My Solutions / answers to tour.golang.org Jan 29, 2016

concurrency-10.go

//Exercise: Web Crawler http://tour.golang.org/concurrency/10
package main

import (
	"fmt"
	"sync"
)

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)
}

type syncStringSet struct {
	set map[string]bool
	mux sync.Mutex
}

// set.add(string) returns true if it does not have string in set, but has now inserted
// the string into the set.
// set.add(url) returns false if it has the url in its cache
func (ss *syncStringSet) add(str string) bool {
	ss.mux.Lock()
	defer ss.mux.Unlock()
	if _, hasEntry := ss.set[str]; hasEntry {
		return false
	}
	ss.set[str] = true
	return true
}

type result struct {
	url  string
	body string
	err  error
}

type crawlError struct {
	error
}

func crawl(
	url string,
	depth int,
	fetcher Fetcher,
	results chan result,
	seen *syncStringSet,
	crawlers *sync.WaitGroup,
) {
	defer crawlers.Done()
	if depth < 1 {
		results <- result{
			url: url,
			err: crawlError{fmt.Errorf("depth exceeded (%d)", depth)},
		}
		return
	}
	if !seen.add(url) {
		results <- result{
			url: url,
			err: crawlError{fmt.Errorf("seen.add(%q) == false", url)},
		}
		return
	}
	body, urls, err := fetcher.Fetch(url)
	for _, u := range urls {
		crawlers.Add(1)
		go crawl(u, depth-1, fetcher, results, seen, crawlers)
	}
	results <- result{url, body, err}
}

func Crawl(url string, depth int, fetcher Fetcher) {
	seen := &syncStringSet{set: make(map[string]bool)}
	crawlers := &sync.WaitGroup{}

	results := make(chan result)

	//expected_msgs.inc()
	crawlers.Add(1)
	go crawl(url, depth, fetcher, results, seen, crawlers)

	go func() {
		crawlers.Wait()
		close(results)
	}()

	for res := range results {
		_, isCrawlError := res.err.(crawlError)
		switch {
		case isCrawlError:
			//fmt.Println(res.err)
		case res.err != nil:
			fmt.Println(res.err)
		default:
			fmt.Printf("found: %s %q\n", res.url, res.body)
		}
	}
}

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/",
		},
	},
}

concurrency-8.go

// Exercise: Equivalent Binary Trees http://tour.golang.org/concurrency/8
// 
// 1. Implement the Walk function.
// 
// 2. Test the Walk function.
// 
// The function tree.New(k) constructs a randomly-structured binary tree
// holding the values k, 2k, 3k, ..., 10k.
// 
// Create a new channel ch and kick off the walker:
// 
// go Walk(tree.New(1), ch)
// 
// Then read and print 10 values from the channel. It should be the numbers
// 1, 2, 3, ..., 10.
// 
// 3. Implement the Same function using Walk to determine whether t1 and t2
// store the same values.
// 
// 4. Test the Same function.
// 
// Same(tree.New(1), tree.New(1)) should return true, and Same(tree.New(1),
// tree.New(2)) should return false.

package main

import (
	"fmt"
	"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.Left != nil {
		Walk(t.Left, ch)
	}
	ch <- t.Value
	if t.Right != nil {
		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:=0; i<10; i++ {
		v1 := <- ch1
		v2 := <- ch2
		if v1 != v2 {
			return false
		}
	}
	return true
}

func main() {
	if Same(tree.New(1), tree.New(1)) {
		fmt.Println("The trees are the same.")
	} else {
		fmt.Println("The trees are NOT the same.")
	}
}
/*
func main() {
	ch := make(chan int)
	go Walk(tree.New(1), ch)
	for i := 0; i < 10; i++ {
		fmt.Println(<-ch)
	}
}
*/

flow-control-8.extra.go

//Exercise: Loops and Functions http://tour.golang.org/flowcontrol/8
// Next, change the loop condition to stop once the value has stopped changing
// (or only changes by a very small delta). See if that's more or fewer
// iterations. How close are you to the math.Sqrt? 

package main

import (
	"fmt"
	"math"
)

func Sqrt(x float64) float64 {
	delta := 0.0001 //terminates at i=7
	//delta := 0.00001 //terminates at i=9
	//delta := 0.000001 //terminates at i=11
	z := float64(1)
	for i := 0; i < 100; i++ {
		z0 := z
		z = z - ((z*z - x) / (2*x))
		if math.Abs(z-z0) < delta {
			fmt.Printf("small enough i=%d, delta=%f\n", i, delta)
			break
		}
	}
	return z
}

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

flow-control-8.go

//Exercise: Loops and Functions http://tour.golang.org/flowcontrol/8
package main

import (
	"fmt"
)

func Sqrt(x float64) float64 {
	z := float64(1)
	for i := 0; i < 10; i++ {
		z = z - ((z*z - x) / (2*x))
	}
	return z
}

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

methods-18.go

//Exercise: Stringers http://tour.golang.org/methods/18
package main

import "fmt"

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

methods-20.go

//Exercise: Errors http://tour.golang.org/methods/20
package main

import (
	"fmt"
	"math"
)

type ErrNegativeSqrt float64

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

func Sqrt(x float64) (float64, error) {
	if x < 0 {
		return math.NaN(), ErrNegativeSqrt(x)
	}
	z := float64(1)
	for i :=1; i<10; i++ {
		z = z - ((z*z)-x)/(2*x)
	}
	return z, nil
}

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

methods-22.go

//Exercise: Readers http://tour.golang.org/methods/22
package main

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

type MyReader struct{}

func (m MyReader) Read(b []byte) (n int, err error) {
	for n = range b {
		b[n] = 'A'
	}
	return
}

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

methods-23.go

//Exercise: rot13Reader http://tour.golang.org/methods/23
package main

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

type rot13Reader struct {
	r io.Reader
}

func (rot *rot13Reader) Read(b []byte) (n int, err error) {
	n, err = rot.r.Read(b)
	for i, v := range b {
		switch {
		case v >= 'A' && v <= 'Z':
			if v < 'N' {
				b[i] += 13
			} else {
				b[i] -= 13
			}
		case v >= 'a' && v <= 'z':
			if v < 'n' {
				b[i] += 13
			} else {
				b[i] -= 13
			}
		}
	}
	return
}

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

methods-25.go

//Exercise: Images http://tour.golang.org/methods/25
package main

import (
	"golang.org/x/tour/pic"
	"image"
	"image/color"
	)

type Image struct{}

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

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

func (i Image) At(x, y int) color.Color {
	return color.RGBA{72, 72, uint8(x^y), 255}
}


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

moretypes-15.go

//Exercise: Slices http://tour.golang.org/moretypes/15
package main

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

func Pic(dx, dy int) [][]uint8 {
	var p = make([]([]uint8), dy)
	for y := 0; y < len(p); y++ {
		p[y] = make([]uint8, dx)
		for x := 0; x < len(p[y]); x++ {
			//p[y][x] = uint8((y + x) / 2)
			//p[y][x] = uint8(y * x)
			p[y][x] = uint8(y ^ x)
        }
    }
	return p
}

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

moretypes-20.go

//Exercise: Maps http://tour.golang.org/moretypes/20
package main

import (
	"golang.org/x/tour/wc"
	"strings"
)

func WordCount(s string) map[string]int {
	wordcount := make(map[string]int)
	for _, w := range strings.Fields(s) {
		wordcount[w]++
	}
	return wordcount
}

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

moretypes-23.go

//Exercise: Fibonacci closure http://tour.golang.org/moretypes/23
package main

import "fmt"

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

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