288 lines
8.3 KiB
JavaScript
288 lines
8.3 KiB
JavaScript
/*
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* bitstream.js
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*
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* Provides readers for bitstreams.
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*
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* Licensed under the MIT License
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*
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* Copyright(c) 2011 Google Inc.
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* Copyright(c) 2011 antimatter15
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*/
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var bitjs = bitjs || {};
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bitjs.io = bitjs.io || {};
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/**
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* This object allows you to peek and consume bits and bytes out of a stream.
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* Note that this stream is optimized, and thus, will *NOT* throw an error if
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* the end of the stream is reached. Only use this in scenarios where you
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* already have all the bits you need.
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*/
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bitjs.io.BitStream = class {
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/**
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* @param {ArrayBuffer} ab An ArrayBuffer object or a Uint8Array.
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* @param {boolean} rtl Whether the stream reads bits from the byte starting
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* from bit 7 to 0 (true) or bit 0 to 7 (false).
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* @param {Number} opt_offset The offset into the ArrayBuffer
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* @param {Number} opt_length The length of this BitStream
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*/
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constructor(ab, rtl, opt_offset, opt_length) {
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if (!(ab instanceof ArrayBuffer)) {
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throw 'Error! BitArray constructed with an invalid ArrayBuffer object';
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}
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const offset = opt_offset || 0;
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const length = opt_length || ab.byteLength;
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/**
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* The bytes in the stream.
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* @type {Uint8Array}
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* @private
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*/
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this.bytes = new Uint8Array(ab, offset, length);
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/**
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* The byte in the stream that we are currently on.
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* @type {Number}
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* @private
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*/
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this.bytePtr = 0;
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/**
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* The bit in the current byte that we will read next (can have values 0 through 7).
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* @type {Number}
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* @private
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*/
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this.bitPtr = 0; // tracks which bit we are on (can have values 0 through 7)
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/**
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* An ever-increasing number.
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* @type {Number}
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* @private
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*/
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this.bitsRead_ = 0;
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this.peekBits = rtl ? this.peekBits_rtl : this.peekBits_ltr;
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}
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/**
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* Returns how many bites have been read in the stream since the beginning of time.
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*/
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getNumBitsRead() {
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return this.bitsRead_;
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}
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/**
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* Returns how many bits are currently in the stream left to be read.
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*/
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getNumBitsLeft() {
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const bitsLeftInByte = 8 - this.bitPtr;
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return (this.bytes.byteLength - this.bytePtr - 1) * 8 + bitsLeftInByte;
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}
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/**
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* byte0 byte1 byte2 byte3
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* 7......0 | 7......0 | 7......0 | 7......0
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*
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* The bit pointer starts at bit0 of byte0 and moves left until it reaches
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* bit7 of byte0, then jumps to bit0 of byte1, etc.
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* @param {number} n The number of bits to peek, must be a positive integer.
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* @param {boolean=} movePointers Whether to move the pointer, defaults false.
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* @return {number} The peeked bits, as an unsigned number.
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*/
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peekBits_ltr(n, opt_movePointers) {
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const NUM = parseInt(n, 10);
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let num = NUM;
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if (n !== num || num <= 0) {
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return 0;
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}
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const BITMASK = bitjs.io.BitStream.BITMASK;
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const movePointers = opt_movePointers || false;
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let bytes = this.bytes;
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let bytePtr = this.bytePtr;
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let bitPtr = this.bitPtr;
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let result = 0;
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let bitsIn = 0;
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// keep going until we have no more bits left to peek at
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while (num > 0) {
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// We overflowed the stream, so just return what we got.
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if (bytePtr >= bytes.length) {
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break;
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}
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const numBitsLeftInThisByte = (8 - bitPtr);
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if (num >= numBitsLeftInThisByte) {
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const mask = (BITMASK[numBitsLeftInThisByte] << bitPtr);
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result |= (((bytes[bytePtr] & mask) >> bitPtr) << bitsIn);
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bytePtr++;
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bitPtr = 0;
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bitsIn += numBitsLeftInThisByte;
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num -= numBitsLeftInThisByte;
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} else {
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const mask = (BITMASK[num] << bitPtr);
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result |= (((bytes[bytePtr] & mask) >> bitPtr) << bitsIn);
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bitPtr += num;
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break;
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}
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}
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if (movePointers) {
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this.bitPtr = bitPtr;
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this.bytePtr = bytePtr;
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this.bitsRead_ += NUM;
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}
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return result;
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}
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/**
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* byte0 byte1 byte2 byte3
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* 7......0 | 7......0 | 7......0 | 7......0
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*
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* The bit pointer starts at bit7 of byte0 and moves right until it reaches
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* bit0 of byte0, then goes to bit7 of byte1, etc.
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* @param {number} n The number of bits to peek. Must be a positive integer.
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* @param {boolean=} movePointers Whether to move the pointer, defaults false.
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* @return {number} The peeked bits, as an unsigned number.
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*/
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peekBits_rtl(n, opt_movePointers) {
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const NUM = parseInt(n, 10);
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let num = NUM;
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if (n !== num || num <= 0) {
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return 0;
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}
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const BITMASK = bitjs.io.BitStream.BITMASK;
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const movePointers = opt_movePointers || false;
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let bytes = this.bytes;
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let bytePtr = this.bytePtr;
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let bitPtr = this.bitPtr;
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let result = 0;
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// keep going until we have no more bits left to peek at
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while (num > 0) {
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// We overflowed the stream, so just return the bits we got.
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if (bytePtr >= bytes.length) {
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break;
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}
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const numBitsLeftInThisByte = (8 - bitPtr);
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if (num >= numBitsLeftInThisByte) {
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result <<= numBitsLeftInThisByte;
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result |= (BITMASK[numBitsLeftInThisByte] & bytes[bytePtr]);
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bytePtr++;
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bitPtr = 0;
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num -= numBitsLeftInThisByte;
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} else {
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result <<= num;
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const numBits = 8 - num - bitPtr;
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result |= ((bytes[bytePtr] & (BITMASK[num] << numBits)) >> numBits);
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bitPtr += num;
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break;
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}
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}
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if (movePointers) {
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this.bitPtr = bitPtr;
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this.bytePtr = bytePtr;
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this.bitsRead_ += NUM;
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}
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return result;
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}
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/**
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* Peek at 16 bits from current position in the buffer.
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* Bit at (bytePtr,bitPtr) has the highest position in returning data.
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* Taken from getbits.hpp in unrar.
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* TODO: Move this out of BitStream and into unrar.
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*/
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getBits() {
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return (((((this.bytes[this.bytePtr] & 0xff) << 16) +
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((this.bytes[this.bytePtr+1] & 0xff) << 8) +
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((this.bytes[this.bytePtr+2] & 0xff))) >>> (8-this.bitPtr)) & 0xffff);
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}
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/**
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* Reads n bits out of the stream, consuming them (moving the bit pointer).
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* @param {number} n The number of bits to read. Must be a positive integer.
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* @return {number} The read bits, as an unsigned number.
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*/
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readBits(n) {
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return this.peekBits(n, true);
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}
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/**
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* This returns n bytes as a sub-array, advancing the pointer if movePointers
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* is true. Only use this for uncompressed blocks as this throws away remaining
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* bits in the current byte.
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* @param {number} n The number of bytes to peek. Must be a positive integer.
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* @param {boolean=} movePointers Whether to move the pointer, defaults false.
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* @return {Uint8Array} The subarray.
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*/
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peekBytes(n, opt_movePointers) {
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const num = parseInt(n, 10);
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if (n !== num || num < 0) {
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throw 'Error! Called peekBytes() with a non-positive integer: ' + n;
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} else if (num === 0) {
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return new Uint8Array();
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}
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// Flush bits until we are byte-aligned.
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// from http://tools.ietf.org/html/rfc1951#page-11
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// "Any bits of input up to the next byte boundary are ignored."
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while (this.bitPtr != 0) {
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this.readBits(1);
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}
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const numBytesLeft = this.getNumBitsLeft() / 8;
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if (num > numBytesLeft) {
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throw 'Error! Overflowed the bit stream! n=' + num + ', bytePtr=' + this.bytePtr +
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', bytes.length=' + this.bytes.length + ', bitPtr=' + this.bitPtr;
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}
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const movePointers = opt_movePointers || false;
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const result = new Uint8Array(num);
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let bytes = this.bytes;
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let ptr = this.bytePtr;
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let bytesLeftToCopy = num;
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while (bytesLeftToCopy > 0) {
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const bytesLeftInStream = bytes.length - ptr;
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const sourceLength = Math.min(bytesLeftToCopy, bytesLeftInStream);
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result.set(bytes.subarray(ptr, ptr + sourceLength), num - bytesLeftToCopy);
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ptr += sourceLength;
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// Overflowed the stream, just return what we got.
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if (ptr >= bytes.length) {
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break;
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}
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bytesLeftToCopy -= sourceLength;
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}
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if (movePointers) {
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this.bytePtr += num;
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this.bitsRead_ += (num * 8);
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}
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return result;
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}
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/**
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* @param {number} n The number of bytes to read.
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* @return {Uint8Array} The subarray.
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*/
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readBytes(n) {
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return this.peekBytes(n, true);
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}
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}
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// mask for getting N number of bits (0-8)
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bitjs.io.BitStream.BITMASK = [0, 0x01, 0x03, 0x07, 0x0F, 0x1F, 0x3F, 0x7F, 0xFF ];
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