From 15f0b88446699bdeaffaf20c42bb31950b50aacc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Kjetil=20=C3=98rbekk?= <kjetil.orbekk@gmail.com>
Date: Wed, 2 May 2012 12:52:21 +0200
Subject: Add CyclicCountDownLatch.

(You may not want to use this.)
---
 .../java/com/orbekk/util/CyclicCountDownLatch.java | 88 ++++++++++++++++++++++
 .../com/orbekk/util/CyclicCountDownLatchTest.java  | 35 +++++++++
 2 files changed, 123 insertions(+)
 create mode 100644 same/src/main/java/com/orbekk/util/CyclicCountDownLatch.java
 create mode 100644 same/src/test/java/com/orbekk/util/CyclicCountDownLatchTest.java

diff --git a/same/src/main/java/com/orbekk/util/CyclicCountDownLatch.java b/same/src/main/java/com/orbekk/util/CyclicCountDownLatch.java
new file mode 100644
index 0000000..a7ce7c4
--- /dev/null
+++ b/same/src/main/java/com/orbekk/util/CyclicCountDownLatch.java
@@ -0,0 +1,88 @@
+/**
+ * Copyright 2012 Kjetil Ørbekk <kjetil.orbekk@gmail.com>
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.orbekk.util;
+
+import java.util.concurrent.locks.AbstractQueuedSynchronizer;
+
+/** This class provides something of a cyclic version of a CountDownLatch.
+ * 
+ * The latch resets when await() completes. It may ignore calls to countDown()
+ * that occur before await() returns (it is not accumulative).
+ * 
+ * Use this class with caution.
+ */
+public class CyclicCountDownLatch {
+    private static class Sync extends AbstractQueuedSynchronizer {
+        final int count;
+        
+        public Sync(int count) {
+            this.count = count;
+            setState(count);
+        }
+        
+        @Override protected boolean tryReleaseShared(int unused) {
+            boolean success = false;
+            while (!success) {
+                int expect = getState();
+                int update = Math.max(expect - 1, 0);
+                success = compareAndSetState(expect, update);
+            }
+            return getState() <= 0;
+        }
+        
+        @Override protected int tryAcquireShared(int unused) {
+            boolean success = false;
+            while (!success) {
+                int expect = getState();
+                if (expect > 0) {
+                    break;
+                }
+                int update = count;
+                success = compareAndSetState(expect, update);
+            }
+            
+            if (success) {
+                // Non-exclusive success. Other threads may attempt to acquire.
+                return 1;
+            } else {
+                // Failure.
+                return -1;
+            }
+        }
+        
+        public int getCount() {
+            return getState();
+        }
+    }
+    
+    private final Sync sync;
+    
+    public CyclicCountDownLatch(int count) {
+        sync = new Sync(count);
+    }
+    
+    public void await() throws InterruptedException {
+        sync.acquireSharedInterruptibly(-1);
+    }
+    
+    public void countDown() {
+        sync.releaseShared(-1);
+    }
+    
+    public int getCount() {
+        return sync.getCount();
+    }
+}
diff --git a/same/src/test/java/com/orbekk/util/CyclicCountDownLatchTest.java b/same/src/test/java/com/orbekk/util/CyclicCountDownLatchTest.java
new file mode 100644
index 0000000..769b7e5
--- /dev/null
+++ b/same/src/test/java/com/orbekk/util/CyclicCountDownLatchTest.java
@@ -0,0 +1,35 @@
+package com.orbekk.util;
+
+import static org.junit.Assert.assertThat;
+import static org.hamcrest.Matchers.*;
+
+import org.junit.Test;
+
+public class CyclicCountDownLatchTest {
+    CyclicCountDownLatch latch = new CyclicCountDownLatch(1);
+
+    @Test public void initialCount() {
+        assertThat(latch.getCount(), is(1));
+    }
+    
+    @Test public void releasesCorrectly() throws Exception {
+        latch.countDown();
+        assertThat(latch.getCount(), is(0));
+        latch.await();
+    }
+    
+    @Test public void testCycle() throws Exception {
+        latch.countDown();
+        latch.await();
+        assertThat(latch.getCount(), is(1));
+        latch.countDown();
+        latch.await();
+    }
+    
+    @Test public void notAccumulative() throws Exception {
+        latch.countDown();
+        latch.countDown();
+        latch.await();
+        assertThat(latch.getCount(), is(1));
+    }
+}
-- 
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