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|
package main
import (
"context"
"fmt"
"net/url"
"sync"
)
/*
main goroutine:
- manages urls channel.
*/
type Packet struct {
url url.URL
depth int
}
func (p Packet) String() string {
return fmt.Sprintf("[%d] %s", p.depth, &p.url)
}
func workers(startURL url.URL, maxConcurrency, maxURLs int) {
worklist := make(chan []Packet)
// Unseen URLs.
packets := make(chan Packet)
go func() {
startPacket := Packet{startURL, 0}
worklist <- []Packet{startPacket}
}()
var wg sync.WaitGroup
ctx, cancel := context.WithCancel(context.Background())
// Create maxConcurrency worker goroutines to demultiplex from
// the urls channel (unseen links.)
for i := range maxConcurrency {
wg.Go(func() {
for p := range packets {
batch := getBatch(p.url)
var ps []Packet
for _, u := range batch {
newPacket := Packet{u, p.depth + 1}
ps = append(ps, newPacket)
}
select {
case <-ctx.Done():
fmt.Printf("exiting early %d\n", i+1)
return
default:
go func() { worklist <- ps }()
}
}
fmt.Printf("terminating %d\n", i+1)
})
}
// The main goroutine deduplicates worklist items and sends
// unseen ones to the crawlers in a fan-out fashion.
seen := make(map[url.URL]int)
count := 1
loop:
for batch := range worklist {
for _, p := range batch {
if _, ok := seen[p.url]; !ok {
fmt.Printf("%d. %s\n", count, p)
count++
seen[p.url] = p.depth
if len(seen) == maxURLs {
break loop
}
packets <- p
}
}
}
close(packets)
cancel()
wg.Wait()
}
|
