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@Blufe(Blufe)inTver Technologies

A Tour of Go の Exercise (2017/4/18現在)

Last updated at Posted at 2017-04-19

きっかけ

ちょっとした理由から目を通してただけだったA Tour of GoのExerciseをやることになったのでどうせだから残すことにしました。
ただあらかじめ言っておくとsyncパッケージを習う前に使ったりしてます。

Exercise: Loops and Functions

package main

import (
    "fmt"
    "math"
)

// DELTA ...
const DELTA = 1e-10

// Sqrt is Square root using Newton's law
func Sqrt(x float64) float64 {
    zn0, zn1 := float64(0), float64(1)
    for math.Abs(zn1-zn0) > DELTA {
        zn0, zn1 = zn1, zn1-((math.Pow(zn1, 2)-x)/(2*zn1))
    }
    return zn1
}

func main() {
    fmt.Println(Sqrt(2))
}

Exercise: Slices

package main

import "golang.org/x/tour/pic"

// Pic https://go-tour-jp.appspot.com/moretypes/18
func Pic(dx, dy int) [][]uint8 {
    data := make([][]uint8, dy)
    for y := range data {
        data[y] = make([]uint8, dx)
        for x := range data[y] {
            data[y][x] = uint8((x + y) / 2)
        }
    }
    return data
}

func main() {
    pic.Show(Pic)
}

$ go run exercise-slices.go | sed -e 's/IMAGE:\(.*\)/<img src="data:image\/png;base64,\1">/g'
<img src="data:image/png;base64,iVBORw0KGgo.....ggg==">

Exercise: Maps

package main

import (
    "strings"

    "golang.org/x/tour/wc"
)

// WordCount https://go-tour-jp.appspot.com/moretypes/23
func WordCount(s string) map[string]int {
    result := map[string]int{}
    for _, word := range strings.Fields(s) {
        result[word]++
    }
    return result
}

func main() {
    wc.Test(WordCount)
}

Exercise: Fibonacci closure

  • exercise-fibonacci-closure.go
  • https://play.golang.org/p/8VDvbbq17G
    • これで実現できるけど、i0 := -1とか逆に繰り返してみただけで、どこから出てきたの？って感じだから良いのかな...
    • if文でn=0とn=1は実現した方が正しいかもしれない
package main

import "fmt"

// fibonacci is a function that returns
// a function that returns an int.
func fibonacci() func() int {
    i0, i1 := -1, 1
    return func() int {
        i0, i1 = i1, (i0 + i1)
        return i1
    }
}

func main() {
    f := fibonacci()
    for i := 0; i < 10; i++ {
        fmt.Println(f())
    }
}

Exercise: Stringers

  • exercise-stringer.go
  • https://play.golang.org/p/8PrLYNy3Jn
    • 問題の難しさが1ランク下がった感あってとっても不安だった(こんなんで良いの？みたいな)
package main

import (
    "fmt"
)

// IPAddr https://go-tour-jp.appspot.com/methods/18
type IPAddr [4]byte

// TODO: Add a "String() string" method to IPAddr.
func (ip IPAddr) String() string {
    return fmt.Sprintf("%d.%d.%d.%d", ip[0], ip[1], ip[2], ip[3])
}

func main() {
    hosts := map[string]IPAddr{
        "loopback":  {127, 0, 0, 1},
        "googleDNS": {8, 8, 8, 8},
    }
    for name, ip := range hosts {
        fmt.Printf("%v: %v\n", name, ip)
    }
}

Exercise: Errors

  • exercise-errors.go
  • https://play.golang.org/p/mMSJpKfY5K
    • fmt.Sprint() は確かに無限ループなんだけど、別にfmt.Sprintf() にしちゃえば良いのではと思った
    • "%v"を第一引数に指定したら同じことが起きるしバグの元だから駄目ですかね
package main

import (
    "fmt"
    "math"
)

// ErrNegativeSqrt https://go-tour-jp.appspot.com/methods/20
type ErrNegativeSqrt float64

func (x ErrNegativeSqrt) Error() string {
    return fmt.Sprintf("cannot Sqrt negative number: %g", x)
}

// Sqrt is Square root using Newton's law
func Sqrt(x float64) (float64, error) {
    if x < 0 {
        return 0, ErrNegativeSqrt(x)
    }

    zn0 := float64(0)
    zn1 := float64(1)
    for math.Abs(zn1-zn0) > 1e-10 {
        zn0, zn1 = zn1, zn1-((math.Pow(zn1, 2)-x)/(2*zn1))
    }
    return zn1, nil
}

func main() {
    fmt.Println(Sqrt(2))
    fmt.Println(Sqrt(-2))
}

Exercise: Readers

package main

import (
    "golang.org/x/tour/reader"
)

// MyReader https://go-tour-jp.appspot.com/methods/22
type MyReader struct{}

func (r MyReader) Read(buf []byte) (int, error) {
    for i := range buf {
        buf[i] = 'A'
    }
    return len(buf), nil
}

func main() {
    reader.Validate(MyReader{})
}

Exercise: rot13Reader

  • exercise-rot-reader.go
  • https://play.golang.org/p/_Z4KDxjZU1
    • 数値(13とか26とか)のマジックナンバーを登場させず、ある程度読めるように書いたらこうなった
package main

import (
    "io"
    "os"
    "strings"
)

type rot13Reader struct {
    r io.Reader
}

// Read https://go-tour-jp.appspot.com/methods/23
func (r3r rot13Reader) Read(buf []byte) (int, error) {
    size, err := r3r.r.Read(buf)
    if err == io.EOF {
        return size, err
    }

    for i, char := range buf {
        switch {
        case 'A' <= char && char <= 'M':
            buf[i] = 'N' + (char - 'A')
        case 'N' <= char && char <= 'Z':
            buf[i] = 'A' + (char - 'N')
        case 'a' <= char && char <= 'm':
            buf[i] = 'n' + (char - 'a')
        case 'n' <= char && char <= 'z':
            buf[i] = 'a' + (char - 'n')
        default:
            continue
        }
    }

    return size, nil
}

func main() {
    s := strings.NewReader("Lbh penpxrq gur pbqr!")
    r := rot13Reader{s}
    io.Copy(os.Stdout, &r)
}

Exercise: Images

package main

import (
    "image"
    "image/color"

    "golang.org/x/tour/pic"
)

// Image https://go-tour-jp.appspot.com/methods/25
type Image struct{}

// ColorModel ...
func (img Image) ColorModel() color.Model {
    return color.RGBAModel
}

// Bounds ...
func (img Image) Bounds() image.Rectangle {
    return image.Rect(0, 0, 256, 256)
}

// At ...
func (img Image) At(x, y int) color.Color {
    v := uint8((x + y) / 2)
    return color.RGBA{v, v, 255, 255}
}

func main() {
    m := Image{}
    pic.ShowImage(m)
}

Exercise: Equivalent Binary Trees

  • exercise-equivalent-binary-trees.go
  • https://play.golang.org/p/7GI8qq1S5b
    • 自由度高かったからsyncパッケージとか勝手に使った
    • ネットで見たらWalk()は他の例の中でもシンプルに書けてると思う
    • Same()は少し長いけど受信と比較の処理を分離させた
      • というか他の例が受信側が直列ばっかりなんだけどそんなので本当に良いんだろうか？
      • と思ってselect文で複数channelsを並べてみた
package main

import (
    "fmt"
    "sync"

    "golang.org/x/tour/tree"
)

// Walk walks the tree t sending all values
// from the tree to the channel ch.
func Walk(t *tree.Tree, ch chan int) {
    if t == nil {
        return
    }

    Walk(t.Left, ch)
    ch <- t.Value
    Walk(t.Right, ch)
}

// Same determines whether the trees
// t1 and t2 contain the same values.
func Same(t1, t2 *tree.Tree) bool {
    done := make(chan bool, 1)
    ch1, ch2 := make(chan int, 1), make(chan int, 1)
    buf1, buf2 := make([]int, 0), make([]int, 0)

    // sender side logic
    wg := new(sync.WaitGroup)
    wg.Add(2)
    go func() {
        Walk(t1, ch1)
        wg.Done()
    }()
    go func() {
        Walk(t2, ch2)
        wg.Done()
    }()
    go func() {
        wg.Wait()
        close(done)
    }()

    // reciever side logic
loop:
    for {
        select {
        case num := <-ch1:
            buf1 = append(buf1, num)

        case num := <-ch2:
            buf2 = append(buf2, num)

        case <-done:
            break loop
        }
    }

    // check values
    if len(buf1) != len(buf2) {
        return false
    }
    for i := range buf1 {
        if buf1[i] != buf2[i] {
            return false
        }
    }
    return true

}

func main() {
    fmt.Println("Walk")
    ch := make(chan int, 1)
    go func() {
        defer close(ch)
        Walk(tree.New(1), ch)
    }()
    for num := range ch {
        fmt.Println(num)
    }

    fmt.Println("Check 1")
    actual1 := Same(tree.New(1), tree.New(1))
    if actual1 {
        fmt.Println("same")
    }

    fmt.Println("Check 2")
    actual2 := Same(tree.New(1), tree.New(2))
    if !actual2 {
        fmt.Println("not same")
    }
}

Exercise: Web Crawler

  • exercise-web-crawler.go
  • https://play.golang.org/p/MjytQIQbuU
    • 実行結果これで合ってるよね？？
    • sync.Mutex{}.Lock()しないといけないのは内部で保持するURLの履歴だけで合ってるよね？
    • クロージャの中で自身を呼ぶ処理はエラーになるようなので先に変数定義だけしてる
    • Crawl()で無理にやってしまったけど、Fetcherインターフェースを持つ側の問題なので間違ったなと後々思った
      • 本来はfakeFetcherが持つmapは初期は空のはずなので簡単なんだけど
      • 今回は初めからfetch結果を持ってしまっているので別に履歴管理をしないといけない
      • それが微妙なのでCrawl()のままにした
package main

import (
    "fmt"
    "sync"
)

// Fetcher https://go-tour-jp.appspot.com/concurrency/9
type Fetcher interface {
    // Fetch returns the body of URL and
    // a slice of URLs found on that page.
    Fetch(url string) (body string, urls []string, err error)
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {
    wg := new(sync.WaitGroup)
    defer wg.Wait()

    mux := new(sync.Mutex)
    history := map[string]int{}

    var checkOrUpdateFetchedURL func(url string) bool
    checkOrUpdateFetchedURL = func(url string) bool {
        mux.Lock()
        defer mux.Unlock()
        if _, ok := history[url]; ok {
            return true
        }
        history[url]++
        return false
    }

    var crawler func(url string, depth int)
    crawler = func(url string, depth int) {
        defer wg.Done()
        if depth <= 0 {
            return
        }

        // Don't fetch the same URL twice.
        if checkOrUpdateFetchedURL(url) {
            return
        }

        body, urls, err := fetcher.Fetch(url)
        if err != nil {
            fmt.Println(err)
            return
        }
        fmt.Printf("found: %s %q\n", url, body)

        // Fetch URLs in parallel.
        for _, u := range urls {
            wg.Add(1)
            go crawler(u, depth-1)
        }

        return
    }

    wg.Add(1)
    go crawler(url, depth)
    return
}

func main() {
    Crawl("http://golang.org/", 4, fetcher)
}

// fakeFetcher is Fetcher that returns canned results.
type fakeFetcher map[string]*fakeResult

type fakeResult struct {
    body string
    urls []string
}

func (f fakeFetcher) Fetch(url string) (string, []string, error) {
    if res, ok := f[url]; ok {
        return res.body, res.urls, nil
    }
    return "", nil, fmt.Errorf("not found: %s", url)
}

// fetcher is a populated fakeFetcher.
var fetcher = fakeFetcher{
    "http://golang.org/": &fakeResult{
        "The Go Programming Language",
        []string{
            "http://golang.org/pkg/",
            "http://golang.org/cmd/",
        },
    },
    "http://golang.org/pkg/": &fakeResult{
        "Packages",
        []string{
            "http://golang.org/",
            "http://golang.org/cmd/",
            "http://golang.org/pkg/fmt/",
            "http://golang.org/pkg/os/",
        },
    },
    "http://golang.org/pkg/fmt/": &fakeResult{
        "Package fmt",
        []string{
            "http://golang.org/",
            "http://golang.org/pkg/",
        },
    },
    "http://golang.org/pkg/os/": &fakeResult{
        "Package os",
        []string{
            "http://golang.org/",
            "http://golang.org/pkg/",
        },
    },
}
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