Concurrency in Golang

channels_and_select.go

package main

import (
	"golang.org/x/tour/tree"
	"fmt"
)

// Solving https://tour.golang.org/concurrency/8

// 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)
	
	var t1Values, t2Values []int
	
  // Constraints specified each tree would only have 10 nodes, so we just count to 20 here
	for i := 0; i < 20; i++ {
		select {
			case x := <-ch1:
				t1Values = append(t1Values, x)
			case y := <-ch2:
				t2Values = append(t2Values, y)
		}
	}
	return equal(t1Values, t2Values)
}

// Needed for equality of slices
func equal(a, b []int) bool {
    if len(a) != len(b) {
        return false
    }
    for i, v := range a {
        if v != b[i] {
            return false
        }
    }
    return true
}

func main() {
	fmt.Println(Same(tree.New(1), tree.New(1))) // true
	fmt.Println(Same(tree.New(2), tree.New(1))) // false
}

waitgroups_and_mutexes.go

package main

import (
	"sync"
	"fmt"
)

// Solving https://tour.golang.org/concurrency/10

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 fetchedUrls struct {
	mutex *sync.Mutex
	fetchedUrls map[string]bool
}

var fetched fetchedUrls

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher, wg *sync.WaitGroup) {
	defer wg.Done()
	// TODO: Fetch URLs in parallel.
	// TODO: Don't fetch the same URL twice.
	// This implementation doesn't do either:
	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)
	for _, u := range urls {
		fetched.mutex.Lock()
		fetched.fetchedUrls[url] = true
		if _, found := fetched.fetchedUrls[u]; !found {
			wg.Add(1)
			go Crawl(u, depth-1, fetcher, wg)
		}
		fetched.mutex.Unlock()
	}
	return
}

func main() {
	fetched = fetchedUrls{mutex: &sync.Mutex{}, fetchedUrls: make(map[string]bool)}
	wg := &sync.WaitGroup{}
	Crawl("https://golang.org/", 4, fetcher, wg)
	wg.Wait()
}

// 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/",
		},
	},
}