sync: WaitGroup

examples/news_fetcher.v

@@ -5,10 +5,9 @@
 import http
 import json
 import sync
-import time
 
 const (
-	NR_THREADS = 8
+	NR_THREADS = 4
 )
 
 struct Story {
@@ -21,7 +20,7 @@ mut:
 	mu      sync.Mutex
 	ids     []int
 	cursor  int
-	list_id int
+	wg		&sync.WaitGroup
 }
 
 fn (f mut Fetcher) fetch() {
@@ -32,6 +31,7 @@ fn (f mut Fetcher) fetch() {
 		}
 		id := f.ids[f.cursor]
 		f.cursor++
+		cursor := f.cursor
 		f.mu.unlock()
 		resp := http.get('https://hacker-news.firebaseio.com/v0/item/${id}.json') or {
 			println('failed to fetch data from /v0/item/${id}.json')
@@ -41,28 +41,36 @@ fn (f mut Fetcher) fetch() {
 			println('failed to decode a story')
 			exit(1)
 		}
-		f.mu.lock()
+		f.wg.done()
-		f.list_id++
-		cursor := f.list_id
-		f.mu.unlock()
 		println('#$cursor) $story.title | $story.url')
 	}
 }
 
-// Fetches top HN stories in 8 coroutines
+// Fetches top HN stories in 4 coroutines
 fn main() {
 	resp := http.get('https://hacker-news.firebaseio.com/v0/topstories.json') or {
 		println('failed to fetch data from /v0/topstories.json')
 		return
 	}
-	ids := json.decode( []int, resp) or {
+	mut ids := json.decode([]int, resp) or {
 		println('failed to decode topstories.json')
 		return
 	}
-	fetcher := &Fetcher{ids: ids}
+	if ids.len > 10 {
+		// ids = ids[:10]
+		mut tmp := [0 ; 10]
+		for i := 0 ; i < 10 ; i++ {
+			tmp[i] = ids[i]
+		}
+		ids = tmp
+	}
+	
+	mut wg := &sync.WaitGroup{}
+	fetcher := &Fetcher{ids: ids, wg: wg} // wg sent via ptr
+	wg.add(ids.len)
 	for i := 0; i < NR_THREADS; i++ {
 		go fetcher.fetch()
 	}
-	time.sleep(5)
+	wg.wait()
 }
 

vlib/sync/waitgroup.v

@@ -0,0 +1,37 @@
+// Copyright (c) 2019 Alexander Medvednikov. All rights reserved.
+// Use of this source code is governed by an MIT license
+// that can be found in the LICENSE file.
+
+module sync
+
+struct WaitGroup {
+mut:
+    mu Mutex
+    finished Mutex
+    active int
+}
+
+pub fn (wg mut WaitGroup) add(delta int) {
+    wg.mu.lock()
+    if wg.active == 0 {
+        wg.finished.lock()
+    }
+    wg.active += delta
+    if wg.active < 0 {
+        panic('Negative number of jobs in waitgroup')
+    }
+    if wg.active == 0 {
+        wg.finished.unlock()
+    }
+    wg.mu.unlock()
+}
+
+pub fn (wg mut WaitGroup) done() {
+    wg.add(-1)
+}
+
+pub fn (wg mut WaitGroup) wait() {
+    wg.finished.lock()
+    wg.finished.unlock()
+}
+