@framekit/webm-writer-esm/src/ArrayBufferDataStream.mjs

WebM Encoder for Canvas Frames

/* eslint no-fallthrough: "off" */
/**
 * A tool for presenting an ArrayBuffer as a stream for writing some simple data types.
 *
 * By Nicholas Sherlock
 *
 */

/*
 * Create an ArrayBuffer of the given length and present it as a writable stream with methods
 * for writing data in different formats.
 */
export function ArrayBufferDataStream(length) {
  this.data = new Uint8Array(length);
  this.pos = 0;
}

ArrayBufferDataStream.prototype.seek = function (toOffset) {
  this.pos = toOffset;
};

ArrayBufferDataStream.prototype.writeBytes = function (arr) {
  for (let i = 0; i < arr.length; i++) {
    this.data[this.pos++] = arr[i];
  }
};

ArrayBufferDataStream.prototype.writeByte = function (b) {
  this.data[this.pos++] = b;
};

//Synonym:
ArrayBufferDataStream.prototype.writeU8 =
  ArrayBufferDataStream.prototype.writeByte;

ArrayBufferDataStream.prototype.writeU16BE = function (u) {
  this.data[this.pos++] = u >> 8;
  this.data[this.pos++] = u;
};

ArrayBufferDataStream.prototype.writeDoubleBE = function (d) {
  let bytes = new Uint8Array(new Float64Array([d]).buffer);

  for (let i = bytes.length - 1; i >= 0; i--) {
    this.writeByte(bytes[i]);
  }
};

ArrayBufferDataStream.prototype.writeFloatBE = function (d) {
  let bytes = new Uint8Array(new Float32Array([d]).buffer);

  for (let i = bytes.length - 1; i >= 0; i--) {
    this.writeByte(bytes[i]);
  }
};

/**
 * Write an ASCII string to the stream
 */
ArrayBufferDataStream.prototype.writeString = function (s) {
  for (let i = 0; i < s.length; i++) {
    this.data[this.pos++] = s.charCodeAt(i);
  }
};

/**
 * Write the given 32-bit integer to the stream as an EBML variable-length integer using the given byte width
 * (use measureEBMLVarInt).
 *
 * No error checking is performed to ensure that the supplied width is correct for the integer.
 *
 * @param i Integer to be written
 * @param width Number of bytes to write to the stream
 */
ArrayBufferDataStream.prototype.writeEBMLVarIntWidth = function (i, width) {
  switch (width) {
    case 1:
      this.writeU8((1 << 7) | i);
      break;
    case 2:
      this.writeU8((1 << 6) | (i >> 8));
      this.writeU8(i);
      break;
    case 3:
      this.writeU8((1 << 5) | (i >> 16));
      this.writeU8(i >> 8);
      this.writeU8(i);
      break;
    case 4:
      this.writeU8((1 << 4) | (i >> 24));
      this.writeU8(i >> 16);
      this.writeU8(i >> 8);
      this.writeU8(i);
      break;
    case 5:
      /*
       * JavaScript converts its doubles to 32-bit integers for bitwise operations, so we need to do a
       * division by 2^32 instead of a right-shift of 32 to retain those top 3 bits
       */
      this.writeU8((1 << 3) | ((i / 4294967296) & 0x7));
      this.writeU8(i >> 24);
      this.writeU8(i >> 16);
      this.writeU8(i >> 8);
      this.writeU8(i);
      break;
    default:
      throw new Error('Bad EBML VINT size ' + width);
  }
};

/**
 * Return the number of bytes needed to encode the given integer as an EBML VINT.
 */
ArrayBufferDataStream.prototype.measureEBMLVarInt = function (val) {
  if (val < (1 << 7) - 1) {
    /* Top bit is set, leaving 7 bits to hold the integer, but we can't store 127 because
     * "all bits set to one" is a reserved value. Same thing for the other cases below:
     */
    return 1;
  } else if (val < (1 << 14) - 1) {
    return 2;
  } else if (val < (1 << 21) - 1) {
    return 3;
  } else if (val < (1 << 28) - 1) {
    return 4;
  } else if (val < 34359738367) {
    // 2 ^ 35 - 1 (can address 32GB)
    return 5;
  } else {
    throw new Error('EBML VINT size not supported ' + val);
  }
};

ArrayBufferDataStream.prototype.writeEBMLVarInt = function (i) {
  this.writeEBMLVarIntWidth(i, this.measureEBMLVarInt(i));
};

/**
 * Write the given unsigned 32-bit integer to the stream in big-endian order using the given byte width.
 * No error checking is performed to ensure that the supplied width is correct for the integer.
 *
 * Omit the width parameter to have it determined automatically for you.
 *
 * @param u Unsigned integer to be written
 * @param width Number of bytes to write to the stream
 */
ArrayBufferDataStream.prototype.writeUnsignedIntBE = function (u, width) {
  if (width === undefined) {
    width = this.measureUnsignedInt(u);
  }

  // Each case falls through:
  switch (width) {
    case 5:
      this.writeU8(Math.floor(u / 4294967296)); // Need to use division to access >32 bits of floating point var
    case 4:
      this.writeU8(u >> 24);
    case 3:
      this.writeU8(u >> 16);
    case 2:
      this.writeU8(u >> 8);
    case 1:
      this.writeU8(u);
      break;
    default:
      throw new Error('Bad UINT size ' + width);
  }
};

/**
 * Return the number of bytes needed to hold the non-zero bits of the given unsigned integer.
 */
ArrayBufferDataStream.prototype.measureUnsignedInt = function (val) {
  // Force to 32-bit unsigned integer
  if (val < 1 << 8) {
    return 1;
  } else if (val < 1 << 16) {
    return 2;
  } else if (val < 1 << 24) {
    return 3;
  } else if (val < 4294967296) {
    return 4;
  } else {
    return 5;
  }
};

/**
 * Return a view on the portion of the buffer from the beginning to the current seek position as a Uint8Array.
 */
ArrayBufferDataStream.prototype.getAsDataArray = function () {
  if (this.pos < this.data.byteLength) {
    return this.data.subarray(0, this.pos);
  } else if (this.pos == this.data.byteLength) {
    return this.data;
  } else {
    throw new Error('ArrayBufferDataStream\'s pos lies beyond end of buffer');
  }
};