Data commit

Task/Metered-concurrency/Go/metered-concurrency-1.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+    "log"
+    "os"
+    "sync"
+    "time"
+)
+
+// counting semaphore implemented with a buffered channel
+type sem chan struct{}
+
+func (s sem) acquire()   { s <- struct{}{} }
+func (s sem) release()   { <-s }
+func (s sem) count() int { return cap(s) - len(s) }
+
+// log package serializes output
+var fmt = log.New(os.Stdout, "", 0)
+
+// library analogy per WP article
+const nRooms = 10
+const nStudents = 20
+
+func main() {
+    rooms := make(sem, nRooms)
+    // WaitGroup used to wait for all students to have studied
+    // before terminating program
+    var studied sync.WaitGroup
+    studied.Add(nStudents)
+    // nStudents run concurrently
+    for i := 0; i < nStudents; i++ {
+        go student(rooms, &studied)
+    }
+    studied.Wait()
+}
+
+func student(rooms sem, studied *sync.WaitGroup) {
+    rooms.acquire()
+    // report per task descrption.  also exercise count operation
+    fmt.Printf("Room entered.  Count is %d.  Studying...\n",
+        rooms.count())
+    time.Sleep(2 * time.Second) // sleep per task description
+    rooms.release()
+    studied.Done() // signal that student is done
+}

Task/Metered-concurrency/Go/metered-concurrency-2.go

@@ -0,0 +1,61 @@
+package main
+
+import (
+    "log"
+    "os"
+    "sync"
+    "time"
+)
+
+var fmt = log.New(os.Stdout, "", 0)
+
+type countSem struct {
+    int
+    sync.Cond
+}
+
+func newCount(n int) *countSem {
+    return &countSem{n, sync.Cond{L: &sync.Mutex{}}}
+}
+
+func (cs *countSem) count() int {
+    cs.L.Lock()
+    c := cs.int
+    cs.L.Unlock()
+    return c
+}
+
+func (cs *countSem) acquire() {
+    cs.L.Lock()
+    cs.int--
+    for cs.int < 0 {
+        cs.Wait()
+    }
+    cs.L.Unlock()
+}
+
+func (cs *countSem) release() {
+    cs.L.Lock()
+    cs.int++
+    cs.L.Unlock()
+    cs.Broadcast()
+}
+
+func main() {
+    librarian := newCount(10)
+    nStudents := 20
+    var studied sync.WaitGroup
+    studied.Add(nStudents)
+    for i := 0; i < nStudents; i++ {
+        go student(librarian, &studied)
+    }
+    studied.Wait()
+}
+
+func student(studyRoom *countSem, studied *sync.WaitGroup) {
+    studyRoom.acquire()
+    fmt.Printf("Room entered.  Count is %d.  Studying...\n", studyRoom.count())
+    time.Sleep(2 * time.Second)
+    studyRoom.release()
+    studied.Done()
+}