Golang Tour of Go: Web Crawler

crawler.go

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)
}

type FetchResult struct {
    Url string
    Body string
    Urls []string
    Depth int
    Error error
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {
    fetched := make(map[string]bool)
    fetchAsync := func(url string, depth int, ch chan *FetchResult) {
        body, urls, err := fetcher.Fetch(url)
        ch <- &FetchResult{url, body, urls, depth, err}
    }
    resultChan := make(chan *FetchResult)

    fetched[url] = true
    go fetchAsync(url, 1, resultChan)

    waitingFor := 1
    for waitingFor > 0 {
        result := <-resultChan
        waitingFor--
        if result.Error != nil {
            fmt.Println(result.Error)
            continue
        }
        fmt.Printf("Found %s %q\n", result.Url, result.Body)
        if result.Depth < depth {
            for _, url := range result.Urls {
                if !fetched[url] {
                    fetched[url] = true
                    go fetchAsync(url, result.Depth + 1, resultChan)
                    waitingFor++
                }
            }
        }
    }
}

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/",
        },
    },
}