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package main
import (
"context"
"fmt"
"net/url"
"sync"
)
/*
CHANNEL/GOROUTINE NOTES
main goroutine:
- manages urls channel.
*/
// Packet accrues data as it passes through our concurrent
// pipeline. Formerly the web crawler only transmitted [url.URL]'s,
// but usingn a compound data type allows us to add URL
// depth-tracking.
type Packet struct {
url url.URL
depth int
}
// String implements the Stringer interface. We need this mainly
// because a [url.URL]'s String method only works when that URL is a
// pointer.
func (p Packet) String() string {
return fmt.Sprintf("[%d] %s", p.depth, &p.url)
}
// workers launches a worker queue for crawling a given Web domain.
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)
// Convert URLs to Packets. In the
// process, bump up the depth by 1.
ps := convertToPackets(batch, p.depth+1)
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)
// Used to prettify the running URL listing.
count := 1
loop:
for batch := range worklist {
for _, p := range batch {
// We're tracking _depth_ with the seen-map
// now, so any unseen URL doesn't have any
// depth-entry registered yet.
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
}
}
}
// We're done writing to the packets channel, so close it.
close(packets)
// There are some in-flight workers as of this point, so
// signal a cancel to them.
cancel()
wg.Wait()
}
// convertToPackets converts the batch of URLs to a slice of Packet
// structs, configuring each one with the given depth.
func convertToPackets(batch []url.URL, depth int) []Packet {
var ps []Packet
for _, u := range batch {
newPacket := Packet{u, depth}
ps = append(ps, newPacket)
}
return ps
}
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