add cond implementation using a channel

mutexes/cond/cond-implementation/channel/main.go

@@ -0,0 +1,37 @@
+package main
+
+import "sync"
+
+func main() {
+}
+
+type cond struct {
+	L  sync.Mutex // used by caller
+	mu sync.Mutex // guards ch
+	ch chan struct{}
+}
+
+func (c *cond) Wait() {
+	c.mu.Lock()
+	ch := c.ch
+	c.mu.Unlock()
+	c.L.Unlock()
+	<-ch
+	c.L.Lock()
+}
+
+func (c *cond) Signal() {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	select {
+	case c.ch <- struct{}{}:
+	default:
+	}
+}
+
+func (c *cond) Broadcast() {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	close(c.ch)
+	c.ch = make(chan struct{})
+}