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+/*
+ * Copyright (C) 2015-2016 Apple Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
+ * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 
+ */
+
+#ifndef WTF_Lock_h
+#define WTF_Lock_h
+
+#include <wtf/LockAlgorithm.h>
+#include <wtf/Locker.h>
+#include <wtf/Noncopyable.h>
+
+namespace TestWebKitAPI {
+struct LockInspector;
+};
+
+namespace WTF {
+
+typedef LockAlgorithm<uint8_t, 1, 2> DefaultLockAlgorithm;
+
+// This is a fully adaptive mutex that only requires 1 byte of storage. It has fast paths that are
+// competetive to a spinlock (uncontended locking is inlined and is just a CAS, microcontention is
+// handled by spinning and yielding), and a slow path that is competetive to std::mutex (if a lock
+// cannot be acquired in a short period of time, the thread is put to sleep until the lock is
+// available again). It uses less memory than a std::mutex. This lock guarantees eventual stochastic
+// fairness, even in programs that relock the lock immediately after unlocking it. Except when there
+// are collisions between this lock and other locks in the ParkingLot, this lock will guarantee that
+// at worst one call to unlock() per millisecond will do a direct hand-off to the thread that is at
+// the head of the queue. When there are collisions, each collision increases the fair unlock delay
+// by one millisecond in the worst case.
+
+// This is a struct without a constructor or destructor so that it can be statically initialized.
+// Use Lock in instance variables.
+struct LockBase {
+    void lock()
+    {
+        if (UNLIKELY(!DefaultLockAlgorithm::lockFastAssumingZero(m_byte)))
+            lockSlow();
+    }
+
+    bool tryLock()
+    {
+        return DefaultLockAlgorithm::tryLock(m_byte);
+    }
+
+    // Need this version for std::unique_lock.
+    bool try_lock()
+    {
+        return tryLock();
+    }
+
+    // Relinquish the lock. Either one of the threads that were waiting for the lock, or some other
+    // thread that happens to be running, will be able to grab the lock. This bit of unfairness is
+    // called barging, and we allow it because it maximizes throughput. However, we bound how unfair
+    // barging can get by ensuring that every once in a while, when there is a thread waiting on the
+    // lock, we hand the lock to that thread directly. Every time unlock() finds a thread waiting,
+    // we check if the last time that we did a fair unlock was more than roughly 1ms ago; if so, we
+    // unlock fairly. Fairness matters most for long critical sections, and this virtually
+    // guarantees that long critical sections always get a fair lock.
+    void unlock()
+    {
+        if (UNLIKELY(!DefaultLockAlgorithm::unlockFastAssumingZero(m_byte)))
+            unlockSlow();
+    }
+
+    // This is like unlock() but it guarantees that we unlock the lock fairly. For short critical
+    // sections, this is much slower than unlock(). For long critical sections, unlock() will learn
+    // to be fair anyway. However, if you plan to relock the lock right after unlocking and you want
+    // to ensure that some other thread runs in the meantime, this is probably the function you
+    // want.
+    void unlockFairly()
+    {
+        if (UNLIKELY(!DefaultLockAlgorithm::unlockFastAssumingZero(m_byte)))
+            unlockFairlySlow();
+    }
+    
+    void safepoint()
+    {
+        if (UNLIKELY(!DefaultLockAlgorithm::safepointFast(m_byte)))
+            safepointSlow();
+    }
+
+    bool isHeld() const
+    {
+        return DefaultLockAlgorithm::isLocked(m_byte);
+    }
+
+    bool isLocked() const
+    {
+        return isHeld();
+    }
+
+protected:
+    friend struct TestWebKitAPI::LockInspector;
+    
+    static const uint8_t isHeldBit = 1;
+    static const uint8_t hasParkedBit = 2;
+    
+    WTF_EXPORT_PRIVATE void lockSlow();
+    WTF_EXPORT_PRIVATE void unlockSlow();
+    WTF_EXPORT_PRIVATE void unlockFairlySlow();
+    WTF_EXPORT_PRIVATE void safepointSlow();
+
+    // Method used for testing only.
+    bool isFullyReset() const
+    {
+        return !m_byte.load();
+    }
+
+    Atomic<uint8_t> m_byte;
+};
+
+class Lock : public LockBase {
+    WTF_MAKE_NONCOPYABLE(Lock);
+    WTF_MAKE_FAST_ALLOCATED;
+public:
+    Lock()
+    {
+        m_byte.store(0, std::memory_order_relaxed);
+    }
+};
+
+typedef LockBase StaticLock;
+typedef Locker<LockBase> LockHolder;
+
+} // namespace WTF
+
+using WTF::Lock;
+using WTF::LockHolder;
+using WTF::StaticLock;
+
+#endif // WTF_Lock_h
+