Make CircBuf are lock-free, threadsafe buffer. (We don't actually

take advantage of this yet.)
2004-03-16 03:06:10 +00:00
parent ab50b4c426
commit eb86c76b9f
2 changed files with 188 additions and 39 deletions
@@ -203,7 +203,7 @@ void RageSound::Update(float delta)
 /* Return the number of bytes available in the input buffer. */
 int RageSound::Bytes_Available() const
 {
-	return databuf.size();
+	return databuf.num_readable();
 }


@@ -277,7 +277,7 @@ int RageSound::FillBuf(int bytes)

 	while(bytes > 0)
 	{
-		if(read_block_size > databuf.capacity() - databuf.size())
+		if(read_block_size > databuf.num_writable())
 			break; /* full */

 		char inbuf[10240];
@@ -347,7 +347,7 @@ int RageSound::GetData(char *buffer, int size)
 	} else {
 		/* Feed data out of our streaming buffer. */
 		ASSERT(Sample);
-		got = min(int(databuf.size()), size);
+		got = min(int(databuf.num_readable()), size);
 		if(buffer)
 			databuf.read(buffer, got);
 	}
@@ -1,67 +1,216 @@
 #ifndef RAGE_UTIL_CIRCULAR_BUFFER
 #define RAGE_UTIL_CIRCULAR_BUFFER

+/* Lock-free circular buffer.  This should be threadsafe if one thread is reading
+ * and another is writing. */
 template<class T>
 class CircBuf
 {
-	basic_string<T> buf;
-	unsigned cnt, start;
-	
+	T *buf;
+	/* read_pos is the position data is read from; write_pos is the position
+	 * data is written to.  If read_pos == write_pos, the buffer is empty.
+	 *
+	 * There will always be at least one position empty, as a completely full
+	 * buffer (read_pos == write_pos) is indistinguishable from an empty buffer.
+	 *
+	 * Invariants: read_pos < size, write_pos < size. */
+	unsigned size;
+
+	/* These are volatile to prevent reads and writes to them from being optimized. */
+	volatile unsigned read_pos, write_pos;
+
 public:
-	CircBuf() { clear(); }
-	unsigned size() const { return cnt; }
-	unsigned capacity() const { return buf.size(); }
+	CircBuf()
+	{
+		buf = NULL;
+		clear();
+	}
+
+	~CircBuf()
+	{
+		delete[] buf;
+	}
+		
+	/* Return the number of elements available to read. */
+	unsigned num_readable() const
+	{
+		const int rpos = read_pos;
+		const int wpos = write_pos;
+		if( rpos < wpos )
+			/* The buffer looks like "eeeeDDDDeeee" (e = empty, D = data). */
+			return wpos - rpos;
+		else if( rpos > wpos )
+			/* The buffer looks like "DDeeeeeeeeDD" (e = empty, D = data). */
+			return size - (rpos - wpos);
+		else // if( rpos == wpos )
+			/* The buffer looks like "eeeeeeeeeeee" (e = empty, D = data). */
+			return 0;
+	}
+	
+	/* Return the number of elements writable.  Note that there must always
+	 * be one */
+	unsigned num_writable() const
+	{
+		const int rpos = read_pos;
+		const int wpos = write_pos;
+
+		int ret;
+		if( rpos < wpos )
+			/* The buffer looks like "eeeeDDDDeeee" (e = empty, D = data). */
+			ret = size - (wpos - rpos);
+		else if( rpos > wpos )
+			/* The buffer looks like "DDeeeeeeeeDD" (e = empty, D = data). */
+			ret = rpos - wpos;
+		else // if( rpos == wpos )
+			/* The buffer looks like "eeeeeeeeeeee" (e = empty, D = data). */
+			ret = size;
+
+		/* Subtract one, to account for the element that we never fill. */
+		return ret - 1;
+	}
+
+	unsigned capacity() const { return size; }

 	void reserve( unsigned n )
 	{
+		/* Reserve an extra byte.  We'll never fill more than n bytes; the extra
+		 * byte is to guarantee that read_pos != write_pos when the buffer is full,
+		 * since that would be ambiguous with an empty buffer. */
 		clear();
-		buf.erase();
-		buf.insert( buf.end(), n, 0 );
+		delete[] buf;
+		buf = new T[n+1];
+		size = n+1;
 	}

 	void clear()
 	{
-		cnt = start = 0;
+		read_pos = write_pos = 0;
 	}

-	void write( const T *buffer, unsigned buffer_size )
+	/* Indicate that n elements have been written. */
+	void advance_write_pointer( int n )
 	{
-		ASSERT( size() + buffer_size <= capacity() ); /* overflow */
+		write_pos = (write_pos + n) % size;
+	}
+	
+	/* Indicate that n elements have been read. */
+	void advance_read_pointer( int n )
+	{
+		read_pos = (read_pos + n) % size;
+	}
+	
+	void get_write_pointers( T *pPointers[2], unsigned pSizes[2] )
+	{
+		const int rpos = read_pos;
+		const int wpos = write_pos;

-		while( buffer_size )
+		if( rpos <= wpos )
 		{
-			unsigned write_pos = start + size();
-			if( write_pos >= buf.size() )
-				write_pos -= buf.size();
-			
-			const int cpy = int(min(buffer_size, buf.size() - write_pos));
-			buf.replace( write_pos, cpy, buffer, cpy );
+			/* The buffer looks like "eeeeDDDDeeee" or "eeeeeeeeeeee" (e = empty, D = data). */
+			pPointers[0] = buf+wpos;
+			pPointers[1] = buf;

-			cnt += cpy;
+			pSizes[0] = size - wpos;
+			pSizes[1] = rpos;
+		}
+		else if( rpos > wpos )
+		{
+			/* The buffer looks like "DDeeeeeeeeDD" (e = empty, D = data). */
+			pPointers[0] = buf+wpos;
+			pPointers[1] = NULL;

-			buffer += cpy;
-			buffer_size -= cpy;
+			pSizes[0] = rpos - wpos;
+			pSizes[1] = 0;
+		}
+
+		/* Subtract one, to account for the element that we never fill. */
+		if( pSizes[1] )
+			--pSizes[1];
+		else
+			--pSizes[0];
+	}
+
+	void get_read_pointers( T *pPointers[2], unsigned pSizes[2] )
+	{
+		const int rpos = read_pos;
+		const int wpos = write_pos;
+
+		if( rpos < wpos )
+		{
+			/* The buffer looks like "eeeeDDDDeeee" (e = empty, D = data). */
+			pPointers[0] = buf+rpos;
+			pPointers[1] = NULL;
+
+			pSizes[0] = wpos - rpos;
+			pSizes[1] = 0;
+		}
+		else if( rpos > wpos )
+		{
+			/* The buffer looks like "DDeeeeeeeeDD" (e = empty, D = data). */
+			pPointers[0] = buf+rpos;
+			pPointers[1] = buf;
+
+			pSizes[0] = size - rpos;
+			pSizes[1] = wpos;
+		}
+		else
+		{
+			/* The buffer looks like "eeeeeeeeeeee" (e = empty, D = data). */
+			pPointers[0] = NULL;
+			pPointers[1] = NULL;
+
+			pSizes[0] = 0;
+			pSizes[1] = 0;
 		}
 	}
-
-	void read( T *buffer, unsigned buffer_size )
+	
+	/* Write buffer_size elements from buffer, and advance the write pointer.  If
+	 * the data will not fit entirely, the write pointer will be unchanged
+	 * and false will be returned. */
+	bool write( const T *buffer, unsigned buffer_size )
 	{
-		ASSERT( size() >= buffer_size ); /* underflow */
+		T *p[2];
+		unsigned sizes[2];
+		get_write_pointers( p, sizes );
+
+		if( buffer_size > sizes[0] + sizes[1] )
+			return false;
 		
-		while( buffer_size )
-		{
-			const unsigned total = static_cast<unsigned>(min(buf.size() - start, size()));
-			const unsigned cpy = min( buffer_size, total );
-			buf.copy( buffer, cpy, start );
+		const int from_first = min( buffer_size, sizes[0] );
+		memcpy( p[0], buffer, from_first*sizeof(T) );
+		if( buffer_size > sizes[0] )
+			memcpy( p[1], buffer+from_first, max(buffer_size-sizes[0], 0u)*sizeof(T) );

-			start += cpy;
-			if( start == buf.size() )
-				start = 0;
-			cnt -= cpy;
+		advance_write_pointer( buffer_size );

-			buffer += cpy;
-			buffer_size -= cpy;
-		}
+		return true;
+	}
+
+	/* Read buffer_size elements from buffer, and advance the read pointer.  If
+	 * the buffer can not be filled completely, the read pointer will be unchanged
+	 * and false will be returned. */
+	bool read( T *buffer, unsigned buffer_size )
+	{
+		T *p[2];
+		unsigned sizes[2];
+		get_read_pointers( p, sizes );
+
+		if( buffer_size > sizes[0] + sizes[1] )
+			return false;
+
+		const int from_first = min( buffer_size, sizes[0] );
+		memcpy( buffer, p[0], from_first*sizeof(T) );
+		if( buffer_size > sizes[0] )
+			memcpy( buffer+from_first, p[1], max(buffer_size-sizes[0], 0u)*sizeof(T) );
+
+		/* Set the data that we just read to 0xFF.  This way, if we're passing pointesr
+		 * through, we can tell if we accidentally get a stale pointer. */
+		memset( p[0], 0xFF, from_first*sizeof(T) );
+		if( buffer_size > sizes[0] )
+			memset( p[1], 0xFF, max(buffer_size-sizes[0], 0u)*sizeof(T) );
+
+		advance_read_pointer( buffer_size );
+		return true;
 	}
 };