jsmpeg/src/buffer.js

JSMpeg.BitBuffer = (function(){ "use strict";

var BitBuffer = function(bufferOrLength, mode) {
	if (typeof(bufferOrLength) === 'object') {
		this.bytes = (bufferOrLength instanceof Uint8Array)
			? bufferOrLength 
			: new Uint8Array(bufferOrLength);

		this.byteLength = this.bytes.length;
	}
	else {
		this.bytes = new Uint8Array(bufferOrLength || 1024*1024);	
		this.byteLength = 0;
	}

	this.mode = mode || BitBuffer.MODE.EXPAND;
	this.index = 0;
};

BitBuffer.prototype.resize = function(size) {
	var newBytes = new Uint8Array(size);
	if (this.byteLength !== 0) {
		this.byteLength = Math.min(this.byteLength, size);
		newBytes.set(this.bytes, 0, this.byteLength);
	}
	this.bytes = newBytes;
	this.index = Math.min(this.index, this.byteLength << 3);
};

BitBuffer.prototype.evict = function(sizeNeeded) {
	var bytePos = this.index >> 3,
		available = this.bytes.length - this.byteLength;
	
	// If the current index is the write position, we can simply reset both
	// to 0. Also reset (and throw away yet unread data) if we won't be able
	// to fit the new data in even after a normal eviction.
	if (
		this.index === this.byteLength << 3 ||
		sizeNeeded > available + bytePos // emergency evac
	) {
		this.byteLength = 0;
		this.index = 0;
		return;
	}
	else if (bytePos === 0) {
		// Nothing read yet - we can't evict anything
		return;
	}
	
	// Some browsers don't support copyWithin() yet - we may have to do 
	// it manually using set and a subarray
	if (this.bytes.copyWithin) {
		this.bytes.copyWithin(0, bytePos, this.byteLength);
	}
	else {
		this.bytes.set(this.bytes.subarray(bytePos, this.byteLength));
	}

	this.byteLength = this.byteLength - bytePos;
	this.index -= bytePos << 3;
	return;
};

BitBuffer.prototype.write = function(buffers) {
	var isArrayOfBuffers = (typeof(buffers[0]) === 'object'),
		totalLength = 0,
		available = this.bytes.length - this.byteLength;

	// Calculate total byte length
	if (isArrayOfBuffers) {
		var totalLength = 0;
		for (var i = 0; i < buffers.length; i++) {
			totalLength += buffers[i].byteLength;
		}
	}
	else {
		totalLength = buffers.byteLength;
	}

	// Do we need to resize or evict?
	if (totalLength > available) {
		if (this.mode === BitBuffer.MODE.EXPAND) {
			var newSize = Math.max(
				this.bytes.length * 2,
				totalLength - available
			);
			this.resize(newSize)
		}
		else {
			this.evict(totalLength);
		}
	}

	if (isArrayOfBuffers) {
		for (var i = 0; i < buffers.length; i++) {
			this.appendSingleBuffer(buffers[i]);
		}
	}
	else {
		this.appendSingleBuffer(buffers);
	}
};

BitBuffer.prototype.appendSingleBuffer = function(buffer) {
	buffer = buffer instanceof Uint8Array
		? buffer 
		: new Uint8Array(buffer);
	
	this.bytes.set(buffer, this.byteLength);
	this.byteLength += buffer.length;
};

BitBuffer.prototype.findNextStartCode = function() {	
	for (var i = (this.index+7 >> 3); i < this.byteLength; i++) {
		if(
			this.bytes[i] == 0x00 &&
			this.bytes[i+1] == 0x00 &&
			this.bytes[i+2] == 0x01
		) {
			this.index = (i+4) << 3;
			return this.bytes[i+3];
		}
	}
	this.index = (this.byteLength << 3);
	return -1;
};

BitBuffer.prototype.findStartCode = function(code) {
	var current = 0;
	while (true) {
		current = this.findNextStartCode();
		if (current === code || current === -1) {
			return current;
		}
	}
	return -1;
};

BitBuffer.prototype.nextBytesAreStartCode = function() {
	var i = (this.index+7 >> 3);
	return (
		i >= this.byteLength || (
			this.bytes[i] == 0x00 && 
			this.bytes[i+1] == 0x00 &&
			this.bytes[i+2] == 0x01
		)
	);
};

BitBuffer.prototype.peek = function(count) {
	var offset = this.index;
	var value = 0;
	while (count) {
		var currentByte = this.bytes[offset >> 3],
			remaining = 8 - (offset & 7), // remaining bits in byte
			read = remaining < count ? remaining : count, // bits in this run
			shift = remaining - read,
			mask = (0xff >> (8-read));

		value = (value << read) | ((currentByte & (mask << shift)) >> shift);

		offset += read;
		count -= read;
	}

	return value;
}

BitBuffer.prototype.read = function(count) {
	var value = this.peek(count);
	this.index += count;
	return value;
};

BitBuffer.prototype.skip = function(count) {
	return (this.index += count);
};

BitBuffer.prototype.rewind = function(count) {
	this.index = Math.max(this.index - count, 0);
};

BitBuffer.prototype.has = function(count) {
	return ((this.byteLength << 3) - this.index) >= count;
};

BitBuffer.MODE = {
	EVICT: 1,
	EXPAND: 2
};

return BitBuffer;

})();