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zyxar/exercise.tour.go

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tour.golang exercise solutions
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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 {
var x, y = 0, 1
return func() (z int) {
z, x, y = x, y, x+y
return
}
}
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/",
},
},
}
@Aternus

Aternus commented Aug 16, 2022

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rot13Reader Solution:
❓ this solution saves an extra loop by returning io.EOF as early as possible.

package tour

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

type rot13Reader struct {
	r io.Reader
}

func (r rot13Reader) Read(b []byte) (int, error) {
	n, err := r.r.Read(b)
	// short circuit: EOF
	if err == io.EOF {
		return 0, io.EOF
	}
	for k, v := range b {
		b[k] = rot13(v)
	}
	return n, err
}

func Rot13ReaderMain() {
	s := strings.NewReader("Lbh penpxrq gur pbqr!")
	r := rot13Reader{s}
	io.Copy(os.Stdout, &r)
}

func rot13(b byte) byte {
	if b >= 'A' && b <= 'Z' {
		b = 'A' + (b-'A'+13)%26
	} else if b >= 'a' && b <= 'z' {
		b = 'a' + (b-'a'+13)%26
	}
	return b
}

@Aternus

Aternus commented Aug 22, 2022

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Images exercise solution:

package tour

import (
	"image"
	"image/color"
)

type Image struct {
	Width  int
	Height int
}

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

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

func (i Image) At(x, y int) color.Color {
	r := uint8(x + y)
	g := uint8(x ^ y)
	b := uint8(x * y)
	//fmt.Println("R:", r, "G:", g, "B:", b)
	return color.RGBA{r, g, b, 255}
}

@AlbertoCoder

AlbertoCoder commented Sep 12, 2022 •
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This is my solution to the Exercise "Stringers":

package main

import "fmt"

type IPAddr [4]byte

// TODO: Add a "String() string" method to IPAddr.

// What I did was I returned the call to Sprintf (in the fmt package) and formatted each of the four byte values in the array
// of bytes, separated with dots:

func (ip IPAddr) String() string{
		
        // The 'Sprintf' function formats according to a format specifier and returns the resulting string.
        // In other words: you first return the resulting string of the function 'Sprintf' and then
       // this method (String()) returns the previously returned string by 'Sprintf':

	return fmt.Sprintf("%v.%v.%v.%v",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 nombre, ip := range hosts {
		fmt.Printf("%v: %v\n", nombre, ip)
	}
}

I hope it helps!

@AlbertoCoder

AlbertoCoder commented Sep 15, 2022

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Excuse me:

In the rot13Reader exercise, how is Read(p []byte) called? I'm a bit confused, since I don't see it being invoked in main()...

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)
}

@americanstone

americanstone commented Oct 4, 2022

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MIT Phd crawl solution

package main

import (
	"fmt"
	"sync"
)

type Cache struct {
	m sync.Mutex
	v map[string]uint
}

func (s *Cache) Exist(key string) bool {
	s.m.Lock()
	defer s.m.Unlock()
	if _, ok := s.v[key]; ok {
		s.v[key]++
		return true
	} else {
		s.v[key] = 1
		return false
	}
}

func (s *Cache) String() string {
	s.m.Lock()
	defer s.m.Unlock()
	var sb string
	for k, v := range s.v {
		sb += fmt.Sprintf("key: %s => value: %q", k, v)
	}
	return fmt.Sprintln(sb)
}

type Fetcher interface {
	Fetch(url string) (body string, urls []string, err error)
}

//
// Serial crawler
//
func SerialCrawl(url string, depth int, fetcher Fetcher, cache *Cache) {
	if cache.Exist(url) {
		return
	}
	//fmt.Printf("cached values %q\n", cache)

	fmt.Println("depth ", depth)
	if depth <= 0 {
		return
	}
	body, urls, err := fetcher.Fetch(url)
	if err != nil {
		fmt.Println(err)
		return
	}
	fmt.Printf("Found: %s %q\n", url, body)
	//fmt.Printf("fetch children %q\n", urls)
	for _, u := range urls {
		//fmt.Printf("recursive crawl url %s\n", u)
		SerialCrawl(u, depth-1, fetcher, cache)
	}
}

//
// Concurrent crawler with shared state and Mutex
//

func ConcurrentMutex(url string, depth int, fetcher Fetcher, cache *Cache) {
	if cache.Exist(url) {
		return
	}
	//fmt.Printf("cached values %q\n", cache)

	if depth <= 0 {
		return
	}

	body, urls, err := fetcher.Fetch(url)
	if err != nil {
		fmt.Println(err)
		return
	}
	fmt.Printf("Found: %s %q\n", url, body)
	//fmt.Printf("fetch children %q\n", urls)
	var done sync.WaitGroup
	for _, u := range urls {
		done.Add(1)
		//fmt.Printf("recursive crawl url %s\n", u)
		u2 := u
		go func() {
			defer done.Done()
			ConcurrentMutex(u2, depth-1, fetcher, cache)
		}()
		//go func(u string) {
		//	defer done.Done()
		//	ConcurrentMutex(u, depth -1,fetcher, cache)
		//}(u)
	}
	done.Wait()
}

//
// Concurrent crawler with channels
//

func worker(url string, depth uint, ch chan []string, fetcher Fetcher, cache *Cache) {
	//fmt.Printf("working on url %s depth %d\n", url, depth)
	body, urls, err := fetcher.Fetch(url)

	if err != nil {
		ch <- []string{}
	} else {
		fmt.Printf("Found: %s %q\n", url, body)
		ch <- urls
	}
}

func master(url string, ch chan []string, depth uint, fetcher Fetcher, cache *Cache) {
	n := 1
	copyDepth := depth
	for urls := range ch {
		// fmt.Printf("dep :%s\n", fmt.Sprintf("%q", copyDepth))
		if copyDepth == 0 {
			break
		}
		for _, url := range urls {
			if !cache.Exist(url) {
				n += 1
				go worker(url, depth, ch, fetcher, cache)
			}
		}
		depth--
		n -= 1
		if n == 0 {
			break
		}
	}
}

func ConcurrentChan(url string, depth uint, fetcher Fetcher, cache *Cache) {
	ch := make(chan []string)
	go func() {
		ch <- []string{url}
	}()
	master(url, ch, depth, fetcher, cache)
}

func main() {
	cache := Cache{v: make(map[string]uint)}
	SerialCrawl("https://golang.org/", 4, fetcher, &cache)

	ConcurrentMutex("https://golang.org/", 4, fetcher, &cache)

	ConcurrentChan("https://golang.org/", 4, fetcher, &cache)

	fmt.Printf("\nCached urls %q\n", cache.v)
}

type fakeFetcher map[string]*fakeResult // to avoid copy value

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)
}

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

@ViktorVoloshko

ViktorVoloshko commented Oct 13, 2023 •
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My relatively simple solution for crawl w/o global variables and channels and with mutex and WaitGroup:

package main

import (
	"fmt"
	"sync"
)

type UrlCache struct {
	urls map[string]bool
	mu   sync.Mutex
}

func (uc *UrlCache) Add(url string) {
	uc.mu.Lock()
	uc.urls[url] = true
	uc.mu.Unlock()
}

func (uc *UrlCache) Get(url string) bool {
	uc.mu.Lock()
	defer uc.mu.Unlock()
	return uc.urls[url]
}

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) {
	var wg sync.WaitGroup
	cache := UrlCache{urls: make(map[string]bool)}
	wg.Add(1)
	go crawl(url, depth, fetcher, &cache, &wg)
	wg.Wait()
}

func crawl(url string, depth int, fetcher Fetcher, cache *UrlCache, wg *sync.WaitGroup) {
	defer wg.Done()
	if depth <= 0 || cache.Get(url) {
		return
	}
	cache.Add(url)
	body, urls, err := fetcher.Fetch(url)
	if err != nil {
		fmt.Println(err)
		return
	}
	fmt.Printf("found: %s %q\n", url, body)
	for _, u := range urls {
		wg.Add(1)
		go crawl(u, depth-1, fetcher, cache, wg)
	}
}

func main() {
	Crawl("https://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{
	"https://golang.org/": &fakeResult{
		"The Go Programming Language",
		[]string{
			"https://golang.org/pkg/",
			"https://golang.org/cmd/",
		},
	},
	"https://golang.org/pkg/": &fakeResult{
		"Packages",
		[]string{
			"https://golang.org/",
			"https://golang.org/cmd/",
			"https://golang.org/pkg/fmt/",
			"https://golang.org/pkg/os/",
		},
	},
	"https://golang.org/pkg/fmt/": &fakeResult{
		"Package fmt",
		[]string{
			"https://golang.org/",
			"https://golang.org/pkg/",
		},
	},
	"https://golang.org/pkg/os/": &fakeResult{
		"Package os",
		[]string{
			"https://golang.org/",
			"https://golang.org/pkg/",
		},
	},
}

@apo1967

apo1967 commented Nov 30, 2023 •
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@Aternus

Reader Solution:

package tour

import (
	"fmt"
)

type MyReader struct{}

func (r MyReader) Read(b []byte) (int, error) {
	bufferLength := len(b)
	if bufferLength <= 0 {
		return 0, fmt.Errorf("buffer length must be greater than 0")
	}
	buffer := make([]byte, bufferLength)
	for k := range buffer {
		buffer[k] = uint8('A')
	}
	n := copy(b, buffer)
	return n, nil
}

Is there a reason why you just don't set the 'A' directly in b? Why creating an internal slice and then copying to b? Just curious...

@dkowsley

dkowsley commented Sep 15, 2024 •
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In the rot13Reader exercise, how is Read(p []byte) called? I'm a bit confused, since I don't see it being invoked in main()...

In the quoted code, we see a call to io.Copy

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

The relevant call chain is io.Copy(dst, src) -> io.copyBuffer(dst, src, nil) -> src.Read(buf) (src == r, in this example).

The function definition for io.Copy and the io.Reader interface it uses are:

type Reader interface {
	Read(p []byte) (n int, err error)
}

func Copy(dst Writer, src Reader) (written int64, err error) {
	return copyBuffer(dst, src, nil)
}

io.copyBuffer has the following definition (I have replaced most of the code with comments to make it easier to read). Take note of the fourth line:

func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
        // if buf is nil we create a temp 32KB buffer for it
	for {
		nr, er := src.Read(buf) // our call to the Read function we created in our example
		// perform writes to dst with error handling and returning if we have finished reading
	}
	return written, err
}

@linhns

linhns commented Jan 15, 2025

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My solution for Web Crawler, heavily inspired by the official solution:

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 URLCache struct {
	mu      sync.Mutex
	fetched map[string]struct{}
}

func (c *URLCache) Add(url string) {
	c.mu.Lock()
	defer c.mu.Unlock()
	c.fetched[url] = struct{}{}
}

func (c *URLCache) Has(url string) bool {
	c.mu.Lock()
	defer c.mu.Unlock()
	_, ok := c.fetched[url]
	return ok
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func crawl(url string, depth int, fetcher Fetcher, urlCache *URLCache) {
	// TODO: Fetch URLs in parallel.
	// TODO: Don't fetch the same URL twice.
	// This implementation doesn't do either:
	if depth <= 0 {
		fmt.Printf("<- Done with %v, maximum depth reached\n", url)
		return
	}

	if urlCache.Has(url) {
		fmt.Printf("<- Done with %v, already fetched\n", url)
		return
	}

	body, urls, err := fetcher.Fetch(url)

	urlCache.Add(url)

	if err != nil {
		fmt.Printf("<- Error on %v: %v\n", url, err)
		return
	}

	fmt.Printf("Found: %s %q\n", url, body)

	var wg sync.WaitGroup
	wg.Add(len(urls))
	for i, u := range urls {
		fmt.Printf("-> Crawling child %v/%v of %v : %v.\n", i, len(urls), url, u)
		go func(url string) {
			defer wg.Done()
			crawl(url, depth-1, fetcher, urlCache)
		}(u)
	}
	wg.Wait()
	fmt.Printf("<- Done with %v\n", url)
}

func Crawl(url string, depth int, fetcher Fetcher) {
	urlCache := URLCache{fetched: make(map[string]struct{})}
	crawl(url, depth, fetcher, &urlCache)
}

func main() {
	Crawl("https://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{
	"https://golang.org/": &fakeResult{
		"The Go Programming Language",
		[]string{
			"https://golang.org/pkg/",
			"https://golang.org/cmd/",
		},
	},
	"https://golang.org/pkg/": &fakeResult{
		"Packages",
		[]string{
			"https://golang.org/",
			"https://golang.org/cmd/",
			"https://golang.org/pkg/fmt/",
			"https://golang.org/pkg/os/",
		},
	},
	"https://golang.org/pkg/fmt/": &fakeResult{
		"Package fmt",
		[]string{
			"https://golang.org/",
			"https://golang.org/pkg/",
		},
	},
	"https://golang.org/pkg/os/": &fakeResult{
		"Package os",
		[]string{
			"https://golang.org/",
			"https://golang.org/pkg/",
		},
	},
}

To avoid creating a global variable, I used an unexported function that takes in a concurrent-safe cache and initialize it in Crawl.

@fenix-hub

fenix-hub commented Apr 17, 2026 •
edited
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@apo1967

@Aternus

Reader Solution:

package tour

import (
	"fmt"
)

type MyReader struct{}

func (r MyReader) Read(b []byte) (int, error) {
	bufferLength := len(b)
	if bufferLength <= 0 {
		return 0, fmt.Errorf("buffer length must be greater than 0")
	}
	buffer := make([]byte, bufferLength)
	for k := range buffer {
		buffer[k] = uint8('A')
	}
	n := copy(b, buffer)
	return n, nil
}

Is there a reason why you just don't set the 'A' directly in b? Why creating an internal slice and then copying to b? Just curious...

Just did the same as you...

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

Can't find a specific reason, other than explicit exercise constraints which are not included in this case, since b will already be a copy of whatever variable the Validate function will use as an argument.

@fenix-hub

fenix-hub commented Apr 17, 2026

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My solution for rot13:

package main

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

type rot13Reader struct {
	r io.Reader
}

func (reader *rot13Reader) Read(b []byte) (int, error) {
	n, err := reader.r.Read(b)
	for i := range n {
		if (b[i] >= 65 && b[i] <= 77 || b[i] >= 97 && b[i] <= 109) {
			b[i] += 13
		} else if (b[i] >= 78 && b[i] <= 90 || b[i] >= 110 && b[i] <= 122) {
			b[i] -= 13
		}
	}
	return n, err
}

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

I've not seen this solution already, which I propose as an efficient/smart alternative: instead of relying on explicit characters, ASCII characters can be just handled by their int value (through the standard Ascii Table here )

@fenix-hub

fenix-hub commented Apr 17, 2026

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My solution for Crawler.
Honestly this seemed the hardest so far: why doesn't the TourGo website instructs the user about the WaitGroup before this exercise? Without it this exercise was almost impossible without changing most of the function definitions.

package main

import (
	"fmt"
	"sync"
)


type Fetcher interface {
	Fetch(url string) (string, []string, error)
}

type FetchResult struct { 
	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) {
	defer wg.Done() // gets executed at the end of Crawl
	
	if depth <= 0 {
		return
	}
	if cache.Check(url) {
		fmt.Printf("Hit! %s\n", url)
		return
	}
	
	body, urls, err := fetcher.Fetch(url)
	cache.Set(url)
	
	if err != nil {
		fmt.Println(err)
		return
	}
	
	fmt.Printf("found: %s %q\n", url, body)
	for _, u := range urls {
		wg.Add(1)
		go Crawl(u, depth-1, fetcher)
	}
	return
}


type Cache struct {
	v map[string]bool // Url -> Body
	m sync.Mutex
}

func (c *Cache) Check(key string) bool {
	c.m.Lock()
	defer c.m.Unlock()
	return c.v[key]  
}

func (c *Cache) Set(key string) {
	c.m.Lock()
	c.v[key] = true
	c.m.Unlock()
}

// 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{
	"https://golang.org/": &fakeResult{
		"The Go Programming Language",
		[]string{
			"https://golang.org/pkg/",
			"https://golang.org/cmd/",
		},
	},
	"https://golang.org/pkg/": &fakeResult{
		"Packages",
		[]string{
			"https://golang.org/",
			"https://golang.org/cmd/",
			"https://golang.org/pkg/fmt/",
			"https://golang.org/pkg/os/",
		},
	},
	"https://golang.org/pkg/fmt/": &fakeResult{
		"Package fmt",
		[]string{
			"https://golang.org/",
			"https://golang.org/pkg/",
		},
	},
	"https://golang.org/pkg/os/": &fakeResult{
		"Package os",
		[]string{
			"https://golang.org/",
			"https://golang.org/pkg/",
		},
	},
}

var cache Cache = Cache{ v: make(map[string]bool) }
var wg sync.WaitGroup     // required to let all launched goroutines finish before main() exits

func main() {
	wg.Add(1)
	go Crawl("https://golang.org/", 4, fetcher)
	wg.Wait()
}

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