); i++ {
+			if i > 0 {
+				p.space()
+			}
+			p.begin()
+			p.string(v.Type().Field(i).Name)
+			p.space()
+			if err := pretty(p, v.Field(i)); err != nil {
+				return err
+			}
+			p.end()
+		}
+		p.end()
+
+	case reflect.Map: // ((key value ...)
+		p.begin()
+		for i, key := range v.MapKeys() {
+			if i > 0 {
+				p.space()
+			}
+			p.begin()
+			if err := pretty(p, key); err != nil {
+				return err
+			}
+			p.space()
+			if err := pretty(p, v.MapIndex(key)); err != nil {
+				return err
+			}
+			p.end()
+		}
+		p.end()
+
+	case reflect.Ptr:
+		return pretty(p, v.Elem())
+
+	default: // float, complex, bool, chan, func, interface
+		return fmt.Errorf("unsupported type: %s", v.Type())
+	}
+	return nil
+}

ch12/sexpr/sexpr_test.go

@@ -0,0 +1,74 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package sexpr
+
+import (
+	"reflect"
+	"testing"
+)
+
+// Test verifies that encoding and decoding a complex data value
+// produces an equal result.
+//
+// The test does not make direct assertions about the encoded output
+// because the output depends on map iteration order, which is
+// nondeterministic.  The output of the t.Log statements can be
+// inspected by running the test with the -v flag:
+//
+// 	$ go test -v gopl.io/ch12/sexpr
+//
+func Test(t *testing.T) {
+	type Movie struct {
+		Title, Subtitle string
+		Year            int
+		Actor           map[string]string
+		Oscars          []string
+		Sequel          *string
+	}
+	strangelove := Movie{
+		Title:    "Dr. Strangelove",
+		Subtitle: "How I Learned to Stop Worrying and Love the Bomb",
+		Year:     1964,
+		Actor: map[string]string{
+			"Dr. Strangelove":            "Peter Sellers",
+			"Grp. Capt. Lionel Mandrake": "Peter Sellers",
+			"Pres. Merkin Muffley":       "Peter Sellers",
+			"Gen. Buck Turgidson":        "George C. Scott",
+			"Brig. Gen. Jack D. Ripper":  "Sterling Hayden",
+			`Maj. T.J. "King" Kong`:      "Slim Pickens",
+		},
+		Oscars: []string{
+			"Best Actor (Nomin.)",
+			"Best Adapted Screenplay (Nomin.)",
+			"Best Director (Nomin.)",
+			"Best Picture (Nomin.)",
+		},
+	}
+
+	// Encode it
+	data, err := Marshal(strangelove)
+	if err != nil {
+		t.Fatalf("Marshal failed: %v", err)
+	}
+	t.Logf("Marshal() = %s\n", data)
+
+	// Decode it
+	var movie Movie
+	if err := Unmarshal(data, &movie); err != nil {
+		t.Fatalf("Unmarshal failed: %v", err)
+	}
+	t.Logf("Unmarshal() = %+v\n", movie)
+
+	// Check equality.
+	if !reflect.DeepEqual(movie, strangelove) {
+		t.Fatal("not equal")
+	}
+
+	// Pretty-print it:
+	data, err = MarshalIndent(strangelove)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("MarshalIdent() = %s\n", data)
+}

ch13/bzip-print/bzip2.c

@@ -0,0 +1,28 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 362.
+// This is the version that appears in print,
+// but it does not comply with the proposed
+// rules for passing pointers between Go and C.
+// (https://github.com/golang/proposal/blob/master/design/12416-cgo-pointers.md)
+// See gopl.io/ch13/bzip for an updated version.
+
+//!+
+/* This file is gopl.io/ch13/bzip/bzip2.c,         */
+/* a simple wrapper for libbzip2 suitable for cgo. */
+#include <bzlib.h>
+
+int bz2compress(bz_stream *s, int action,
+                char *in, unsigned *inlen, char *out, unsigned *outlen) {
+  s->next_in = in;
+  s->avail_in = *inlen;
+  s->next_out = out;
+  s->avail_out = *outlen;
+  int r = BZ2_bzCompress(s, action);
+  *inlen -= s->avail_in;
+  *outlen -= s->avail_out;
+  return r;
+}
+
+//!-

ch13/bzip-print/bzip2.go

@@ -0,0 +1,96 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 362.
+// This is the version that appears in print,
+// but it does not comply with the proposed
+// rules for passing pointers between Go and C.
+// (https://github.com/golang/proposal/blob/master/design/12416-cgo-pointers.md)
+// See gopl.io/ch13/bzip for an updated version.
+//!+
+
+// Package bzip provides a writer that uses bzip2 compression (bzip.org).
+package bzip
+
+/*
+#cgo CFLAGS: -I/usr/include
+#cgo LDFLAGS: -L/usr/lib -lbz2
+#include <bzlib.h>
+int bz2compress(bz_stream *s, int action,
+                char *in, unsigned *inlen, char *out, unsigned *outlen);
+*/
+import "C"
+
+import (
+	"io"
+	"unsafe"
+)
+
+type writer struct {
+	w      io.Writer // underlying output stream
+	stream *C.bz_stream
+	outbuf [64 * 1024]byte
+}
+
+// NewWriter returns a writer for bzip2-compressed streams.
+func NewWriter(out io.Writer) io.WriteCloser {
+	const (
+		blockSize  = 9
+		verbosity  = 0
+		workFactor = 30
+	)
+	w := &writer{w: out, stream: new(C.bz_stream)}
+	C.BZ2_bzCompressInit(w.stream, blockSize, verbosity, workFactor)
+	return w
+}
+
+//!-
+
+//!+write
+func (w *writer) Write(data []byte) (int, error) {
+	if w.stream == nil {
+		panic("closed")
+	}
+	var total int // uncompressed bytes written
+
+	for len(data) > 0 {
+		inlen, outlen := C.uint(len(data)), C.uint(cap(w.outbuf))
+		C.bz2compress(w.stream, C.BZ_RUN,
+			(*C.char)(unsafe.Pointer(&data[0])), &inlen,
+			(*C.char)(unsafe.Pointer(&w.outbuf)), &outlen)
+		total += int(inlen)
+		data = data[inlen:]
+		if _, err := w.w.Write(w.outbuf[:outlen]); err != nil {
+			return total, err
+		}
+	}
+	return total, nil
+}
+
+//!-write
+
+//!+close
+// Close flushes the compressed data and closes the stream.
+// It does not close the underlying io.Writer.
+func (w *writer) Close() error {
+	if w.stream == nil {
+		panic("closed")
+	}
+	defer func() {
+		C.BZ2_bzCompressEnd(w.stream)
+		w.stream = nil
+	}()
+	for {
+		inlen, outlen := C.uint(0), C.uint(cap(w.outbuf))
+		r := C.bz2compress(w.stream, C.BZ_FINISH, nil, &inlen,
+			(*C.char)(unsafe.Pointer(&w.outbuf)), &outlen)
+		if _, err := w.w.Write(w.outbuf[:outlen]); err != nil {
+			return err
+		}
+		if r == C.BZ_STREAM_END {
+			return nil
+		}
+	}
+}
+
+//!-close

ch13/bzip-print/bzip2_test.go

@@ -0,0 +1,40 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package bzip_test
+
+import (
+	"bytes"
+	"compress/bzip2" // reader
+	"io"
+	"testing"
+
+	"gopl.io/ch13/bzip" // writer
+)
+
+func TestBzip2(t *testing.T) {
+	var compressed, uncompressed bytes.Buffer
+	w := bzip.NewWriter(&compressed)
+
+	// Write a repetitive message in a million pieces,
+	// compressing one copy but not the other.
+	tee := io.MultiWriter(w, &uncompressed)
+	for i := 0; i < 1000000; i++ {
+		io.WriteString(tee, "hello")
+	}
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	// Check the size of the compressed stream.
+	if got, want := compressed.Len(), 255; got != want {
+		t.Errorf("1 million hellos compressed to %d bytes, want %d", got, want)
+	}
+
+	// Decompress and compare with original.
+	var decompressed bytes.Buffer
+	io.Copy(&decompressed, bzip2.NewReader(&compressed))
+	if !bytes.Equal(uncompressed.Bytes(), decompressed.Bytes()) {
+		t.Error("decompression yielded a different message")
+	}
+}

ch13/bzip/bzip2.c

@@ -0,0 +1,32 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 362.
+//
+// The version of this program that appeared in the first and second
+// printings did not comply with the proposed rules for passing
+// pointers between Go and C, described here:
+// https://github.com/golang/proposal/blob/master/design/12416-cgo-pointers.md
+//
+// The version below, which appears in the third printing,
+// has been corrected.  See bzip2.go for explanation.
+
+//!+
+/* This file is gopl.io/ch13/bzip/bzip2.c,         */
+/* a simple wrapper for libbzip2 suitable for cgo. */
+#include <bzlib.h>
+
+int bz2compress(bz_stream *s, int action,
+                char *in, unsigned *inlen, char *out, unsigned *outlen) {
+  s->next_in = in;
+  s->avail_in = *inlen;
+  s->next_out = out;
+  s->avail_out = *outlen;
+  int r = BZ2_bzCompress(s, action);
+  *inlen -= s->avail_in;
+  *outlen -= s->avail_out;
+  s->next_in = s->next_out = NULL;
+  return r;
+}
+
+//!-

ch13/bzip/bzip2.go

@@ -0,0 +1,111 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 362.
+//
+// The version of this program that appeared in the first and second
+// printings did not comply with the proposed rules for passing
+// pointers between Go and C, described here:
+// https://github.com/golang/proposal/blob/master/design/12416-cgo-pointers.md
+//
+// The rules forbid a C function like bz2compress from storing 'in'
+// and 'out' (pointers to variables allocated by Go) into the Go
+// variable 's', even temporarily.
+//
+// The version below, which appears in the third printing, has been
+// corrected.  To comply with the rules, the bz_stream variable must
+// be allocated by C code.  We have introduced two C functions,
+// bz2alloc and bz2free, to allocate and free instances of the
+// bz_stream type.  Also, we have changed bz2compress so that before
+// it returns, it clears the fields of the bz_stream that contain
+// pointers to Go variables.
+
+//!+
+
+// Package bzip provides a writer that uses bzip2 compression (bzip.org).
+package bzip
+
+/*
+#cgo CFLAGS: -I/usr/include
+#cgo LDFLAGS: -L/usr/lib -lbz2
+#include <bzlib.h>
+#include <stdlib.h>
+bz_stream* bz2alloc() { return calloc(1, sizeof(bz_stream)); }
+int bz2compress(bz_stream *s, int action,
+                char *in, unsigned *inlen, char *out, unsigned *outlen);
+void bz2free(bz_stream* s) { free(s); }
+*/
+import "C"
+
+import (
+	"io"
+	"unsafe"
+)
+
+type writer struct {
+	w      io.Writer // underlying output stream
+	stream *C.bz_stream
+	outbuf [64 * 1024]byte
+}
+
+// NewWriter returns a writer for bzip2-compressed streams.
+func NewWriter(out io.Writer) io.WriteCloser {
+	const blockSize = 9
+	const verbosity = 0
+	const workFactor = 30
+	w := &writer{w: out, stream: C.bz2alloc()}
+	C.BZ2_bzCompressInit(w.stream, blockSize, verbosity, workFactor)
+	return w
+}
+
+//!-
+
+//!+write
+func (w *writer) Write(data []byte) (int, error) {
+	if w.stream == nil {
+		panic("closed")
+	}
+	var total int // uncompressed bytes written
+
+	for len(data) > 0 {
+		inlen, outlen := C.uint(len(data)), C.uint(cap(w.outbuf))
+		C.bz2compress(w.stream, C.BZ_RUN,
+			(*C.char)(unsafe.Pointer(&data[0])), &inlen,
+			(*C.char)(unsafe.Pointer(&w.outbuf)), &outlen)
+		total += int(inlen)
+		data = data[inlen:]
+		if _, err := w.w.Write(w.outbuf[:outlen]); err != nil {
+			return total, err
+		}
+	}
+	return total, nil
+}
+
+//!-write
+
+//!+close
+// Close flushes the compressed data and closes the stream.
+// It does not close the underlying io.Writer.
+func (w *writer) Close() error {
+	if w.stream == nil {
+		panic("closed")
+	}
+	defer func() {
+		C.BZ2_bzCompressEnd(w.stream)
+		C.bz2free(w.stream)
+		w.stream = nil
+	}()
+	for {
+		inlen, outlen := C.uint(0), C.uint(cap(w.outbuf))
+		r := C.bz2compress(w.stream, C.BZ_FINISH, nil, &inlen,
+			(*C.char)(unsafe.Pointer(&w.outbuf)), &outlen)
+		if _, err := w.w.Write(w.outbuf[:outlen]); err != nil {
+			return err
+		}
+		if r == C.BZ_STREAM_END {
+			return nil
+		}
+	}
+}
+
+//!-close

ch13/bzip/bzip2_test.go

@@ -0,0 +1,40 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package bzip_test
+
+import (
+	"bytes"
+	"compress/bzip2" // reader
+	"io"
+	"testing"
+
+	"gopl.io/ch13/bzip" // writer
+)
+
+func TestBzip2(t *testing.T) {
+	var compressed, uncompressed bytes.Buffer
+	w := bzip.NewWriter(&compressed)
+
+	// Write a repetitive message in a million pieces,
+	// compressing one copy but not the other.
+	tee := io.MultiWriter(w, &uncompressed)
+	for i := 0; i < 1000000; i++ {
+		io.WriteString(tee, "hello")
+	}
+	if err := w.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	// Check the size of the compressed stream.
+	if got, want := compressed.Len(), 255; got != want {
+		t.Errorf("1 million hellos compressed to %d bytes, want %d", got, want)
+	}
+
+	// Decompress and compare with original.
+	var decompressed bytes.Buffer
+	io.Copy(&decompressed, bzip2.NewReader(&compressed))
+	if !bytes.Equal(uncompressed.Bytes(), decompressed.Bytes()) {
+		t.Error("decompression yielded a different message")
+	}
+}

ch13/bzipper/main.go

@@ -0,0 +1,29 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 365.
+
+//!+
+
+// Bzipper reads input, bzip2-compresses it, and writes it out.
+package main
+
+import (
+	"io"
+	"log"
+	"os"
+
+	"gopl.io/ch13/bzip"
+)
+
+func main() {
+	w := bzip.NewWriter(os.Stdout)
+	if _, err := io.Copy(w, os.Stdin); err != nil {
+		log.Fatalf("bzipper: %v\n", err)
+	}
+	if err := w.Close(); err != nil {
+		log.Fatalf("bzipper: close: %v\n", err)
+	}
+}
+
+//!-

ch13/equal/equal.go

@@ -0,0 +1,127 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 359.
+
+// Package equal provides a deep equivalence relation for arbitrary values.
+package equal
+
+import (
+	"reflect"
+	"unsafe"
+)
+
+//!+
+func equal(x, y reflect.Value, seen map[comparison]bool) bool {
+	if !x.IsValid() || !y.IsValid() {
+		return x.IsValid() == y.IsValid()
+	}
+	if x.Type() != y.Type() {
+		return false
+	}
+
+	// ...cycle check omitted (shown later)...
+
+	//!-
+	//!+cyclecheck
+	// cycle check
+	if x.CanAddr() && y.CanAddr() {
+		xptr := unsafe.Pointer(x.UnsafeAddr())
+		yptr := unsafe.Pointer(y.UnsafeAddr())
+		if xptr == yptr {
+			return true // identical references
+		}
+		c := comparison{xptr, yptr, x.Type()}
+		if seen[c] {
+			return true // already seen
+		}
+		seen[c] = true
+	}
+	//!-cyclecheck
+	//!+
+	switch x.Kind() {
+	case reflect.Bool:
+		return x.Bool() == y.Bool()
+
+	case reflect.String:
+		return x.String() == y.String()
+
+	// ...numeric cases omitted for brevity...
+
+	//!-
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32,
+		reflect.Int64:
+		return x.Int() == y.Int()
+
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32,
+		reflect.Uint64, reflect.Uintptr:
+		return x.Uint() == y.Uint()
+
+	case reflect.Float32, reflect.Float64:
+		return x.Float() == y.Float()
+
+	case reflect.Complex64, reflect.Complex128:
+		return x.Complex() == y.Complex()
+	//!+
+	case reflect.Chan, reflect.UnsafePointer, reflect.Func:
+		return x.Pointer() == y.Pointer()
+
+	case reflect.Ptr, reflect.Interface:
+		return equal(x.Elem(), y.Elem(), seen)
+
+	case reflect.Array, reflect.Slice:
+		if x.Len() != y.Len() {
+			return false
+		}
+		for i := 0; i < x.Len(); i++ {
+			if !equal(x.Index(i), y.Index(i), seen) {
+				return false
+			}
+		}
+		return true
+
+	// ...struct and map cases omitted for brevity...
+	//!-
+	case reflect.Struct:
+		for i, n := 0, x.NumField(); i < n; i++ {
+			if !equal(x.Field(i), y.Field(i), seen) {
+				return false
+			}
+		}
+		return true
+
+	case reflect.Map:
+		if x.Len() != y.Len() {
+			return false
+		}
+		for _, k := range x.MapKeys() {
+			if !equal(x.MapIndex(k), y.MapIndex(k), seen) {
+				return false
+			}
+		}
+		return true
+		//!+
+	}
+	panic("unreachable")
+}
+
+//!-
+
+//!+comparison
+// Equal reports whether x and y are deeply equal.
+//!-comparison
+//
+// Map keys are always compared with ==, not deeply.
+// (This matters for keys containing pointers or interfaces.)
+//!+comparison
+func Equal(x, y interface{}) bool {
+	seen := make(map[comparison]bool)
+	return equal(reflect.ValueOf(x), reflect.ValueOf(y), seen)
+}
+
+type comparison struct {
+	x, y unsafe.Pointer
+	t    reflect.Type
+}
+
+//!-comparison

ch13/equal/equal_test.go

@@ -0,0 +1,133 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package equal
+
+import (
+	"bytes"
+	"fmt"
+	"testing"
+)
+
+func TestEqual(t *testing.T) {
+	one, oneAgain, two := 1, 1, 2
+
+	type CyclePtr *CyclePtr
+	var cyclePtr1, cyclePtr2 CyclePtr
+	cyclePtr1 = &cyclePtr1
+	cyclePtr2 = &cyclePtr2
+
+	type CycleSlice []CycleSlice
+	var cycleSlice = make(CycleSlice, 1)
+	cycleSlice[0] = cycleSlice
+
+	ch1, ch2 := make(chan int), make(chan int)
+	var ch1ro <-chan int = ch1
+
+	type mystring string
+
+	var iface1, iface1Again, iface2 interface{} = &one, &oneAgain, &two
+
+	for _, test := range []struct {
+		x, y interface{}
+		want bool
+	}{
+		// basic types
+		{1, 1, true},
+		{1, 2, false},   // different values
+		{1, 1.0, false}, // different types
+		{"foo", "foo", true},
+		{"foo", "bar", false},
+		{mystring("foo"), "foo", false}, // different types
+		// slices
+		{[]string{"foo"}, []string{"foo"}, true},
+		{[]string{"foo"}, []string{"bar"}, false},
+		{[]string{}, []string(nil), true},
+		// slice cycles
+		{cycleSlice, cycleSlice, true},
+		// maps
+		{
+			map[string][]int{"foo": {1, 2, 3}},
+			map[string][]int{"foo": {1, 2, 3}},
+			true,
+		},
+		{
+			map[string][]int{"foo": {1, 2, 3}},
+			map[string][]int{"foo": {1, 2, 3, 4}},
+			false,
+		},
+		{
+			map[string][]int{},
+			map[string][]int(nil),
+			true,
+		},
+		// pointers
+		{&one, &one, true},
+		{&one, &two, false},
+		{&one, &oneAgain, true},
+		{new(bytes.Buffer), new(bytes.Buffer), true},
+		// pointer cycles
+		{cyclePtr1, cyclePtr1, true},
+		{cyclePtr2, cyclePtr2, true},
+		{cyclePtr1, cyclePtr2, true}, // they're deeply equal
+		// functions
+		{(func())(nil), (func())(nil), true},
+		{(func())(nil), func() {}, false},
+		{func() {}, func() {}, false},
+		// arrays
+		{[...]int{1, 2, 3}, [...]int{1, 2, 3}, true},
+		{[...]int{1, 2, 3}, [...]int{1, 2, 4}, false},
+		// channels
+		{ch1, ch1, true},
+		{ch1, ch2, false},
+		{ch1ro, ch1, false}, // NOTE: not equal
+		// interfaces
+		{&iface1, &iface1, true},
+		{&iface1, &iface2, false},
+		{&iface1Again, &iface1, true},
+	} {
+		if Equal(test.x, test.y) != test.want {
+			t.Errorf("Equal(%v, %v) = %t",
+				test.x, test.y, !test.want)
+		}
+	}
+}
+
+func Example_equal() {
+	//!+
+	fmt.Println(Equal([]int{1, 2, 3}, []int{1, 2, 3}))        // "true"
+	fmt.Println(Equal([]string{"foo"}, []string{"bar"}))      // "false"
+	fmt.Println(Equal([]string(nil), []string{}))             // "true"
+	fmt.Println(Equal(map[string]int(nil), map[string]int{})) // "true"
+	//!-
+
+	// Output:
+	// true
+	// false
+	// true
+	// true
+}
+
+func Example_equalCycle() {
+	//!+cycle
+	// Circular linked lists a -> b -> a and c -> c.
+	type link struct {
+		value string
+		tail  *link
+	}
+	a, b, c := &link{value: "a"}, &link{value: "b"}, &link{value: "c"}
+	a.tail, b.tail, c.tail = b, a, c
+	fmt.Println(Equal(a, a)) // "true"
+	fmt.Println(Equal(b, b)) // "true"
+	fmt.Println(Equal(c, c)) // "true"
+	fmt.Println(Equal(a, b)) // "false"
+	fmt.Println(Equal(a, c)) // "false"
+	//!-cycle
+
+	// Output:
+	// true
+	// true
+	// true
+	// false
+	// false
+}

ch13/unsafeptr/main.go

@@ -0,0 +1,38 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 357.
+
+// Package unsafeptr demonstrates basic use of unsafe.Pointer.
+package main
+
+import (
+	"fmt"
+	"unsafe"
+)
+
+func main() {
+	//!+main
+	var x struct {
+		a bool
+		b int16
+		c []int
+	}
+
+	// equivalent to pb := &x.b
+	pb := (*int16)(unsafe.Pointer(
+		uintptr(unsafe.Pointer(&x)) + unsafe.Offsetof(x.b)))
+	*pb = 42
+
+	fmt.Println(x.b) // "42"
+	//!-main
+}
+
+/*
+//!+wrong
+	// NOTE: subtly incorrect!
+	tmp := uintptr(unsafe.Pointer(&x)) + unsafe.Offsetof(x.b)
+	pb := (*int16)(unsafe.Pointer(tmp))
+	*pb = 42
+//!-wrong
+*/

ch2/boiling/main.go

@@ -0,0 +1,22 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 29.
+//!+
+
+// Boiling prints the boiling point of water.
+package main
+
+import "fmt"
+
+const boilingF = 212.0
+
+func main() {
+	var f = boilingF
+	var c = (f - 32) * 5 / 9
+	fmt.Printf("boiling point = %g°F or %g°C\n", f, c)
+	// Output:
+	// boiling point = 212°F or 100°C
+}
+
+//!-

ch2/cf/main.go

@@ -0,0 +1,32 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 43.
+//!+
+
+// Cf converts its numeric argument to Celsius and Fahrenheit.
+package main
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+
+	"gopl.io/ch2/tempconv"
+)
+
+func main() {
+	for _, arg := range os.Args[1:] {
+		t, err := strconv.ParseFloat(arg, 64)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "cf: %v\n", err)
+			os.Exit(1)
+		}
+		f := tempconv.Fahrenheit(t)
+		c := tempconv.Celsius(t)
+		fmt.Printf("%s = %s, %s = %s\n",
+			f, tempconv.FToC(f), c, tempconv.CToF(c))
+	}
+}
+
+//!-

ch2/echo4/main.go

@@ -0,0 +1,27 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 33.
+//!+
+
+// Echo4 prints its command-line arguments.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"strings"
+)
+
+var n = flag.Bool("n", false, "omit trailing newline")
+var sep = flag.String("s", " ", "separator")
+
+func main() {
+	flag.Parse()
+	fmt.Print(strings.Join(flag.Args(), *sep))
+	if !*n {
+		fmt.Println()
+	}
+}
+
+//!-

ch2/ftoc/main.go

@@ -0,0 +1,22 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 29.
+//!+
+
+// Ftoc prints two Fahrenheit-to-Celsius conversions.
+package main
+
+import "fmt"
+
+func main() {
+	const freezingF, boilingF = 32.0, 212.0
+	fmt.Printf("%g°F = %g°C\n", freezingF, fToC(freezingF)) // "32°F = 0°C"
+	fmt.Printf("%g°F = %g°C\n", boilingF, fToC(boilingF))   // "212°F = 100°C"
+}
+
+func fToC(f float64) float64 {
+	return (f - 32) * 5 / 9
+}
+
+//!-

ch2/popcount/main.go

@@ -0,0 +1,31 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 45.
+
+// (Package doc comment intentionally malformed to demonstrate golint.)
+//!+
+package popcount
+
+// pc[i] is the population count of i.
+var pc [256]byte
+
+func init() {
+	for i := range pc {
+		pc[i] = pc[i/2] + byte(i&1)
+	}
+}
+
+// PopCount returns the population count (number of set bits) of x.
+func PopCount(x uint64) int {
+	return int(pc[byte(x>>(0*8))] +
+		pc[byte(x>>(1*8))] +
+		pc[byte(x>>(2*8))] +
+		pc[byte(x>>(3*8))] +
+		pc[byte(x>>(4*8))] +
+		pc[byte(x>>(5*8))] +
+		pc[byte(x>>(6*8))] +
+		pc[byte(x>>(7*8))])
+}
+
+//!-

ch2/popcount/popcount_test.go

@@ -0,0 +1,89 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package popcount_test
+
+import (
+	"testing"
+
+	"gopl.io/ch2/popcount"
+)
+
+// -- Alternative implementations --
+
+func BitCount(x uint64) int {
+	// Hacker's Delight, Figure 5-2.
+	x = x - ((x >> 1) & 0x5555555555555555)
+	x = (x & 0x3333333333333333) + ((x >> 2) & 0x3333333333333333)
+	x = (x + (x >> 4)) & 0x0f0f0f0f0f0f0f0f
+	x = x + (x >> 8)
+	x = x + (x >> 16)
+	x = x + (x >> 32)
+	return int(x & 0x7f)
+}
+
+func PopCountByClearing(x uint64) int {
+	n := 0
+	for x != 0 {
+		x = x & (x - 1) // clear rightmost non-zero bit
+		n++
+	}
+	return n
+}
+
+func PopCountByShifting(x uint64) int {
+	n := 0
+	for i := uint(0); i < 64; i++ {
+		if x&(1<<i) != 0 {
+			n++
+		}
+	}
+	return n
+}
+
+// -- Benchmarks --
+
+func BenchmarkPopCount(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		popcount.PopCount(0x1234567890ABCDEF)
+	}
+}
+
+func BenchmarkBitCount(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		BitCount(0x1234567890ABCDEF)
+	}
+}
+
+func BenchmarkPopCountByClearing(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		PopCountByClearing(0x1234567890ABCDEF)
+	}
+}
+
+func BenchmarkPopCountByShifting(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		PopCountByShifting(0x1234567890ABCDEF)
+	}
+}
+
+// Go 1.6, 2.67GHz Xeon
+// $ go test -cpu=4 -bench=. gopl.io/ch2/popcount
+// BenchmarkPopCount-4                  200000000         6.30 ns/op
+// BenchmarkBitCount-4                  300000000         4.15 ns/op
+// BenchmarkPopCountByClearing-4        30000000         45.2 ns/op
+// BenchmarkPopCountByShifting-4        10000000        153 ns/op
+//
+// Go 1.6, 2.5GHz Intel Core i5
+// $ go test -cpu=4 -bench=. gopl.io/ch2/popcount
+// BenchmarkPopCount-4                  200000000         7.52 ns/op
+// BenchmarkBitCount-4                  500000000         3.36 ns/op
+// BenchmarkPopCountByClearing-4        50000000         34.3 ns/op
+// BenchmarkPopCountByShifting-4        20000000        108 ns/op
+//
+// Go 1.7, 3.5GHz Xeon
+// $ go test -cpu=4 -bench=. gopl.io/ch2/popcount
+// BenchmarkPopCount-12                 2000000000        0.28 ns/op
+// BenchmarkBitCount-12                 2000000000        0.27 ns/op
+// BenchmarkPopCountByClearing-12       100000000        18.5 ns/op
+// BenchmarkPopCountByShifting-12       20000000         70.1 ns/op

ch2/tempconv/conv.go

@@ -0,0 +1,16 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 41.
+
+//!+
+
+package tempconv
+
+// CToF converts a Celsius temperature to Fahrenheit.
+func CToF(c Celsius) Fahrenheit { return Fahrenheit(c*9/5 + 32) }
+
+// FToC converts a Fahrenheit temperature to Celsius.
+func FToC(f Fahrenheit) Celsius { return Celsius((f - 32) * 5 / 9) }
+
+//!-

ch2/tempconv/tempconv.go

@@ -0,0 +1,23 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+//!+
+
+// Package tempconv performs Celsius and Fahrenheit conversions.
+package tempconv
+
+import "fmt"
+
+type Celsius float64
+type Fahrenheit float64
+
+const (
+	AbsoluteZeroC Celsius = -273.15
+	FreezingC     Celsius = 0
+	BoilingC      Celsius = 100
+)
+
+func (c Celsius) String() string    { return fmt.Sprintf("%g°C", c) }
+func (f Fahrenheit) String() string { return fmt.Sprintf("%g°F", f) }
+
+//!-

ch2/tempconv0/celsius.go

@@ -0,0 +1,27 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 39.
+//!+
+
+// Package tempconv performs Celsius and Fahrenheit temperature computations.
+package tempconv
+
+import "fmt"
+
+type Celsius float64
+type Fahrenheit float64
+
+const (
+	AbsoluteZeroC Celsius = -273.15
+	FreezingC     Celsius = 0
+	BoilingC      Celsius = 100
+)
+
+func CToF(c Celsius) Fahrenheit { return Fahrenheit(c*9/5 + 32) }
+
+func FToC(f Fahrenheit) Celsius { return Celsius((f - 32) * 5 / 9) }
+
+//!-
+
+func (c Celsius) String() string { return fmt.Sprintf("%g°C", c) }

ch2/tempconv0/tempconv_test.go

@@ -0,0 +1,45 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package tempconv
+
+import "fmt"
+
+func Example_one() {
+	{
+		//!+arith
+		fmt.Printf("%g\n", BoilingC-FreezingC) // "100" °C
+		boilingF := CToF(BoilingC)
+		fmt.Printf("%g\n", boilingF-CToF(FreezingC)) // "180" °F
+		//!-arith
+	}
+	/*
+		//!+arith
+		fmt.Printf("%g\n", boilingF-FreezingC)       // compile error: type mismatch
+		//!-arith
+	*/
+
+	// Output:
+	// 100
+	// 180
+}
+
+func Example_two() {
+	//!+printf
+	c := FToC(212.0)
+	fmt.Println(c.String()) // "100°C"
+	fmt.Printf("%v\n", c)   // "100°C"; no need to call String explicitly
+	fmt.Printf("%s\n", c)   // "100°C"
+	fmt.Println(c)          // "100°C"
+	fmt.Printf("%g\n", c)   // "100"; does not call String
+	fmt.Println(float64(c)) // "100"; does not call String
+	//!-printf
+
+	// Output:
+	// 100°C
+	// 100°C
+	// 100°C
+	// 100°C
+	// 100
+	// 100
+}

ch3/basename1/main.go

@@ -0,0 +1,44 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 72.
+
+// Basename1 reads file names from stdin and prints the base name of each one.
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+)
+
+func main() {
+	input := bufio.NewScanner(os.Stdin)
+	for input.Scan() {
+		fmt.Println(basename(input.Text()))
+	}
+	// NOTE: ignoring potential errors from input.Err()
+}
+
+//!+
+// basename removes directory components and a .suffix.
+// e.g., a => a, a.go => a, a/b/c.go => c, a/b.c.go => b.c
+func basename(s string) string {
+	// Discard last '/' and everything before.
+	for i := len(s) - 1; i >= 0; i-- {
+		if s[i] == '/' {
+			s = s[i+1:]
+			break
+		}
+	}
+	// Preserve everything before last '.'.
+	for i := len(s) - 1; i >= 0; i-- {
+		if s[i] == '.' {
+			s = s[:i]
+			break
+		}
+	}
+	return s
+}
+
+//!-

ch3/basename2/main.go

@@ -0,0 +1,36 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 72.
+
+// Basename2 reads file names from stdin and prints the base name of each one.
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+)
+
+func main() {
+	input := bufio.NewScanner(os.Stdin)
+	for input.Scan() {
+		fmt.Println(basename(input.Text()))
+	}
+	// NOTE: ignoring potential errors from input.Err()
+}
+
+// basename removes directory components and a trailing .suffix.
+// e.g., a => a, a.go => a, a/b/c.go => c, a/b.c.go => b.c
+//!+
+func basename(s string) string {
+	slash := strings.LastIndex(s, "/") // -1 if "/" not found
+	s = s[slash+1:]
+	if dot := strings.LastIndex(s, "."); dot >= 0 {
+		s = s[:dot]
+	}
+	return s
+}
+
+//!-

ch3/comma/main.go

@@ -0,0 +1,40 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 73.
+
+// Comma prints its argument numbers with a comma at each power of 1000.
+//
+// Example:
+// 	$ go build gopl.io/ch3/comma
+//	$ ./comma 1 12 123 1234 1234567890
+// 	1
+// 	12
+// 	123
+// 	1,234
+// 	1,234,567,890
+//
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	for i := 1; i < len(os.Args); i++ {
+		fmt.Printf("  %s\n", comma(os.Args[i]))
+	}
+}
+
+//!+
+// comma inserts commas in a non-negative decimal integer string.
+func comma(s string) string {
+	n := len(s)
+	if n <= 3 {
+		return s
+	}
+	return comma(s[:n-3]) + "," + s[n-3:]
+}
+
+//!-

ch3/mandelbrot/main.go

@@ -0,0 +1,84 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 61.
+//!+
+
+// Mandelbrot emits a PNG image of the Mandelbrot fractal.
+package main
+
+import (
+	"image"
+	"image/color"
+	"image/png"
+	"math/cmplx"
+	"os"
+)
+
+func main() {
+	const (
+		xmin, ymin, xmax, ymax = -2, -2, +2, +2
+		width, height          = 1024, 1024
+	)
+
+	img := image.NewRGBA(image.Rect(0, 0, width, height))
+	for py := 0; py < height; py++ {
+		y := float64(py)/height*(ymax-ymin) + ymin
+		for px := 0; px < width; px++ {
+			x := float64(px)/width*(xmax-xmin) + xmin
+			z := complex(x, y)
+			// Image point (px, py) represents complex value z.
+			img.Set(px, py, mandelbrot(z))
+		}
+	}
+	png.Encode(os.Stdout, img) // NOTE: ignoring errors
+}
+
+func mandelbrot(z complex128) color.Color {
+	const iterations = 200
+	const contrast = 15
+
+	var v complex128
+	for n := uint8(0); n < iterations; n++ {
+		v = v*v + z
+		if cmplx.Abs(v) > 2 {
+			return color.Gray{255 - contrast*n}
+		}
+	}
+	return color.Black
+}
+
+//!-
+
+// Some other interesting functions:
+
+func acos(z complex128) color.Color {
+	v := cmplx.Acos(z)
+	blue := uint8(real(v)*128) + 127
+	red := uint8(imag(v)*128) + 127
+	return color.YCbCr{192, blue, red}
+}
+
+func sqrt(z complex128) color.Color {
+	v := cmplx.Sqrt(z)
+	blue := uint8(real(v)*128) + 127
+	red := uint8(imag(v)*128) + 127
+	return color.YCbCr{128, blue, red}
+}
+
+// f(x) = x^4 - 1
+//
+// z' = z - f(z)/f'(z)
+//    = z - (z^4 - 1) / (4 * z^3)
+//    = z - (z - 1/z^3) / 4
+func newton(z complex128) color.Color {
+	const iterations = 37
+	const contrast = 7
+	for i := uint8(0); i < iterations; i++ {
+		z -= (z - 1/(z*z*z)) / 4
+		if cmplx.Abs(z*z*z*z-1) < 1e-6 {
+			return color.Gray{255 - contrast*i}
+		}
+	}
+	return color.Black
+}

ch3/netflag/netflag.go

@@ -0,0 +1,30 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 77.
+
+// Netflag demonstrates an integer type used as a bit field.
+package main
+
+import (
+	"fmt"
+	. "net"
+)
+
+//!+
+func IsUp(v Flags) bool     { return v&FlagUp == FlagUp }
+func TurnDown(v *Flags)     { *v &^= FlagUp }
+func SetBroadcast(v *Flags) { *v |= FlagBroadcast }
+func IsCast(v Flags) bool   { return v&(FlagBroadcast|FlagMulticast) != 0 }
+
+func main() {
+	var v Flags = FlagMulticast | FlagUp
+	fmt.Printf("%b %t\n", v, IsUp(v)) // "10001 true"
+	TurnDown(&v)
+	fmt.Printf("%b %t\n", v, IsUp(v)) // "10000 false"
+	SetBroadcast(&v)
+	fmt.Printf("%b %t\n", v, IsUp(v))   // "10010 false"
+	fmt.Printf("%b %t\n", v, IsCast(v)) // "10010 true"
+}
+
+//!-

ch3/printints/main.go

@@ -0,0 +1,33 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 74.
+
+// Printints demonstrates the use of bytes.Buffer to format a string.
+package main
+
+import (
+	"bytes"
+	"fmt"
+)
+
+//!+
+// intsToString is like fmt.Sprint(values) but adds commas.
+func intsToString(values []int) string {
+	var buf bytes.Buffer
+	buf.WriteByte('[')
+	for i, v := range values {
+		if i > 0 {
+			buf.WriteString(", ")
+		}
+		fmt.Fprintf(&buf, "%d", v)
+	}
+	buf.WriteByte(']')
+	return buf.String()
+}
+
+func main() {
+	fmt.Println(intsToString([]int{1, 2, 3})) // "[1, 2, 3]"
+}
+
+//!-

ch3/surface/main.go

@@ -0,0 +1,62 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 58.
+//!+
+
+// Surface computes an SVG rendering of a 3-D surface function.
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+const (
+	width, height = 600, 320            // canvas size in pixels
+	cells         = 100                 // number of grid cells
+	xyrange       = 30.0                // axis ranges (-xyrange..+xyrange)
+	xyscale       = width / 2 / xyrange // pixels per x or y unit
+	zscale        = height * 0.4        // pixels per z unit
+	angle         = math.Pi / 6         // angle of x, y axes (=30°)
+)
+
+var sin30, cos30 = math.Sin(angle), math.Cos(angle) // sin(30°), cos(30°)
+
+func main() {
+	fmt.Printf("<svg xmlns='http://www.w3.org/2000/svg' "+
+		"style='stroke: grey; fill: white; stroke-width: 0.7' "+
+		"width='%d' height='%d'>", width, height)
+	for i := 0; i < cells; i++ {
+		for j := 0; j < cells; j++ {
+			ax, ay := corner(i+1, j)
+			bx, by := corner(i, j)
+			cx, cy := corner(i, j+1)
+			dx, dy := corner(i+1, j+1)
+			fmt.Printf("<polygon points='%g,%g %g,%g %g,%g %g,%g'/>\n",
+				ax, ay, bx, by, cx, cy, dx, dy)
+		}
+	}
+	fmt.Println("</svg>")
+}
+
+func corner(i, j int) (float64, float64) {
+	// Find point (x,y) at corner of cell (i,j).
+	x := xyrange * (float64(i)/cells - 0.5)
+	y := xyrange * (float64(j)/cells - 0.5)
+
+	// Compute surface height z.
+	z := f(x, y)
+
+	// Project (x,y,z) isometrically onto 2-D SVG canvas (sx,sy).
+	sx := width/2 + (x-y)*cos30*xyscale
+	sy := height/2 + (x+y)*sin30*xyscale - z*zscale
+	return sx, sy
+}
+
+func f(x, y float64) float64 {
+	r := math.Hypot(x, y) // distance from (0,0)
+	return math.Sin(r) / r
+}
+
+//!-

ch4/append/main.go

@@ -0,0 +1,79 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 88.
+
+// Append illustrates the behavior of the built-in append function.
+package main
+
+import "fmt"
+
+func appendslice(x []int, y ...int) []int {
+	var z []int
+	zlen := len(x) + len(y)
+	if zlen <= cap(x) {
+		// There is room to expand the slice.
+		z = x[:zlen]
+	} else {
+		// There is insufficient space.
+		// Grow by doubling, for amortized linear complexity.
+		zcap := zlen
+		if zcap < 2*len(x) {
+			zcap = 2 * len(x)
+		}
+		z = make([]int, zlen, zcap)
+		copy(z, x)
+	}
+	copy(z[len(x):], y)
+	return z
+}
+
+//!+append
+func appendInt(x []int, y int) []int {
+	var z []int
+	zlen := len(x) + 1
+	if zlen <= cap(x) {
+		// There is room to grow.  Extend the slice.
+		z = x[:zlen]
+	} else {
+		// There is insufficient space.  Allocate a new array.
+		// Grow by doubling, for amortized linear complexity.
+		zcap := zlen
+		if zcap < 2*len(x) {
+			zcap = 2 * len(x)
+		}
+		z = make([]int, zlen, zcap)
+		copy(z, x) // a built-in function; see text
+	}
+	z[len(x)] = y
+	return z
+}
+
+//!-append
+
+//!+growth
+func main() {
+	var x, y []int
+	for i := 0; i < 10; i++ {
+		y = appendInt(x, i)
+		fmt.Printf("%d  cap=%d\t%v\n", i, cap(y), y)
+		x = y
+	}
+}
+
+//!-growth
+
+/*
+//!+output
+0  cap=1   [0]
+1  cap=2   [0 1]
+2  cap=4   [0 1 2]
+3  cap=4   [0 1 2 3]
+4  cap=8   [0 1 2 3 4]
+5  cap=8   [0 1 2 3 4 5]
+6  cap=8   [0 1 2 3 4 5 6]
+7  cap=8   [0 1 2 3 4 5 6 7]
+8  cap=16  [0 1 2 3 4 5 6 7 8]
+9  cap=16  [0 1 2 3 4 5 6 7 8 9]
+//!-output
+*/

ch4/autoescape/main.go

@@ -0,0 +1,30 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 117.
+
+// Autoescape demonstrates automatic HTML escaping in html/template.
+package main
+
+import (
+	"html/template"
+	"log"
+	"os"
+)
+
+//!+
+func main() {
+	const templ = `<p>A: {{.A}}</p><p>B: {{.B}}</p>`
+	t := template.Must(template.New("escape").Parse(templ))
+	var data struct {
+		A string        // untrusted plain text
+		B template.HTML // trusted HTML
+	}
+	data.A = "<b>Hello!</b>"
+	data.B = "<b>Hello!</b>"
+	if err := t.Execute(os.Stdout, data); err != nil {
+		log.Fatal(err)
+	}
+}
+
+//!-

ch4/charcount/main.go

@@ -0,0 +1,56 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 97.
+//!+
+
+// Charcount computes counts of Unicode characters.
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"os"
+	"unicode"
+	"unicode/utf8"
+)
+
+func main() {
+	counts := make(map[rune]int)    // counts of Unicode characters
+	var utflen [utf8.UTFMax + 1]int // count of lengths of UTF-8 encodings
+	invalid := 0                    // count of invalid UTF-8 characters
+
+	in := bufio.NewReader(os.Stdin)
+	for {
+		r, n, err := in.ReadRune() // returns rune, nbytes, error
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "charcount: %v\n", err)
+			os.Exit(1)
+		}
+		if r == unicode.ReplacementChar && n == 1 {
+			invalid++
+			continue
+		}
+		counts[r]++
+		utflen[n]++
+	}
+	fmt.Printf("rune\tcount\n")
+	for c, n := range counts {
+		fmt.Printf("%q\t%d\n", c, n)
+	}
+	fmt.Print("\nlen\tcount\n")
+	for i, n := range utflen {
+		if i > 0 {
+			fmt.Printf("%d\t%d\n", i, n)
+		}
+	}
+	if invalid > 0 {
+		fmt.Printf("\n%d invalid UTF-8 characters\n", invalid)
+	}
+}
+
+//!-

ch4/dedup/main.go

@@ -0,0 +1,33 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 96.
+
+// Dedup prints only one instance of each line; duplicates are removed.
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+)
+
+//!+
+func main() {
+	seen := make(map[string]bool) // a set of strings
+	input := bufio.NewScanner(os.Stdin)
+	for input.Scan() {
+		line := input.Text()
+		if !seen[line] {
+			seen[line] = true
+			fmt.Println(line)
+		}
+	}
+
+	if err := input.Err(); err != nil {
+		fmt.Fprintf(os.Stderr, "dedup: %v\n", err)
+		os.Exit(1)
+	}
+}
+
+//!-

ch4/embed/main.go

@@ -0,0 +1,46 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 106.
+
+// Embed demonstrates basic struct embedding.
+package main
+
+import "fmt"
+
+type Point struct{ X, Y int }
+
+type Circle struct {
+	Point
+	Radius int
+}
+
+type Wheel struct {
+	Circle
+	Spokes int
+}
+
+func main() {
+	var w Wheel
+	//!+
+	w = Wheel{Circle{Point{8, 8}, 5}, 20}
+
+	w = Wheel{
+		Circle: Circle{
+			Point:  Point{X: 8, Y: 8},
+			Radius: 5,
+		},
+		Spokes: 20, // NOTE: trailing comma necessary here (and at Radius)
+	}
+
+	fmt.Printf("%#v\n", w)
+	// Output:
+	// Wheel{Circle:Circle{Point:Point{X:8, Y:8}, Radius:5}, Spokes:20}
+
+	w.X = 42
+
+	fmt.Printf("%#v\n", w)
+	// Output:
+	// Wheel{Circle:Circle{Point:Point{X:42, Y:8}, Radius:5}, Spokes:20}
+	//!-
+}

ch4/github/github.go

@@ -0,0 +1,35 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 110.
+//!+
+
+// Package github provides a Go API for the GitHub issue tracker.
+// See https://developer.github.com/v3/search/#search-issues.
+package github
+
+import "time"
+
+const IssuesURL = "https://api.github.com/search/issues"
+
+type IssuesSearchResult struct {
+	TotalCount int `json:"total_count"`
+	Items      []*Issue
+}
+
+type Issue struct {
+	Number    int
+	HTMLURL   string `json:"html_url"`
+	Title     string
+	State     string
+	User      *User
+	CreatedAt time.Time `json:"created_at"`
+	Body      string    // in Markdown format
+}
+
+type User struct {
+	Login   string
+	HTMLURL string `json:"html_url"`
+}
+
+//!-

ch4/github/search.go

@@ -0,0 +1,53 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+//!+
+
+package github
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/url"
+	"strings"
+)
+
+// SearchIssues queries the GitHub issue tracker.
+func SearchIssues(terms []string) (*IssuesSearchResult, error) {
+	q := url.QueryEscape(strings.Join(terms, " "))
+	resp, err := http.Get(IssuesURL + "?q=" + q)
+	if err != nil {
+		return nil, err
+	}
+	//!-
+	// For long-term stability, instead of http.Get, use the
+	// variant below which adds an HTTP request header indicating
+	// that only version 3 of the GitHub API is acceptable.
+	//
+	//   req, err := http.NewRequest("GET", IssuesURL+"?q="+q, nil)
+	//   if err != nil {
+	//       return nil, err
+	//   }
+	//   req.Header.Set(
+	//       "Accept", "application/vnd.github.v3.text-match+json")
+	//   resp, err := http.DefaultClient.Do(req)
+	//!+
+
+	// We must close resp.Body on all execution paths.
+	// (Chapter 5 presents 'defer', which makes this simpler.)
+	if resp.StatusCode != http.StatusOK {
+		resp.Body.Close()
+		return nil, fmt.Errorf("search query failed: %s", resp.Status)
+	}
+
+	var result IssuesSearchResult
+	if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
+		resp.Body.Close()
+		return nil, err
+	}
+	resp.Body.Close()
+	return &result, nil
+}
+
+//!-

ch4/graph/main.go

@@ -0,0 +1,43 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 99.
+
+// Graph shows how to use a map of maps to represent a directed graph.
+package main
+
+import "fmt"
+
+//!+
+var graph = make(map[string]map[string]bool)
+
+func addEdge(from, to string) {
+	edges := graph[from]
+	if edges == nil {
+		edges = make(map[string]bool)
+		graph[from] = edges
+	}
+	edges[to] = true
+}
+
+func hasEdge(from, to string) bool {
+	return graph[from][to]
+}
+
+//!-
+
+func main() {
+	addEdge("a", "b")
+	addEdge("c", "d")
+	addEdge("a", "d")
+	addEdge("d", "a")
+	fmt.Println(hasEdge("a", "b"))
+	fmt.Println(hasEdge("c", "d"))
+	fmt.Println(hasEdge("a", "d"))
+	fmt.Println(hasEdge("d", "a"))
+	fmt.Println(hasEdge("x", "b"))
+	fmt.Println(hasEdge("c", "d"))
+	fmt.Println(hasEdge("x", "d"))
+	fmt.Println(hasEdge("d", "x"))
+
+}

ch4/issues/main.go

@@ -0,0 +1,52 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 112.
+//!+
+
+// Issues prints a table of GitHub issues matching the search terms.
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+
+	"gopl.io/ch4/github"
+)
+
+//!+
+func main() {
+	result, err := github.SearchIssues(os.Args[1:])
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("%d issues:\n", result.TotalCount)
+	for _, item := range result.Items {
+		fmt.Printf("#%-5d %9.9s %.55s\n",
+			item.Number, item.User.Login, item.Title)
+	}
+}
+
+//!-
+
+/*
+//!+textoutput
+$ go build gopl.io/ch4/issues
+$ ./issues repo:golang/go is:open json decoder
+13 issues:
+#5680    eaigner encoding/json: set key converter on en/decoder
+#6050  gopherbot encoding/json: provide tokenizer
+#8658  gopherbot encoding/json: use bufio
+#8462  kortschak encoding/json: UnmarshalText confuses json.Unmarshal
+#5901        rsc encoding/json: allow override type marshaling
+#9812  klauspost encoding/json: string tag not symmetric
+#7872  extempora encoding/json: Encoder internally buffers full output
+#9650    cespare encoding/json: Decoding gives errPhase when unmarshalin
+#6716  gopherbot encoding/json: include field name in unmarshal error me
+#6901  lukescott encoding/json, encoding/xml: option to treat unknown fi
+#6384    joeshaw encoding/json: encode precise floating point integers u
+#6647    btracey x/tools/cmd/godoc: display type kind of each named type
+#4237  gjemiller encoding/base64: URLEncoding padding is optional
+//!-textoutput
+*/

ch4/issueshtml/main.go

@@ -0,0 +1,52 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 115.
+
+// Issueshtml prints an HTML table of issues matching the search terms.
+package main
+
+import (
+	"log"
+	"os"
+
+	"gopl.io/ch4/github"
+)
+
+//!+template
+import "html/template"
+
+var issueList = template.Must(template.New("issuelist").Parse(`
+<h1>{{.TotalCount}} issues</h1>
+<table>
+<tr style='text-align: left'>
+  <th>#</th>
+  <th>State</th>
+  <th>User</th>
+  <th>Title</th>
+</tr>
+{{range .Items}}
+<tr>
+  <td><a href='{{.HTMLURL}}'>{{.Number}}</a></td>
+  <td>{{.State}}</td>
+  <td><a href='{{.User.HTMLURL}}'>{{.User.Login}}</a></td>
+  <td><a href='{{.HTMLURL}}'>{{.Title}}</a></td>
+</tr>
+{{end}}
+</table>
+`))
+
+//!-template
+
+//!+
+func main() {
+	result, err := github.SearchIssues(os.Args[1:])
+	if err != nil {
+		log.Fatal(err)
+	}
+	if err := issueList.Execute(os.Stdout, result); err != nil {
+		log.Fatal(err)
+	}
+}
+
+//!-

ch4/issuesreport/main.go

@@ -0,0 +1,89 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 113.
+
+// Issuesreport prints a report of issues matching the search terms.
+package main
+
+import (
+	"log"
+	"os"
+	"text/template"
+	"time"
+
+	"gopl.io/ch4/github"
+)
+
+//!+template
+const templ = `{{.TotalCount}} issues:
+{{range .Items}}----------------------------------------
+Number: {{.Number}}
+User:   {{.User.Login}}
+Title:  {{.Title | printf "%.64s"}}
+Age:    {{.CreatedAt | daysAgo}} days
+{{end}}`
+
+//!-template
+
+//!+daysAgo
+func daysAgo(t time.Time) int {
+	return int(time.Since(t).Hours() / 24)
+}
+
+//!-daysAgo
+
+//!+exec
+var report = template.Must(template.New("issuelist").
+	Funcs(template.FuncMap{"daysAgo": daysAgo}).
+	Parse(templ))
+
+func main() {
+	result, err := github.SearchIssues(os.Args[1:])
+	if err != nil {
+		log.Fatal(err)
+	}
+	if err := report.Execute(os.Stdout, result); err != nil {
+		log.Fatal(err)
+	}
+}
+
+//!-exec
+
+func noMust() {
+	//!+parse
+	report, err := template.New("report").
+		Funcs(template.FuncMap{"daysAgo": daysAgo}).
+		Parse(templ)
+	if err != nil {
+		log.Fatal(err)
+	}
+	//!-parse
+	result, err := github.SearchIssues(os.Args[1:])
+	if err != nil {
+		log.Fatal(err)
+	}
+	if err := report.Execute(os.Stdout, result); err != nil {
+		log.Fatal(err)
+	}
+}
+
+/*
+//!+output
+$ go build gopl.io/ch4/issuesreport
+$ ./issuesreport repo:golang/go is:open json decoder
+13 issues:
+----------------------------------------
+Number: 5680
+User:   eaigner
+Title:  encoding/json: set key converter on en/decoder
+Age:    750 days
+----------------------------------------
+Number: 6050
+User:   gopherbot
+Title:  encoding/json: provide tokenizer
+Age:    695 days
+----------------------------------------
+...
+//!-output
+*/

ch4/movie/main.go

@@ -0,0 +1,104 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 108.
+
+// Movie prints Movies as JSON.
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"log"
+)
+
+//!+
+type Movie struct {
+	Title  string
+	Year   int  `json:"released"`
+	Color  bool `json:"color,omitempty"`
+	Actors []string
+}
+
+var movies = []Movie{
+	{Title: "Casablanca", Year: 1942, Color: false,
+		Actors: []string{"Humphrey Bogart", "Ingrid Bergman"}},
+	{Title: "Cool Hand Luke", Year: 1967, Color: true,
+		Actors: []string{"Paul Newman"}},
+	{Title: "Bullitt", Year: 1968, Color: true,
+		Actors: []string{"Steve McQueen", "Jacqueline Bisset"}},
+	// ...
+}
+
+//!-
+
+func main() {
+	{
+		//!+Marshal
+		data, err := json.Marshal(movies)
+		if err != nil {
+			log.Fatalf("JSON marshaling failed: %s", err)
+		}
+		fmt.Printf("%s\n", data)
+		//!-Marshal
+	}
+
+	{
+		//!+MarshalIndent
+		data, err := json.MarshalIndent(movies, "", "    ")
+		if err != nil {
+			log.Fatalf("JSON marshaling failed: %s", err)
+		}
+		fmt.Printf("%s\n", data)
+		//!-MarshalIndent
+
+		//!+Unmarshal
+		var titles []struct{ Title string }
+		if err := json.Unmarshal(data, &titles); err != nil {
+			log.Fatalf("JSON unmarshaling failed: %s", err)
+		}
+		fmt.Println(titles) // "[{Casablanca} {Cool Hand Luke} {Bullitt}]"
+		//!-Unmarshal
+	}
+}
+
+/*
+//!+output
+[{"Title":"Casablanca","released":1942,"Actors":["Humphrey Bogart","Ingr
+id Bergman"]},{"Title":"Cool Hand Luke","released":1967,"color":true,"Ac
+tors":["Paul Newman"]},{"Title":"Bullitt","released":1968,"color":true,"
+Actors":["Steve McQueen","Jacqueline Bisset"]}]
+//!-output
+*/
+
+/*
+//!+indented
+[
+    {
+        "Title": "Casablanca",
+        "released": 1942,
+        "Actors": [
+            "Humphrey Bogart",
+            "Ingrid Bergman"
+        ]
+    },
+    {
+        "Title": "Cool Hand Luke",
+        "released": 1967,
+        "color": true,
+        "Actors": [
+            "Paul Newman"
+        ]
+    },
+    {
+        "Title": "Bullitt",
+        "released": 1968,
+        "color": true,
+        "Actors": [
+            "Steve McQueen",
+            "Jacqueline Bisset"
+        ]
+    }
+]
+//!-indented
+*/

ch4/nonempty/main.go

@@ -0,0 +1,47 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 91.
+
+//!+nonempty
+
+// Nonempty is an example of an in-place slice algorithm.
+package main
+
+import "fmt"
+
+// nonempty returns a slice holding only the non-empty strings.
+// The underlying array is modified during the call.
+func nonempty(strings []string) []string {
+	i := 0
+	for _, s := range strings {
+		if s != "" {
+			strings[i] = s
+			i++
+		}
+	}
+	return strings[:i]
+}
+
+//!-nonempty
+
+func main() {
+	//!+main
+	data := []string{"one", "", "three"}
+	fmt.Printf("%q\n", nonempty(data)) // `["one" "three"]`
+	fmt.Printf("%q\n", data)           // `["one" "three" "three"]`
+	//!-main
+}
+
+//!+alt
+func nonempty2(strings []string) []string {
+	out := strings[:0] // zero-length slice of original
+	for _, s := range strings {
+		if s != "" {
+			out = append(out, s)
+		}
+	}
+	return out
+}
+
+//!-alt

ch4/rev/main.go

@@ -0,0 +1,60 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 86.
+
+// Rev reverses a slice.
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+)
+
+func main() {
+	//!+array
+	a := [...]int{0, 1, 2, 3, 4, 5}
+	reverse(a[:])
+	fmt.Println(a) // "[5 4 3 2 1 0]"
+	//!-array
+
+	//!+slice
+	s := []int{0, 1, 2, 3, 4, 5}
+	// Rotate s left by two positions.
+	reverse(s[:2])
+	reverse(s[2:])
+	reverse(s)
+	fmt.Println(s) // "[2 3 4 5 0 1]"
+	//!-slice
+
+	// Interactive test of reverse.
+	input := bufio.NewScanner(os.Stdin)
+outer:
+	for input.Scan() {
+		var ints []int
+		for _, s := range strings.Fields(input.Text()) {
+			x, err := strconv.ParseInt(s, 10, 64)
+			if err != nil {
+				fmt.Fprintln(os.Stderr, err)
+				continue outer
+			}
+			ints = append(ints, int(x))
+		}
+		reverse(ints)
+		fmt.Printf("%v\n", ints)
+	}
+	// NOTE: ignoring potential errors from input.Err()
+}
+
+//!+rev
+// reverse reverses a slice of ints in place.
+func reverse(s []int) {
+	for i, j := 0, len(s)-1; i < j; i, j = i+1, j-1 {
+		s[i], s[j] = s[j], s[i]
+	}
+}
+
+//!-rev

ch4/sha256/main.go

@@ -0,0 +1,25 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 83.
+
+// The sha256 command computes the SHA256 hash (an array) of a string.
+package main
+
+import "fmt"
+
+//!+
+import "crypto/sha256"
+
+func main() {
+	c1 := sha256.Sum256([]byte("x"))
+	c2 := sha256.Sum256([]byte("X"))
+	fmt.Printf("%x\n%x\n%t\n%T\n", c1, c2, c1 == c2, c1)
+	// Output:
+	// 2d711642b726b04401627ca9fbac32f5c8530fb1903cc4db02258717921a4881
+	// 4b68ab3847feda7d6c62c1fbcbeebfa35eab7351ed5e78f4ddadea5df64b8015
+	// false
+	// [32]uint8
+}
+
+//!-

ch4/treesort/sort.go

@@ -0,0 +1,50 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 101.
+
+// Package treesort provides insertion sort using an unbalanced binary tree.
+package treesort
+
+//!+
+type tree struct {
+	value       int
+	left, right *tree
+}
+
+// Sort sorts values in place.
+func Sort(values []int) {
+	var root *tree
+	for _, v := range values {
+		root = add(root, v)
+	}
+	appendValues(values[:0], root)
+}
+
+// appendValues appends the elements of t to values in order
+// and returns the resulting slice.
+func appendValues(values []int, t *tree) []int {
+	if t != nil {
+		values = appendValues(values, t.left)
+		values = append(values, t.value)
+		values = appendValues(values, t.right)
+	}
+	return values
+}
+
+func add(t *tree, value int) *tree {
+	if t == nil {
+		// Equivalent to return &tree{value: value}.
+		t = new(tree)
+		t.value = value
+		return t
+	}
+	if value < t.value {
+		t.left = add(t.left, value)
+	} else {
+		t.right = add(t.right, value)
+	}
+	return t
+}
+
+//!-

ch4/treesort/sort_test.go

@@ -0,0 +1,23 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+package treesort_test
+
+import (
+	"math/rand"
+	"sort"
+	"testing"
+
+	"gopl.io/ch4/treesort"
+)
+
+func TestSort(t *testing.T) {
+	data := make([]int, 50)
+	for i := range data {
+		data[i] = rand.Int() % 50
+	}
+	treesort.Sort(data)
+	if !sort.IntsAreSorted(data) {
+		t.Errorf("not sorted: %v", data)
+	}
+}

ch5/defer1/defer.go

@@ -0,0 +1,48 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 150.
+
+// Defer1 demonstrates a deferred call being invoked during a panic.
+package main
+
+import "fmt"
+
+//!+f
+func main() {
+	f(3)
+}
+
+func f(x int) {
+	fmt.Printf("f(%d)\n", x+0/x) // panics if x == 0
+	defer fmt.Printf("defer %d\n", x)
+	f(x - 1)
+}
+
+//!-f
+
+/*
+//!+stdout
+f(3)
+f(2)
+f(1)
+defer 1
+defer 2
+defer 3
+//!-stdout
+
+//!+stderr
+panic: runtime error: integer divide by zero
+main.f(0)
+        src/gopl.io/ch5/defer1/defer.go:14
+main.f(1)
+        src/gopl.io/ch5/defer1/defer.go:16
+main.f(2)
+        src/gopl.io/ch5/defer1/defer.go:16
+
+main.f(3)
+        src/gopl.io/ch5/defer1/defer.go:16
+main.main()
+        src/gopl.io/ch5/defer1/defer.go:10
+//!-stderr
+*/

ch5/defer2/defer.go

@@ -0,0 +1,51 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 151.
+
+// Defer2 demonstrates a deferred call to runtime.Stack during a panic.
+package main
+
+import (
+	"fmt"
+	"os"
+	"runtime"
+)
+
+//!+
+func main() {
+	defer printStack()
+	f(3)
+}
+
+func printStack() {
+	var buf [4096]byte
+	n := runtime.Stack(buf[:], false)
+	os.Stdout.Write(buf[:n])
+}
+
+//!-
+
+func f(x int) {
+	fmt.Printf("f(%d)\n", x+0/x) // panics if x == 0
+	defer fmt.Printf("defer %d\n", x)
+	f(x - 1)
+}
+
+/*
+//!+printstack
+goroutine 1 [running]:
+main.printStack()
+	src/gopl.io/ch5/defer2/defer.go:20
+main.f(0)
+	src/gopl.io/ch5/defer2/defer.go:27
+main.f(1)
+	src/gopl.io/ch5/defer2/defer.go:29
+main.f(2)
+	src/gopl.io/ch5/defer2/defer.go:29
+main.f(3)
+	src/gopl.io/ch5/defer2/defer.go:29
+main.main()
+	src/gopl.io/ch5/defer2/defer.go:15
+//!-printstack
+*/

ch5/fetch/main.go

@@ -0,0 +1,54 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 148.
+
+// Fetch saves the contents of a URL into a local file.
+package main
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"path"
+)
+
+//!+
+// Fetch downloads the URL and returns the
+// name and length of the local file.
+func fetch(url string) (filename string, n int64, err error) {
+	resp, err := http.Get(url)
+	if err != nil {
+		return "", 0, err
+	}
+	defer resp.Body.Close()
+
+	local := path.Base(resp.Request.URL.Path)
+	if local == "/" {
+		local = "index.html"
+	}
+	f, err := os.Create(local)
+	if err != nil {
+		return "", 0, err
+	}
+	n, err = io.Copy(f, resp.Body)
+	// Close file, but prefer error from Copy, if any.
+	if closeErr := f.Close(); err == nil {
+		err = closeErr
+	}
+	return local, n, err
+}
+
+//!-
+
+func main() {
+	for _, url := range os.Args[1:] {
+		local, n, err := fetch(url)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "fetch %s: %v\n", url, err)
+			continue
+		}
+		fmt.Fprintf(os.Stderr, "%s => %s (%d bytes).\n", url, local, n)
+	}
+}

ch5/findlinks1/main.go

@@ -0,0 +1,76 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 122.
+//!+main
+
+// Findlinks1 prints the links in an HTML document read from standard input.
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"golang.org/x/net/html"
+)
+
+func main() {
+	doc, err := html.Parse(os.Stdin)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "findlinks1: %v\n", err)
+		os.Exit(1)
+	}
+	for _, link := range visit(nil, doc) {
+		fmt.Println(link)
+	}
+}
+
+//!-main
+
+//!+visit
+// visit appends to links each link found in n and returns the result.
+func visit(links []string, n *html.Node) []string {
+	if n.Type == html.ElementNode && n.Data == "a" {
+		for _, a := range n.Attr {
+			if a.Key == "href" {
+				links = append(links, a.Val)
+			}
+		}
+	}
+	for c := n.FirstChild; c != nil; c = c.NextSibling {
+		links = visit(links, c)
+	}
+	return links
+}
+
+//!-visit
+
+/*
+//!+html
+package html
+
+type Node struct {
+	Type                    NodeType
+	Data                    string
+	Attr                    []Attribute
+	FirstChild, NextSibling *Node
+}
+
+type NodeType int32
+
+const (
+	ErrorNode NodeType = iota
+	TextNode
+	DocumentNode
+	ElementNode
+	CommentNode
+	DoctypeNode
+)
+
+type Attribute struct {
+	Key, Val string
+}
+
+func Parse(r io.Reader) (*Node, error)
+//!-html
+*/

ch5/findlinks2/main.go

@@ -0,0 +1,69 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 125.
+
+// Findlinks2 does an HTTP GET on each URL, parses the
+// result as HTML, and prints the links within it.
+//
+// Usage:
+//	findlinks url ...
+package main
+
+import (
+	"fmt"
+	"net/http"
+	"os"
+
+	"golang.org/x/net/html"
+)
+
+// visit appends to links each link found in n, and returns the result.
+func visit(links []string, n *html.Node) []string {
+	if n.Type == html.ElementNode && n.Data == "a" {
+		for _, a := range n.Attr {
+			if a.Key == "href" {
+				links = append(links, a.Val)
+			}
+		}
+	}
+	for c := n.FirstChild; c != nil; c = c.NextSibling {
+		links = visit(links, c)
+	}
+	return links
+}
+
+//!+
+func main() {
+	for _, url := range os.Args[1:] {
+		links, err := findLinks(url)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "findlinks2: %v\n", err)
+			continue
+		}
+		for _, link := range links {
+			fmt.Println(link)
+		}
+	}
+}
+
+// findLinks performs an HTTP GET request for url, parses the
+// response as HTML, and extracts and returns the links.
+func findLinks(url string) ([]string, error) {
+	resp, err := http.Get(url)
+	if err != nil {
+		return nil, err
+	}
+	if resp.StatusCode != http.StatusOK {
+		resp.Body.Close()
+		return nil, fmt.Errorf("getting %s: %s", url, resp.Status)
+	}
+	doc, err := html.Parse(resp.Body)
+	resp.Body.Close()
+	if err != nil {
+		return nil, fmt.Errorf("parsing %s as HTML: %v", url, err)
+	}
+	return visit(nil, doc), nil
+}
+
+//!-

ch5/findlinks3/findlinks.go

@@ -0,0 +1,56 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 139.
+
+// Findlinks3 crawls the web, starting with the URLs on the command line.
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+
+	"gopl.io/ch5/links"
+)
+
+//!+breadthFirst
+// breadthFirst calls f for each item in the worklist.
+// Any items returned by f are added to the worklist.
+// f is called at most once for each item.
+func breadthFirst(f func(item string) []string, worklist []string) {
+	seen := make(map[string]bool)
+	for len(worklist) > 0 {
+		items := worklist
+		worklist = nil
+		for _, item := range items {
+			if !seen[item] {
+				seen[item] = true
+				worklist = append(worklist, f(item)...)
+			}
+		}
+	}
+}
+
+//!-breadthFirst
+
+//!+crawl
+func crawl(url string) []string {
+	fmt.Println(url)
+	list, err := links.Extract(url)
+	if err != nil {
+		log.Print(err)
+	}
+	return list
+}
+
+//!-crawl
+
+//!+main
+func main() {
+	// Crawl the web breadth-first,
+	// starting from the command-line arguments.
+	breadthFirst(crawl, os.Args[1:])
+}
+
+//!-main

ch5/links/links.go

@@ -0,0 +1,67 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 138.
+//!+Extract
+
+// Package links provides a link-extraction function.
+package links
+
+import (
+	"fmt"
+	"net/http"
+
+	"golang.org/x/net/html"
+)
+
+// Extract makes an HTTP GET request to the specified URL, parses
+// the response as HTML, and returns the links in the HTML document.
+func Extract(url string) ([]string, error) {
+	resp, err := http.Get(url)
+	if err != nil {
+		return nil, err
+	}
+	if resp.StatusCode != http.StatusOK {
+		resp.Body.Close()
+		return nil, fmt.Errorf("getting %s: %s", url, resp.Status)
+	}
+
+	doc, err := html.Parse(resp.Body)
+	resp.Body.Close()
+	if err != nil {
+		return nil, fmt.Errorf("parsing %s as HTML: %v", url, err)
+	}
+
+	var links []string
+	visitNode := func(n *html.Node) {
+		if n.Type == html.ElementNode && n.Data == "a" {
+			for _, a := range n.Attr {
+				if a.Key != "href" {
+					continue
+				}
+				link, err := resp.Request.URL.Parse(a.Val)
+				if err != nil {
+					continue // ignore bad URLs
+				}
+				links = append(links, link.String())
+			}
+		}
+	}
+	forEachNode(doc, visitNode, nil)
+	return links, nil
+}
+
+//!-Extract
+
+// Copied from gopl.io/ch5/outline2.
+func forEachNode(n *html.Node, pre, post func(n *html.Node)) {
+	if pre != nil {
+		pre(n)
+	}
+	for c := n.FirstChild; c != nil; c = c.NextSibling {
+		forEachNode(c, pre, post)
+	}
+	if post != nil {
+		post(n)
+	}
+}

ch5/outline/main.go

@@ -0,0 +1,36 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 123.
+
+// Outline prints the outline of an HTML document tree.
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"golang.org/x/net/html"
+)
+
+//!+
+func main() {
+	doc, err := html.Parse(os.Stdin)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "outline: %v\n", err)
+		os.Exit(1)
+	}
+	outline(nil, doc)
+}
+
+func outline(stack []string, n *html.Node) {
+	if n.Type == html.ElementNode {
+		stack = append(stack, n.Data) // push tag
+		fmt.Println(stack)
+	}
+	for c := n.FirstChild; c != nil; c = c.NextSibling {
+		outline(stack, c)
+	}
+}
+
+//!-

ch5/outline2/outline.go

@@ -0,0 +1,80 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 133.
+
+// Outline prints the outline of an HTML document tree.
+package main
+
+import (
+	"fmt"
+	"net/http"
+	"os"
+
+	"golang.org/x/net/html"
+)
+
+func main() {
+	for _, url := range os.Args[1:] {
+		outline(url)
+	}
+}
+
+func outline(url string) error {
+	resp, err := http.Get(url)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+
+	doc, err := html.Parse(resp.Body)
+	if err != nil {
+		return err
+	}
+
+	//!+call
+	forEachNode(doc, startElement, endElement)
+	//!-call
+
+	return nil
+}
+
+//!+forEachNode
+// forEachNode calls the functions pre(x) and post(x) for each node
+// x in the tree rooted at n. Both functions are optional.
+// pre is called before the children are visited (preorder) and
+// post is called after (postorder).
+func forEachNode(n *html.Node, pre, post func(n *html.Node)) {
+	if pre != nil {
+		pre(n)
+	}
+
+	for c := n.FirstChild; c != nil; c = c.NextSibling {
+		forEachNode(c, pre, post)
+	}
+
+	if post != nil {
+		post(n)
+	}
+}
+
+//!-forEachNode
+
+//!+startend
+var depth int
+
+func startElement(n *html.Node) {
+	if n.Type == html.ElementNode {
+		fmt.Printf("%*s<%s>\n", depth*2, "", n.Data)
+		depth++
+	}
+}
+
+func endElement(n *html.Node) {
+	if n.Type == html.ElementNode {
+		depth--
+		fmt.Printf("%*s</%s>\n", depth*2, "", n.Data)
+	}
+}
+
+//!-startend

ch5/squares/main.go

@@ -0,0 +1,30 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 135.
+
+// The squares program demonstrates a function value with state.
+package main
+
+import "fmt"
+
+//!+
+// squares returns a function that returns
+// the next square number each time it is called.
+func squares() func() int {
+	var x int
+	return func() int {
+		x++
+		return x * x
+	}
+}
+
+func main() {
+	f := squares()
+	fmt.Println(f()) // "1"
+	fmt.Println(f()) // "4"
+	fmt.Println(f()) // "9"
+	fmt.Println(f()) // "16"
+}
+
+//!-

ch5/sum/main.go

@@ -0,0 +1,33 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 142.
+
+// The sum program demonstrates a variadic function.
+package main
+
+import "fmt"
+
+//!+
+func sum(vals ...int) int {
+	total := 0
+	for _, val := range vals {
+		total += val
+	}
+	return total
+}
+
+//!-
+
+func main() {
+	//!+main
+	fmt.Println(sum())           //  "0"
+	fmt.Println(sum(3))          //  "3"
+	fmt.Println(sum(1, 2, 3, 4)) //  "10"
+	//!-main
+
+	//!+slice
+	values := []int{1, 2, 3, 4}
+	fmt.Println(sum(values...)) // "10"
+	//!-slice
+}

ch5/title1/title.go

@@ -0,0 +1,82 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 144.
+
+// Title1 prints the title of an HTML document specified by a URL.
+package main
+
+/*
+//!+output
+$ go build gopl.io/ch5/title1
+$ ./title1 http://gopl.io
+The Go Programming Language
+$ ./title1 https://golang.org/doc/effective_go.html
+Effective Go - The Go Programming Language
+$ ./title1 https://golang.org/doc/gopher/frontpage.png
+title: https://golang.org/doc/gopher/frontpage.png
+    has type image/png, not text/html
+//!-output
+*/
+
+import (
+	"fmt"
+	"net/http"
+	"os"
+	"strings"
+
+	"golang.org/x/net/html"
+)
+
+// Copied from gopl.io/ch5/outline2.
+func forEachNode(n *html.Node, pre, post func(n *html.Node)) {
+	if pre != nil {
+		pre(n)
+	}
+	for c := n.FirstChild; c != nil; c = c.NextSibling {
+		forEachNode(c, pre, post)
+	}
+	if post != nil {
+		post(n)
+	}
+}
+
+//!+
+func title(url string) error {
+	resp, err := http.Get(url)
+	if err != nil {
+		return err
+	}
+
+	// Check Content-Type is HTML (e.g., "text/html; charset=utf-8").
+	ct := resp.Header.Get("Content-Type")
+	if ct != "text/html" && !strings.HasPrefix(ct, "text/html;") {
+		resp.Body.Close()
+		return fmt.Errorf("%s has type %s, not text/html", url, ct)
+	}
+
+	doc, err := html.Parse(resp.Body)
+	resp.Body.Close()
+	if err != nil {
+		return fmt.Errorf("parsing %s as HTML: %v", url, err)
+	}
+
+	visitNode := func(n *html.Node) {
+		if n.Type == html.ElementNode && n.Data == "title" &&
+			n.FirstChild != nil {
+			fmt.Println(n.FirstChild.Data)
+		}
+	}
+	forEachNode(doc, visitNode, nil)
+	return nil
+}
+
+//!-
+
+func main() {
+	for _, arg := range os.Args[1:] {
+		if err := title(arg); err != nil {
+			fmt.Fprintf(os.Stderr, "title: %v\n", err)
+		}
+	}
+}

ch5/title2/title.go

@@ -0,0 +1,72 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 145.
+
+// Title2 prints the title of an HTML document specified by a URL.
+// It uses defer to simplify closing the response body stream.
+package main
+
+import (
+	"fmt"
+	"net/http"
+	"os"
+	"strings"
+
+	"golang.org/x/net/html"
+)
+
+// Copied from gopl.io/ch5/outline2.
+func forEachNode(n *html.Node, pre, post func(n *html.Node)) {
+	if pre != nil {
+		pre(n)
+	}
+	for c := n.FirstChild; c != nil; c = c.NextSibling {
+		forEachNode(c, pre, post)
+	}
+	if post != nil {
+		post(n)
+	}
+}
+
+//!+
+func title(url string) error {
+	resp, err := http.Get(url)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+
+	ct := resp.Header.Get("Content-Type")
+	if ct != "text/html" && !strings.HasPrefix(ct, "text/html;") {
+		return fmt.Errorf("%s has type %s, not text/html", url, ct)
+	}
+
+	doc, err := html.Parse(resp.Body)
+	if err != nil {
+		return fmt.Errorf("parsing %s as HTML: %v", url, err)
+	}
+
+	// ...print doc's title element...
+	//!-
+	visitNode := func(n *html.Node) {
+		if n.Type == html.ElementNode && n.Data == "title" &&
+			n.FirstChild != nil {
+			fmt.Println(n.FirstChild.Data)
+		}
+	}
+	forEachNode(doc, visitNode, nil)
+	//!+
+
+	return nil
+}
+
+//!-
+
+func main() {
+	for _, arg := range os.Args[1:] {
+		if err := title(arg); err != nil {
+			fmt.Fprintf(os.Stderr, "title: %v\n", err)
+		}
+	}
+}

ch5/title3/title.go

@@ -0,0 +1,99 @@
+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 153.
+
+// Title3 prints the title of an HTML document specified by a URL.
+package main
+
+import (
+	"fmt"
+	"net/http"
+	"os"
+	"strings"
+
+	"golang.org/x/net/html"
+)
+
+// Copied from gopl.io/ch5/outline2.
+func forEachNode(n *html.Node, pre, post func(n *html.Node)) {
+	if pre != nil {
+		pre(n)
+	}
+	for c := n.FirstChild; c != nil; c = c.NextSibling {
+		forEachNode(c, pre, post)
+	}
+	if post != nil {
+		post(n)
+	}
+}
+
+//!+
+// soleTitle returns the text of the first non-empty title element
+// in doc, and an error if there was not exactly one.
+func soleTitle(doc *html.Node) (title string, err error) {
+	type bailout struct{}
+
+	defer func() {
+		switch p := recover(); p {
+		case nil:
+			// no panic
+		case bailout{}:
+			// "expected" panic
+			err = fmt.Errorf("multiple title elements")
+		default:
+			panic(p) // unexpected panic; carry on panicking
+		}
+	}()
+
+	// Bail out of recursion if we find more than one non-empty title.
+	forEachNode(doc, func(n *html.Node) {
+		if n.Type == html.ElementNode && n.Data == "title" &&
+			n.FirstChild != nil {
+			if title != "" {
+				panic(bailout{}) // multiple title elements
+			}
+			title = n.FirstChild.Data
+		}
+	}, nil)
+	if title == "" {
+		return "", fmt.Errorf("no title element")
+	}
+	return title, nil
+}
+
+//!-
+
+func title(url string) error {
+	resp, err := http.Get(url)
+	if err != nil {
+		return err
+	}
+
+	// Check Content-Type is HTML (e.g., "text/html; charset=utf-8").
+	ct := resp.Header.Get("Content-Type")
+	if ct != "text/html" && !strings.HasPrefix(ct, "text/html;") {
+		resp.Body.Close()
+		return fmt.Errorf("%s has type %s, not text/html", url, ct)
+	}
+
+	doc, err := html.Parse(resp.Body)
+	resp.Body.Close()
+	if err != nil {
+		return fmt.Errorf("parsing %s as HTML: %v", url, err)
+	}
+	title, err := soleTitle(doc)
+	if err != nil {
+		return err
+	}
+	fmt.Println(title)
+	return nil
+}
+
+func main() {
+	for _, arg := range os.Args[1:] {
+		if err := title(arg); err != nil {
+			fmt.Fprintf(os.Stderr, "title: %v\n", err)
+		}
+	}
+}