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parallel-library/cps/static/js/io/bytestream.js

196 lines
5.6 KiB
JavaScript

/*
* bytestream.js
*
* Provides readers for byte streams.
*
* Licensed under the MIT License
*
* Copyright(c) 2011 Google Inc.
* Copyright(c) 2011 antimatter15
*/
/* global bitjs, Uint8Array */
var bitjs = bitjs || {};
bitjs.io = bitjs.io || {};
(function() {
/**
* This object allows you to peek and consume bytes as numbers and strings
* out of an ArrayBuffer. In this buffer, everything must be byte-aligned.
*
* @param {ArrayBuffer} ab The ArrayBuffer object.
* @param {number=} optOffset The offset into the ArrayBuffer
* @param {number=} optLength The length of this BitStream
* @constructor
*/
bitjs.io.ByteStream = function(ab, optOffset, optLength) {
var offset = optOffset || 0;
var length = optLength || ab.byteLength;
this.bytes = new Uint8Array(ab, offset, length);
this.ptr = 0;
};
/**
* Peeks at the next n bytes as an unsigned number but does not advance the
* pointer
* TODO: This apparently cannot read more than 4 bytes as a number?
* @param {number} n The number of bytes to peek at.
* @return {number} The n bytes interpreted as an unsigned number.
*/
bitjs.io.ByteStream.prototype.peekNumber = function(n) {
// TODO: return error if n would go past the end of the stream?
if (n <= 0 || typeof n !== typeof 1) {
return -1;
}
var result = 0;
// read from last byte to first byte and roll them in
var curByte = this.ptr + n - 1;
while (curByte >= this.ptr) {
result <<= 8;
result |= this.bytes[curByte];
--curByte;
}
return result;
};
/**
* Returns the next n bytes as an unsigned number (or -1 on error)
* and advances the stream pointer n bytes.
* @param {number} n The number of bytes to read.
* @return {number} The n bytes interpreted as an unsigned number.
*/
bitjs.io.ByteStream.prototype.readNumber = function(n) {
var num = this.peekNumber(n);
this.ptr += n;
return num;
};
/**
* Returns the next n bytes as a signed number but does not advance the
* pointer.
* @param {number} n The number of bytes to read.
* @return {number} The bytes interpreted as a signed number.
*/
bitjs.io.ByteStream.prototype.peekSignedNumber = function(n) {
var num = this.peekNumber(n);
var HALF = Math.pow(2, (n * 8) - 1);
var FULL = HALF * 2;
if (num >= HALF) num -= FULL;
return num;
};
/**
* Returns the next n bytes as a signed number and advances the stream pointer.
* @param {number} n The number of bytes to read.
* @return {number} The bytes interpreted as a signed number.
*/
bitjs.io.ByteStream.prototype.readSignedNumber = function(n) {
var num = this.peekSignedNumber(n);
this.ptr += n;
return num;
};
/**
* ToDo: Returns the next n bytes as a signed number and advances the stream pointer.
* @param {number} n The number of bytes to read.
* @return {number} The bytes interpreted as a signed number.
*/
bitjs.io.ByteStream.prototype.movePointer = function(n) {
this.ptr += n;
// end of buffer reached
if ((this.bytes.byteLength - this.ptr) < 0 ) {
this.ptr = this.bytes.byteLength;
}
}
/**
* ToDo: Returns the next n bytes as a signed number and advances the stream pointer.
* @param {number} n The number of bytes to read.
* @return {number} The bytes interpreted as a signed number.
*/
bitjs.io.ByteStream.prototype.moveTo = function(n) {
if ( n < 0 ) {
n = 0;
}
this.ptr = n;
// end of buffer reached
if ((this.bytes.byteLength - this.ptr) < 0 ) {
this.ptr = this.bytes.byteLength;
}
}
/**
* This returns n bytes as a sub-array, advancing the pointer if movePointers
* is true.
* @param {number} n The number of bytes to read.
* @param {boolean} movePointers Whether to move the pointers.
* @return {Uint8Array} The subarray.
*/
bitjs.io.ByteStream.prototype.peekBytes = function(n, movePointers) {
if (n <= 0 || typeof n !== typeof 1) {
return null;
}
var result = this.bytes.subarray(this.ptr, this.ptr + n);
if (movePointers) {
this.ptr += n;
}
return result;
};
/**
* Reads the next n bytes as a sub-array.
* @param {number} n The number of bytes to read.
* @return {Uint8Array} The subarray.
*/
bitjs.io.ByteStream.prototype.readBytes = function(n) {
return this.peekBytes(n, true);
};
/**
* Peeks at the next n bytes as a string but does not advance the pointer.
* @param {number} n The number of bytes to peek at.
* @return {string} The next n bytes as a string.
*/
bitjs.io.ByteStream.prototype.peekString = function(n) {
if (n <= 0 || typeof n !== typeof 1) {
return "";
}
var result = "";
for (var p = this.ptr, end = this.ptr + n; p < end; ++p) {
result += String.fromCharCode(this.bytes[p]);
}
return result;
};
/**
* Returns the next n bytes as an ASCII string and advances the stream pointer
* n bytes.
* @param {number} n The number of bytes to read.
* @return {string} The next n bytes as a string.
*/
bitjs.io.ByteStream.prototype.readString = function(n) {
var strToReturn = this.peekString(n);
this.ptr += n;
return strToReturn;
};
})();