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epubreader/public/bitjs/io/bytestream.js

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2020-04-21 20:37:42 +00:00
/*
* bytestream.js
*
* Provides readers for byte streams.
*
* Licensed under the MIT License
*
* Copyright(c) 2011 Google Inc.
* Copyright(c) 2011 antimatter15
*/
var bitjs = bitjs || {};
bitjs.io = bitjs.io || {};
/**
* 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.
*/
bitjs.io.ByteStream = class {
/**
* @param {ArrayBuffer} ab The ArrayBuffer object.
* @param {number=} opt_offset The offset into the ArrayBuffer
* @param {number=} opt_length The length of this BitStream
*/
constructor(ab, opt_offset, opt_length) {
const offset = opt_offset || 0;
const length = opt_length || 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.
*/
peekNumber(n) {
// TODO: return error if n would go past the end of the stream?
if (n <= 0 || typeof n != typeof 1) {
return -1;
}
let result = 0;
// read from last byte to first byte and roll them in
let 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.
*/
readNumber(n) {
const 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.
*/
peekSignedNumber(n) {
let num = this.peekNumber(n);
const HALF = Math.pow(2, (n * 8) - 1);
const 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.
*/
readSignedNumber(n) {
const num = this.peekSignedNumber(n);
this.ptr += n;
return num;
}
/**
* 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.
*/
peekBytes(n, movePointers) {
if (n <= 0 || typeof n != typeof 1) {
return null;
}
const 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.
*/
readBytes(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.
*/
peekString(n) {
if (n <= 0 || typeof n != typeof 1) {
return "";
}
let result = "";
for (let 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.
*/
readString(n) {
const strToReturn = this.peekString(n);
this.ptr += n;
return strToReturn;
}
}