2 files changed, 153 insertions, 0 deletions

diff --git a/src/unstable/readwritemutex.cpp b/src/unstable/readwritemutex.cpp
new file mode 100644
index 0000000..b0a3b77
--- /dev/null
+++ b/src/unstable/readwritemutex.cpp
@@ -0,0 +1,78 @@
+#include "bu/readwritemutex.h"
+
+Bu::ReadWriteMutex::ReadWriteMutex() :
+    iCounter( 0 ),
+    bWantWrite( false )
+{
+}
+
+Bu::ReadWriteMutex::~ReadWriteMutex()
+{
+}
+
+void Bu::ReadWriteMutex::lockRead()
+{
+    // Check to see if someone wants to write
+    cWrite.lock();
+    if( bWantWrite )
+    {
+        // If so, wait patiently for them to finish
+        cWrite.wait();
+    }
+    cWrite.unlock();
+
+    //  Now lock the read counter
+    mRead.lock();
+    iCounter++;
+    // If the lock counter is one, we just locked for the first time,
+    // so we lock the writer.
+    if( iCounter == 1 )
+        mWrite.lock();
+    mRead.unlock();
+}
+
+void Bu::ReadWriteMutex::unlockRead()
+{
+    // Lock the read counter
+    mRead.lock();
+    iCounter--;
+    // If we just decremented the counter back to zero then we can
+    // release the write lock
+    if( iCounter == 0 )
+        mWrite.unlock();
+    mRead.unlock();
+}
+
+void Bu::ReadWriteMutex::lockWrite()
+{
+    // Lock the read counter
+    mRead.lock();
+    if( iCounter > 0 )
+    {
+        // If there is an active read in progress then we set the bWantWrite
+        // flag to make sure no more readers start working.
+        cWrite.lock();
+        bWantWrite = true;
+        cWrite.unlock();
+    }
+    mRead.unlock();
+
+    // Lock the write lock
+    mWrite.lock();
+}
+
+void Bu::ReadWriteMutex::unlockWrite()
+{
+    // Just always set the bWantWrite flag to false at this point, as long
+    // as we're locked.
+    cWrite.lock();
+    bWantWrite = false;
+    cWrite.unlock();
+
+    // Release all waiting readers, they won't actually do much until we
+    // unlock the write lock though
+    cWrite.broadcast();
+
+    // Unlock the write lock
+    mWrite.unlock();
+}
diff --git a/src/unstable/readwritemutex.h b/src/unstable/readwritemutex.h
new file mode 100644
index 0000000..9e07047
--- /dev/null
+++ b/src/unstable/readwritemutex.h
@@ -0,0 +1,75 @@
+#ifndef BU_READ_WRITE_MUTEX_H
+#define BU_READ_WRITE_MUTEX_H
+
+#include "bu/mutex.h"
+#include "bu/condition.h"
+
+namespace Bu
+{
+    /**
+     * Mutex designed for situations where overlapped reading is safe, but
+     * overlapped writing isn't.  There are many, many good examples of this
+     * including most data structures, streams, etc. etc.
+     *
+     * Use this just like a normal mutex except that you use the
+     * lockRead/unlockRead and lockWrite/unlockWrite functions depending on
+     * weather the section of code your locking is reading data or changing
+     * data.
+     *
+     * This particular mutex is designed so that while a read operation is
+     * happening other read operations can also happen, but no write operations
+     * can occur.  While a write is happening, no other write or read operation
+     * can continue.  There is an extra feature to ensure writes get a chance
+     * to complete, when a lockWrite is issued, all current read operations
+     * continue, but future read operations block until the write is complete.
+     */
+    class ReadWriteMutex
+    {
+    public:
+        ReadWriteMutex();
+        virtual ~ReadWriteMutex();
+
+        /**
+         * Lock the mutex for reading.  Multiple code sections can hold a read
+         * lock at the same time, but write locks will wait for all read locks
+         * to be released before continuing.  Read locks will also wait if
+         * there is an active write lock.
+         *
+         * It is very important to not make any changes to your data within
+         * a read lock.
+         */
+        void lockRead();
+
+        /**
+         * Release a read lock.
+         */
+        void unlockRead();
+
+        /**
+         * Lock the mutex for writing.  Only one code section can have a write
+         * lock at any given time.  No code sections can be in a locked read
+         * section while a write lock is held.  When a write lock is requested
+         * all following read locks will block until the write operation is
+         * started, ensuring writes always get a chance to execute.
+         *
+         * Within a write locked code section feel free to change your data
+         * and read your data.  It is imparative to spend as little time as
+         * possible in a write-locked section.
+         */
+        void lockWrite();
+
+        /**
+         * Release a write lock.
+         */
+        void unlockWrite();
+
+    private:
+        Bu::Mutex mRead;
+        int iCounter;
+        Bu::Mutex mWrite;
+        Bu::Condition cWrite;
+        bool bWantWrite;
+    };
+};
+
+#endif