1 files changed, 72 insertions, 0 deletions
diff --git a/docs/examples/pilib/semaphore.scala b/docs/examples/pilib/semaphore.scala
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+++ b/docs/examples/pilib/semaphore.scala
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+package examples.pilib;
+
+/** Solution of exercise session 6 (first question). */
+object semaphore {
+
+  import scala.concurrent.pilib._;
+
+  class Signal extends Chan[unit] {
+    def send = write(());
+    def receive = read;
+  }
+
+  /** Interface. */
+  trait Semaphore {
+    def get: unit;
+    def release: unit;
+  }
+
+  /** First implementation. */
+  class Sem1 extends Semaphore {
+
+    private val g = new Signal;
+    private val r = new Signal;
+
+    def get: unit = g.send;
+    def release: unit = r.send;
+
+    private def Sched: unit = choice (
+      g * (x => { r.receive; Sched }),
+      r * (x => Sched)
+    );
+    spawn< Sched >;
+  }
+
+  /** Second implementation. */
+  class Sem2 extends Semaphore {
+
+    private val a = new Signal;
+    private val na = new Signal;
+
+    def get: unit = { a.receive; spawn< na.send > }
+    def release: unit = choice (
+      a * (x => spawn< a.send >),
+      na * (x => spawn< a.send >)
+    );
+    spawn< a.send >;
+  }
+
+  /** Test program. */
+  def main(args: Array[String]): unit = {
+    val random = new java.util.Random();
+    val sem = new Sem2;
+    def mutex(p: => unit): unit = { sem.get; p; sem.release }
+
+    spawn< {
+      Thread.sleep(1 + random.nextInt(100));
+      mutex( {
+	System.out.println("a1");
+	Thread.sleep(1 + random.nextInt(100));
+	System.out.println("a2")
+      } )
+    } | {
+      Thread.sleep(1 + random.nextInt(100));
+      mutex( {
+	System.out.println("b1");
+	Thread.sleep(1 + random.nextInt(100));
+	System.out.println("b2")
+      } )
+    } >;
+  }
+}
+