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// URLs implements a breadth-first search webcrawler based on the
// example given in section 8.6 of The Go Programming Language.
package main
import (
"context"
"flag"
"fmt"
"log"
"net/url"
)
func main() {
maxConcurrency := flag.Int("c", 0, "Maximum number of concurrent queue pushes")
startRawURL := flag.String("url", "", "Entry-point URL")
maxURLs := flag.Int("max", 0, "Maximum number of URLs to collect (omitted or 0 means no limit)")
flag.Parse()
if *maxConcurrency == 0 {
log.Fatal("Missing -c argument")
}
if *maxConcurrency < 1 {
log.Fatalf("Invalid -c argument: %d", *maxConcurrency)
}
if *startRawURL == "" {
log.Fatal("Missing -url argument")
}
if *maxURLs < 0 {
log.Fatalf("Invalid -max argument: %d", *maxURLs)
}
startURL, err := url.Parse(*startRawURL)
if err != nil {
log.Fatal(err)
}
crawler(*startURL, *maxConcurrency, *maxURLs)
}
func crawler(startURL url.URL, maxConcurrency, maxURLs int) {
/* Main logic starts here. */
worklist := make(chan []url.URL)
go func() {
worklist <- []url.URL{startURL}
}()
sem := make(chan struct{}, maxConcurrency)
ctx, cancel := context.WithCancel(context.Background())
seen := make(map[url.URL]bool)
i := 1
// FIXME: unfortunately, this example leaks, but I don't know
// how to fix that yet.
loop:
for list := range worklist {
for _, u := range list {
if maxURLs > 0 && len(seen) == maxURLs {
break loop
}
if !seen[u] {
fmt.Printf("%d. %s\n", i, &u)
i++
seen[u] = true
go func() {
sem <- struct{}{}
defer func() { <-sem }()
more := getMoreURLs(ctx, u)
if len(more) > 0 {
worklist <- more
}
}()
}
}
}
// We broke the range loop, meaning there should be no more
// pending getMoreURLs jobs anyway.
cancel()
// For now, print out some diagnostics that prove that there
// are still pending sends on the worklist channel.
for batch := range worklist {
fmt.Printf("%d\n", len(batch))
for _, u := range batch {
fmt.Printf("-- %s\n", &u)
}
}
}
func getMoreURLs(ctx context.Context, u url.URL) []url.URL {
select {
case <-ctx.Done():
return nil
default:
}
doc, err := fetch(u)
if err != nil {
log.Print(err)
return nil
}
return findURLs(u, doc)
}
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