epubreader/public/bitjs/io/bytestream.js

/*
 * 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;
  }
}