webcrawler.go

     1  //go:build OMIT
     2  
     3  // Copyright 2012 The Go Authors.  All rights reserved.
     4  // Use of this source code is governed by a BSD-style
     5  // license that can be found in the LICENSE file.
     6  
     7  package main
     8  
     9  import (
    10  	"errors"
    11  	"fmt"
    12  	"sync"
    13  )
    14  
    15  type Fetcher interface {
    16  	// Fetch returns the body of URL and
    17  	// a slice of URLs found on that page.
    18  	Fetch(url string) (body string, urls []string, err error)
    19  }
    20  
    21  // fetched tracks URLs that have been (or are being) fetched.
    22  // The lock must be held while reading from or writing to the map.
    23  // See https://golang.org/ref/spec#Struct_types section on embedded types.
    24  var fetched = struct {
    25  	m map[string]error
    26  	sync.Mutex
    27  }{m: make(map[string]error)}
    28  
    29  var loading = errors.New("url load in progress") // sentinel value
    30  
    31  // Crawl uses fetcher to recursively crawl
    32  // pages starting with url, to a maximum of depth.
    33  func Crawl(url string, depth int, fetcher Fetcher) {
    34  	if depth <= 0 {
    35  		fmt.Printf("<- Done with %v, depth 0.\n", url)
    36  		return
    37  	}
    38  
    39  	fetched.Lock()
    40  	if _, ok := fetched.m[url]; ok {
    41  		fetched.Unlock()
    42  		fmt.Printf("<- Done with %v, already fetched.\n", url)
    43  		return
    44  	}
    45  	// We mark the url to be loading to avoid others reloading it at the same time.
    46  	fetched.m[url] = loading
    47  	fetched.Unlock()
    48  
    49  	// We load it concurrently.
    50  	body, urls, err := fetcher.Fetch(url)
    51  
    52  	// And update the status in a synced zone.
    53  	fetched.Lock()
    54  	fetched.m[url] = err
    55  	fetched.Unlock()
    56  
    57  	if err != nil {
    58  		fmt.Printf("<- Error on %v: %v\n", url, err)
    59  		return
    60  	}
    61  	fmt.Printf("Found: %s %q\n", url, body)
    62  	done := make(chan bool)
    63  	for i, u := range urls {
    64  		fmt.Printf("-> Crawling child %v/%v of %v : %v.\n", i, len(urls), url, u)
    65  		go func(url string) {
    66  			Crawl(url, depth-1, fetcher)
    67  			done <- true
    68  		}(u)
    69  	}
    70  	for i, u := range urls {
    71  		fmt.Printf("<- [%v] %v/%v Waiting for child %v.\n", url, i, len(urls), u)
    72  		<-done
    73  	}
    74  	fmt.Printf("<- Done with %v\n", url)
    75  }
    76  
    77  func main() {
    78  	Crawl("https://golang.org/", 4, fetcher)
    79  
    80  	fmt.Println("Fetching stats\n--------------")
    81  	for url, err := range fetched.m {
    82  		if err != nil {
    83  			fmt.Printf("%v failed: %v\n", url, err)
    84  		} else {
    85  			fmt.Printf("%v was fetched\n", url)
    86  		}
    87  	}
    88  }
    89  
    90  // fakeFetcher is Fetcher that returns canned results.
    91  type fakeFetcher map[string]*fakeResult
    92  
    93  type fakeResult struct {
    94  	body string
    95  	urls []string
    96  }
    97  
    98  func (f *fakeFetcher) Fetch(url string) (string, []string, error) {
    99  	if res, ok := (*f)[url]; ok {
   100  		return res.body, res.urls, nil
   101  	}
   102  	return "", nil, fmt.Errorf("not found: %s", url)
   103  }
   104  
   105  // fetcher is a populated fakeFetcher.
   106  var fetcher = &fakeFetcher{
   107  	"https://golang.org/": &fakeResult{
   108  		"The Go Programming Language",
   109  		[]string{
   110  			"https://golang.org/pkg/",
   111  			"https://golang.org/cmd/",
   112  		},
   113  	},
   114  	"https://golang.org/pkg/": &fakeResult{
   115  		"Packages",
   116  		[]string{
   117  			"https://golang.org/",
   118  			"https://golang.org/cmd/",
   119  			"https://golang.org/pkg/fmt/",
   120  			"https://golang.org/pkg/os/",
   121  		},
   122  	},
   123  	"https://golang.org/pkg/fmt/": &fakeResult{
   124  		"Package fmt",
   125  		[]string{
   126  			"https://golang.org/",
   127  			"https://golang.org/pkg/",
   128  		},
   129  	},
   130  	"https://golang.org/pkg/os/": &fakeResult{
   131  		"Package os",
   132  		[]string{
   133  			"https://golang.org/",
   134  			"https://golang.org/pkg/",
   135  		},
   136  	},
   137  }
   138
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