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1 | var InDescription = require('./in_description');
var OutDescription = require('./out_description');
/*global Uint8Array, ArrayBuffer*/
/**
* Manages the packing/unpacking of values as a set number of bits
* @class Bitstream
* @constructor
* @param {Array} buffer an array of 7-bit integers representing the intial
* data for this Bitstream
*/
function Bitstream(buffer) {
this.arr = [];
this._nbits = 0;
this._index = 0;
if (buffer) {
this.fromArrayBuffer(buffer);
}
}
Bitstream.prototype = {
constructor: Bitstream,
/**
* @method bitsLeft
* @return {Number} the number of bits which can be read without causing an
* overread
*/
bitsLeft: function () {
return this._nbits - this._index;
},
/**
* Empty the buffer and reset the index
* @method empty
*/
empty: function () {
this.arr = [];
this._index = 0;
this._nbits = 0;
},
/**
* Move the _index to the first position >= the current index which is the
* beginning of a cell. Used to burn off padding when processing data from
* "appendData" since it pads to the nearest multiple of CELLSIZE
* @method align
*/
align: function () {
var delta = this._index % 7;
if (delta === 0) {
return;
}
this._advance(7 - delta);
},
/**
* Set the `n` bits starting at `offset` to contain the unsigned integer
* `value`
* @method _setBits
* @private
* @param {Number} offset The zero-based bit offset to start at
* @param {Number} n The number of bits to pack the value in to
* @param {Number} value The value to pack. Will be cast to an unsigned
* integer and truncated or padded to n bits
*/
_setBits: function (offset, n, value) {
var bits;
var cell;
var cellOffset;
var mask;
var nbits;
cell = Math.floor(offset / 7);
cellOffset = offset % 7;
while (n > 0) {
// determine how many bits will fit into the current cell
nbits = Math.min(n, 7 - cellOffset);
// make an all-set bitmask with length of nbits
mask = (1 << nbits) - 1;
// get the next nbits bits from the value
bits = value & mask;
// move the bits and mask to the correct cell offset
bits <<= cellOffset;
mask <<= cellOffset;
// set the cells bits
this.arr[cell] = (this.arr[cell] & (~mask)) | bits;
// prepare for next iteration
value >>= nbits;
n -= nbits;
cellOffset = 0;
cell++;
}
},
/**
* Return the value of the first n bits starting at offset
* @private
* @method _getBits
* @param {Number} offset The zero-based bit offset to start at
* @param {Number} n The number of bits to unpack the value from
* @return {Number} The unsigned value after unpacking
*/
_getBits: function (offset, n) {
var bits;
var cell;
var cellOffset;
var mask;
var nbits;
var value;
var valueOffset;
cell = Math.floor(offset / 7);
cellOffset = offset % 7;
value = 0;
valueOffset = 0;
while (n > 0) {
// determine how many bits can be retrieved from this cell
nbits = Math.min(n, 7 - cellOffset);
// make an all-set bitmask with length of nbits
mask = (1 << nbits) - 1;
mask <<= cellOffset;
bits = this.arr[cell] & mask;
bits >>= cellOffset;
value |= bits << valueOffset;
// prepare for next iteration
n -= nbits;
cellOffset = 0;
cell++;
valueOffset += nbits;
}
return value;
},
/**
* Convert the data in this `Bitstream` to a `Uint8Array` containing an
* `ArrayBuffer` suitable for transmitting this data over a binary websocket
* @method toArrayBuffer
* @return A `Uint8Array` containing the data in this Bitstream
*/
toArrayBuffer: function () {
var buf = new ArrayBuffer(this.arr.length);
var arr = new Uint8Array(buf);
var i;
for (i = 0; i < this.arr.length; i++) {
arr[i] = this.arr[i];
}
return arr;
},
/**
* Populate the data in this Bitstream from an ArrayBuffer received over a
* binary websocket
* @method fromArrayBuffer
*/
fromArrayBuffer: function (buffer) {
this.empty();
this.appendData(buffer);
},
/**
* Append data from an ArrayBuffer received over a binary websocket to this
* Bitstream
* @method appendData
*/
appendData: function (buffer) {
var i;
for (i = 0; i < buffer.length; i++) {
this.arr.push(buffer[i]);
}
this._extend(i * 7);
},
/**
* Append UTF-8 encoded data from a string to this Bitstream
* @method appendChars
*/
appendChars: function (chars) {
var data = [];
var i;
for (i = 0; i < chars.length; i++) {
data.push(chars.charCodeAt(i));
}
this.appendData(data);
},
/**
* Convert the data to a valid UTF-8 string
* @method toChars
*/
toChars: function () {
var i;
var result = '';
for (i = 0; i < this.arr.length; i++) {
result += String.fromCharCode(this.arr[i]);
}
return result;
},
/**
* Read an unsigned integer *without* consuming any bits
* @method peekUInt
* @param {Number} bits The number of bits to unpack
*/
peekUInt: function (bits) {
var result = this._getBits(this._index, bits);
return result;
},
/**
* Read an unsigned integer consuming the specified number of bits
* @method readUInt
* @param {Number} bits The number of bits to unpack
*/
readUInt: function (bits) {
var result = this.peekUInt(bits);
this._advance(bits);
return result;
},
/**
* Write an unsigned integer using the specified number of bits
* @method writeUInt
* @param {Number} value Value to write.
* @param {Number} bits The number of bits to unpack
*/
writeUInt: function (value, bits) {
this._setBits(this._index, bits, value);
this._extend(bits);
},
/**
* Read a signed integer without consuming any bits
* @method peekSInt
* @param {Number} bits The number of bits to peek at
*/
peekSInt: function (bits) {
var result = this._getBits(this._index, bits - 1);
result *= this._getBits(this._index + bits - 1, 1) ? -1 : 1;
return result;
},
/**
* Read a signed integer consuming the specified number of bits
* @method readSInt
* @param {Number} bits The number of bits to unpack
*/
readSInt: function (bits) {
var result = this.peekSInt(bits);
this._advance(bits);
return result;
},
/**
* write a signed integer using the specified number of bits
* @method writeSInt
* @param {Number} value Value to write. Will be truncated or padded
* to the specified number of bits
* @param {Number} bits The number of bits to unpack
*/
writeSInt: function (value, bits) {
this._setBits(this._index, bits - 1, Math.abs(value));
this._extend(bits - 1);
this._setBits(this._index, 1, value < 0);
this._extend(1);
},
/**
* Read a normalized float without consuming any bits
* @method peekFloat
* @param {Number} bits The number of bits to peek at
*/
peekFloat: function (bits) {
// We unpack the signed integer representation divided by the
// maximum value a number with this number of bits can hold. By
// definition the result is in the range [0.0, 1.0]
var result = this._getBits(this._index, bits - 1) / (Math.pow(2, bits - 1) - 1);
// Then the sign bit
result *= this._getBits(this._index + bits - 1, 1) ? -1 : 1;
return result;
},
/**
* Read a float value, consuming the specified number of bits
* @method readFloat
* @param {Number} bits The number of bits to unpack
*/
readFloat: function (bits) {
var result = this.peekFloat(bits);
this._advance(bits);
return result;
},
/**
* Write a normalized float in the range `[0.0, 1.0]` using the specified
* number of bits
* @method writeFloat
* @param {Number} value Value to write.
* @param {Number} bits The number of bits to unpack
*/
writeFloat: function (value, bits) {
var absValue = Math.abs(value);
// The absolute normalized value * the maximum value of this bitlength
this._setBits(this._index, bits - 1, absValue * (Math.pow(2, bits - 1) - 1));
this._extend(bits - 1);
// Then the sign bit
this._setBits(this._index, 1, (value < 0));
this._extend(1);
},
/**
* Read a zero-terminated string value
* @method readString
* @return the String read from the bitstream
*/
readString: function () {
var chars = [];
var c = this.readUInt(8);
while (c !== 0) {
chars.push(c);
c = this.readUInt(8);
}
return String.fromCharCode.apply(false, chars);
},
/**
* Write a zero-terminated string
* number of bits
* @method writeString
* @param {String} value Value to write.
*/
writeString: function (value) {
var i;
for (i = 0; i < value.length; i++) {
this.writeUInt(value.charCodeAt(i), 8);
}
this.writeUInt(0, 8);
},
/**
* Pack an object with a `.serialize()` method into this bitstream
* @method pack
* @param {NetObject} obj The object to serialize
*/
pack: function (obj) {
var description = new InDescription();
description._bitStream = this;
description._target = obj;
this._serializeObject(obj, description);
},
/**
* Unpack an object with a .serialize() method from this bitstream
* @method unpack
* @param {NetObject} obj The object to deserialize to
*/
unpack: function (obj) {
var description = new OutDescription();
description._bitStream = this;
description._target = obj;
this._serializeObject(obj, description);
return description._target;
},
/**
* Calls all serialize methods in this object's prototype chain with
* `description` as its argument. This allows packing and unpacking classes
* which use prototypal inheritance.
* @method _serializeObject
* @param {NetObject} obj The object to serialize
* @param {Description} description The description to serialize through
* @private
*/
_serializeObject: function (obj, description) {
// We'll walk the prototype chain looking for .serialize methods,
// and call them in order from child-most to parent-most
// (arguably backwards, but it's easier to code)
var proto = obj.constructor.prototype;
var serialize = obj.serialize;
while (serialize && (typeof serialize === 'function')) {
serialize.call(obj, description);
proto = Object.getPrototypeOf(proto);
serialize = proto ? proto.serialize : false;
}
},
/**
* See if the contents and byte length of the buffer of this Bitstream
* and `other` are exactly equal
* @method equals
* @param {Bitstream} other The bitstream to compare with
* @return {Boolean} `true` if the bistreams are effectively equal
*/
equals: function (other) {
var i;
if (other.arr.length !== this.arr.length) {
return false;
}
for (i = 0; i < this.arr.length; i++) {
if (this.arr[i] !== other.arr[i]) {
return false;
}
}
return true;
},
/**
* Advance the head by the specified number of bits and check for overread
* @method _advance
* @private
* @param {Number} bits The number of bits to advance the index by
*/
_advance: function (bits) {
this._index += bits;
if (this._index > this._nbits) {
throw new Error('Bitstream overread');
}
},
/**
* Extend the buffer size by the specified number of bits. Also
* advances the index
* @method _extend
* @private
* @param {Number} bits The number of bits to expand the buffer by
*/
_extend: function (bits) {
this._nbits += bits;
this._index += bits;
}
};
/**
* Create a bitstream from a valid UTF-8 string
* @static
* @method fromChars
*/
Bitstream.fromChars = function (str) {
var chars = [];
var i;
var bs;
for (i = 0; i < str.length; i++) {
chars.push(str.charCodeAt(i));
}
bs = new Bitstream(chars);
return bs;
};
module.exports = Bitstream; |