ch8: add exercise8.6

Author: linehk

ch8/exercise8.6/main.go

@@ -0,0 +1,66 @@
+package main
+
+import (
+	"fmt"
+	"github.com/linehk/gopl/ch5/links"
+	"log"
+	"os"
+)
+
+var tokens = make(chan struct{}, 20)
+
+type work struct {
+	url   string
+	depth int
+}
+
+func crawl(w work) []work {
+	fmt.Printf("depth: %d, url: %s\n", w.depth, w.url)
+
+	if w.depth >= 3 {
+		return nil
+	}
+
+	tokens <- struct{}{} // acquire a token
+	urls, err := links.Extract(w.url)
+	<-tokens // release the token
+	if err != nil {
+		log.Print(err)
+	}
+
+	var works []work
+	for _, url := range urls {
+		works = append(works, work{url, w.depth + 1})
+	}
+	return works
+}
+
+func main() {
+	worklist := make(chan []work)
+	var n int // number of pending sends to worklist
+
+	// Start with the command-line arguments.
+	n++
+	go func() {
+		var works []work
+		for _, url := range os.Args[1:] {
+			works = append(works, work{url, 1})
+		}
+		worklist <- works
+	}()
+
+	// Crawl the web concurrently.
+	seen := make(map[string]bool)
+	for ; n > 0; n-- {
+		works := <-worklist
+		for _, w := range works {
+			if !seen[w.url] {
+				seen[w.url] = true
+				n++
+				go func(w work) {
+					worklist <- crawl(w)
+				}(w)
+			}
+		}
+	}
+}