bit-array.js

/**
 * JavaScript BitArray - v0.2.0
 *
 * Licensed under the revised BSD License.
 * Copyright 2010-2012 Bram Stein
 * All rights reserved.
 */

/**
 * Creates a new empty BitArray with the given length or initialises the BitArray with the given hex representation.
 */
var BitArray = function (size, hex) {
 this.values = [];

 if (hex) {
 hex = hex.slice(/^0x/.exec(hex) ? 2 : 0);

 if (hex.length * 4 > this.length) {
 throw 'Hex value is too large for this bit array.'
 } else if (hex.length * 4 < this.length) {
 // pad it
 while(hex.length * 4 < this.length) {
 hex = '0' + hex;
 }
 }

 for (var i = 0; i < (hex.length / 8); i++) {
 var slice = hex.slice(i * 8, i * 8 + 8);
 this.values[i] = parseInt(slice, 16);
 }
 } else {
 for (var i = 0; i < Math.ceil(size / 32); i += 1) {
 this.values[i] = 0;
 }
 }
};

/**
 * Returns the total number of bits in this BitArray.
 */
BitArray.prototype.size = function () {
	return this.values.length * 32;
};

/**
 * Sets the bit at index to a value (boolean.)
 */
BitArray.prototype.set = function (index, value) {
	var i = Math.floor(index / 32);
	// Since "undefined | 1 << index" is equivalent to "0 | 1 << index" we do not need to initialise the array explicitly here.
	if (value) {
		this.values[i] |= 1 << index - i * 32;
	} else {
		this.values[i] &= ~(1 << index - i * 32);
	}
	return this;
};

/**
 * Toggles the bit at index. If the bit is on, it is turned off. Likewise, if the bit is off it is turned on.
 */
BitArray.prototype.toggle = function (index) {
	var i = Math.floor(index / 32);
	this.values[i] ^= 1 << index - i * 32;
	return this;
};

/**
 * Returns the value of the bit at index (boolean.)
 */
BitArray.prototype.get = function (index) {
	var i = Math.floor(index / 32);
	return !!(this.values[i] & (1 << index - i * 32));
};

/**
 * Resets the BitArray so that it is empty and can be re-used.
 */
BitArray.prototype.reset = function () {
	this.values = [];
	return this;
};

/**
 * Returns a copy of this BitArray.
 */
BitArray.prototype.copy = function () {
	var cp = new BitArray();
	cp.length = this.length;
	cp.values = [].concat(this.values);
	return cp;
};

/**
 * Returns true if this BitArray equals another. Two BitArrays are considered
 * equal if both have the same length and bit pattern.
 */
BitArray.prototype.equals = function (x) {
	return this.values.length === x.values.length &&
		this.values.every(function (value, index) {
		return value === x.values[index];
	});
};

/**
 * Returns the JSON representation of this BitArray.
 */
BitArray.prototype.toJSON = function () {
	return JSON.stringify(this.values);
};

/**
 * Returns a string representation of the BitArray with bits
 * in logical order.
 */
BitArray.prototype.toString = function () {
	return this.toArray().map(function (value) {
		return value ? '1' : '0';
	}).join('');
};

/**
 * Returns a string representation of the BitArray with bits
 * in mathemetical order.
 */
BitArray.prototype.toBinaryString = function () {
	return this.toArray().map(function (value) {
		return value ? '1' : '0';
	}).reverse().join('');
};

/**
 * Returns a hexadecimal string representation of the BitArray
 * with bits in logical order.
 */
BitArray.prototype.toHexString = function () {
 var result = [];

 for (var i = 0; i < this.values.length; i += 1) {
 result.push(('00000000' + (this.values[i] >>> 0).toString(16)).slice(-8));
 }
 return result.join('');
};

/**
 * Convert the BitArray to an Array of boolean values.
 */
BitArray.prototype.toArray = function () {
	var result = [];

 for (var i = 0; i < this.values.length * 32; i += 1) {
 result.push(this.get(i));
 }
	return result;
};

/**
 * Returns the total number of bits set to one in this BitArray.
 */
BitArray.prototype.count = function () {
	var total = 0;

	// If we remove the toggle method we could efficiently cache the number of bits without calculating it on the fly.
	this.values.forEach(function (x) {
		// See: http://bits.stephan-brumme.com/countBits.html for an explanation
		x = x - ((x >> 1) & 0x55555555);
		x = (x & 0x33333333) + ((x >> 2) & 0x33333333);
		x = x + (x >> 4);
		x &= 0xF0F0F0F;

		total += (x * 0x01010101) >> 24;
	});
	return total;
};

/**
 * Inverts this BitArray.
 */
BitArray.prototype.not = function () {
	this.values = this.values.map(function (v) {
		return ~v;
	});
	return this;
};

/**
 * Bitwise OR on the values of this BitArray using BitArray x.
 */
BitArray.prototype.or = function (x) {
	if (this.values.length !== x.values.length) {
		throw 'Arguments must be of the same length.';
	}
	this.values = this.values.map(function (v, i) {
		return v | x.values[i];
	});
	return this;
};

/**
 * Bitwise AND on the values of this BitArray using BitArray x.
 */
BitArray.prototype.and = function (x) {
	if (this.values.length !== x.values.length) {
		throw 'Arguments must be of the same length.';
	}
	this.values = this.values.map(function (v, i) {
		return v & x.values[i];
	});
	return this;
};

/**
 * Bitwise XOR on the values of this BitArray using BitArray x.
 */
BitArray.prototype.xor = function (x) {
	if (this.values.length !== x.values.length) {
		throw 'Arguments must be of the same length.';
	}
	this.values = this.values.map(function (v, i) {
		return v ^ x.values[i];
	});
	return this;
};

module.exports = BitArray;