horse/scripts/crc32.js

(function () {
	'use strict';

	var table = [],
		poly = 0xEDB88320; // reverse polynomial

	// build the table
	function makeTable() {
		var c, n, k;

		for (n = 0; n < 256; n += 1) {
			c = n;
			for (k = 0; k < 8; k += 1) {
				if (c & 1) {
					c = poly ^ (c >>> 1);
				} else {
					c = c >>> 1;
				}
			}
			table[n] = c >>> 0;
		}
	}

	function strToArr(str) {
		// sweet hack to turn string into a 'byte' array
		return Array.prototype.map.call(str, function (c) {
			return c.charCodeAt(0);
		});
	}

	/*
	 * Compute CRC of array directly.
	 *
	 * This is slower for repeated calls, so append mode is not supported.
	 */
	function crcDirect(arr) {
		var crc = -1, // initial contents of LFBSR
			i, j, l, temp;

		for (i = 0, l = arr.length; i < l; i += 1) {
			temp = (crc ^ arr[i]) & 0xff;

			// read 8 bits one at a time
			for (j = 0; j < 8; j += 1) {
				if ((temp & 1) === 1) {
					temp = (temp >>> 1) ^ poly;
				} else {
					temp = (temp >>> 1);
				}
			}
			crc = (crc >>> 8) ^ temp;
		}

		// flip bits
		return crc ^ -1;
	}

	/*
	 * Compute CRC with the help of a pre-calculated table.
	 *
	 * This supports append mode, if the second parameter is set.
	 */
	function crcTable(arr, append) {
		var crc, i, l;

		// if we're in append mode, don't reset crc
		// if arr is null or undefined, reset table and return
		if (typeof crcTable.crc === 'undefined' || !append || !arr) {
			crcTable.crc = 0 ^ -1;

			if (!arr) {
				return;
			}
		}

		// store in temp variable for minor speed gain
		crc = crcTable.crc;

		for (i = 0, l = arr.length; i < l; i += 1) {
			crc = (crc >>> 8) ^ table[(crc ^ arr[i]) & 0xff];
		}

		crcTable.crc = crc;

		return crc ^ -1;
	}

	// build the table
	// this isn't that costly, and most uses will be for table assisted mode
	makeTable();

	module.exports = function (val, direct) {
		var val = (typeof val === 'string') ? strToArr(val) : val,
			ret = direct ? crcDirect(val) : crcTable(val);

		// convert to 2's complement hex
		return (ret >>> 0).toString(16);
	};
	module.exports.direct = crcDirect;
	module.exports.table = crcTable;
}());