util/CircularBuffer: new buffer class

src/util/CircularBuffer.hxx

@@ -0,0 +1,165 @@
+/*
+ * Copyright (C) 2014 Max Kellermann <max@duempel.org>
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * - Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * - 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 THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * ``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 THE
+ * FOUNDATION 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 CIRCULAR_BUFFER_HPP
+#define CIRCULAR_BUFFER_HPP
+
+#include "WritableBuffer.hxx"
+
+#include <assert.h>
+#include <stddef.h>
+
+/**
+ * A circular buffer.
+ *
+ * This class does not manage buffer memory.  It will not allocate or
+ * free any memory, it only manages the contents of an existing
+ * buffer given to the constructor.
+ *
+ * Everything between #head and #tail is valid data (may wrap around).
+ * If both are equal, then the buffer is empty.  Due to this
+ * implementation detail, the buffer is empty when #size-1 items are
+ * stored; the last buffer cell cannot be used.
+ */
+template<typename T>
+class CircularBuffer {
+public:
+	typedef WritableBuffer<T> Range;
+	typedef typename Range::pointer_type pointer_type;
+	typedef typename Range::size_type size_type;
+
+protected:
+	/**
+	 * The next index to be read.
+	 */
+	size_type head;
+
+	/**
+	 * The next index to be written to.
+	 */
+	size_type tail;
+
+	const size_type size;
+	const pointer_type data;
+
+public:
+	constexpr CircularBuffer(pointer_type _data, size_type _size)
+		:head(0), tail(0), size(_size), data(_data) {}
+
+	CircularBuffer(const CircularBuffer &other) = delete;
+
+protected:
+	constexpr size_type Next(size_type i) const {
+		return i + 1 == size
+			? 0
+			: i + 1;
+	}
+
+public:
+	void Clear() {
+		head = tail = 0;
+	}
+
+	size_type GetSize() const {
+		return size;
+	}
+
+	constexpr bool IsEmpty() const {
+		return head == tail;
+	}
+
+	constexpr bool IsFull() const {
+		return Next(tail) == head;
+	}
+
+	/**
+	 * Prepares writing.  Returns a buffer range which may be written.
+	 * When you are finished, call Append().
+	 */
+	Range Write() {
+		assert(head < size);
+		assert(tail < size);
+
+		size_type end = tail < head
+			? head - 1
+			/* the "head==0" is there so we don't write
+			   the last cell, as this situation cannot be
+			   represented by head/tail */
+			: size - (head == 0);
+
+		return Range(data + tail, end - tail);
+	}
+
+	/**
+	 * Expands the tail of the buffer, after data has been written
+	 * to the buffer returned by Write().
+	 */
+	void Append(size_type n) {
+		assert(head < size);
+		assert(tail < size);
+		assert(n < size);
+		assert(tail + n <= size);
+		assert(head <= tail || tail + n < head);
+
+		tail += n;
+
+		if (tail == size) {
+			assert(head > 0);
+			tail = 0;
+		}
+	}
+
+	/**
+	 * Return a buffer range which may be read.  The buffer pointer is
+	 * writable, to allow modifications while parsing.
+	 */
+	Range Read() {
+		assert(head < size);
+		assert(tail < size);
+
+		return Range(data + head, (tail < head ? size : tail) - head);
+	}
+
+	/**
+	 * Marks a chunk as consumed.
+	 */
+	void Consume(size_type n) {
+		assert(head < size);
+		assert(tail < size);
+		assert(n < size);
+		assert(head + n <= size);
+		assert(tail < head || head + n <= tail);
+
+		head += n;
+		if (head == size)
+			head = 0;
+	}
+};
+
+#endif