tour.golang exercise solutions

## exercise.tour.go
/* Exercise: Loops and Functions #43 */
package main

import (
	"fmt"
	"math"
)

func Sqrt(x float64) float64 {
	z := float64(2.)
	s := float64(0)
	for {
		z = z - (z*z - x)/(2*z)
		if math.Abs(s-z) < 1e-15 {
			break
		}
		s = z
	}
	return s
}

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

/******************************************************************************************************/

/* Exercise: Maps #44 */
package main

import (
	"tour/wc"
	"strings"
)

func WordCount(s string) map[string]int {
	ss := strings.Fields(s)
	num := len(ss)
	ret := make(map[string]int)
	for i := 0; i < num; i++ {
		(ret[ss[i]])++
	}
	return ret
}

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

/******************************************************************************************************/

/* Exercise: Slices #45 */
package main

import "tour/pic"

func Pic(dx, dy int) [][]uint8 {
	ret := make([][]uint8, dy)
	for i := 0; i < dy; i++ {
		ret[i] = make([]uint8, dx)
		for j := 0; j < dx; j++ {
			ret[i][j] = uint8(i^j+(i+j)/2)
		}
	}
	return ret
}

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

/******************************************************************************************************/

/* Exercise: Fibonacci closure #46 */
package main

import "fmt"

// fibonacci is a function that returns
// a function that returns an int.
func fibonacci() func() int {
	x := 0
	y := 1
	return func() int {
		x,y = y,x+y
		return x
	}
}

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

/******************************************************************************************************/

/* Advanced Exercise: Complex cube roots #47 */
package main

import (
	"fmt"
	"math/cmplx"
)

func Cbrt(x complex128) complex128 {
	z := complex128(2)
	s := complex128(0)
	for {
		z = z - (cmplx.Pow(z,3) - x)/(3 * (z * z))
		if cmplx.Abs(s-z) < 1e-17 {
			break
		}
		s = z
	}
	return z
}

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

/******************************************************************************************************/

/* Exercise: Errors #57 */
package main

import (
	"fmt"
)

type ErrNegativeSqrt float64

func (e ErrNegativeSqrt) Error() string {
	return fmt.Sprintf("cannot Sqrt negativ number: %g", float64(e))
}

func Sqrt(f float64) (float64, error) {
	if f < 0 {
		return 0, ErrNegativeSqrt(f)
	}
	return 0, nil
}

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

/******************************************************************************************************/

/* Exercise: Images #58 */
package main

import (
	"image"
	"tour/pic"
	"image/color"
)

type Image struct{
	Width, Height int
	colr uint8
}

func (r *Image) Bounds() image.Rectangle {
	return image.Rect(0, 0, r.Width, r.Height)
}

func (r *Image) ColorModel() color.Model {
	return color.RGBAModel
}

func (r *Image) At(x, y int) color.Color {
	return color.RGBA{r.colr+uint8(x), r.colr+uint8(y), 255, 255}
}

func main() {
	m := Image{100, 100, 128}
	pic.ShowImage(&m)
}

/******************************************************************************************************/
/* Exercise: Rot13 Reader #59: 'You cracked the code!' */
package main

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

type rot13Reader struct {
	r io.Reader
}

func (rot *rot13Reader) Read(p []byte) (n int, err error) {
	n,err = rot.r.Read(p)
	for i := 0; i < len(p); i++ {
		if (p[i] >= 'A' && p[i] < 'N') || (p[i] >='a' && p[i] < 'n') {
			p[i] += 13
		} else if (p[i] > 'M' && p[i] <= 'Z') || (p[i] > 'm' && p[i] <= 'z'){
			p[i] -= 13
		}
	}
	return
}

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

/******************************************************************************************************/

/* Exercise: Equivalent Binary Trees #67 */
package main

import (
	"tour/tree"
	"fmt"
)

// Walk walks the tree t sending all values
// from the tree to the channel ch.
func Walk(t *tree.Tree, ch chan int) {
	_walk(t, ch)
	close(ch)

}

func _walk(t *tree.Tree, ch chan int) {
	if t != nil {
		_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 {
	ch1 := make(chan int)
	ch2 := make(chan int)
	go Walk(t1, ch1)
	go Walk(t2, ch2)
	for i := range ch1 {
		if i != <- ch2 {
			return false
		}
	}
	return true
}

func main() {
	//tree.New(2)
	ch := make(chan int)
	go Walk(tree.New(1), ch)
	for v := range ch {
		fmt.Print(v)
	}
	fmt.Println(Same(tree.New(1), tree.New(1)))
	fmt.Println(Same(tree.New(1), tree.New(2)))
}

/******************************************************************************************************/

/* Exercise: Web Crawler #69 */
package main

import (
    "fmt"
)

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)
}

var store map[string]bool

func Krawl(url string, fetcher Fetcher, Urls chan []string) {
    body, urls, err := fetcher.Fetch(url)
    if err != nil {
        fmt.Println(err)
    } else {
        fmt.Printf("found: %s %q\n", url, body)
    }
    Urls <- urls
}

func Crawl(url string, depth int, fetcher Fetcher) {
    Urls := make(chan []string)
    go Krawl(url, fetcher, Urls)
    band := 1
    store[url] = true // init for level 0 done
    for i := 0; i < depth; i++ {
        for band > 0 {
            band--
            next := <- Urls
            for _, url := range next {
                if _, done := store[url] ; !done {
                    store[url] = true
                    band++
                    go Krawl(url, fetcher, Urls)
                }
            }
        }
    }
    return
}

func main() {
    store = make(map[string]bool)
    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/",
        },
    },
}