alias/utils/fnvHash.js

A grunt path hashing task

'use strict';

/**
 * FNV1a 32bit hashing with optional reduction to 16-32 bits
 * FNV is a simple and fast NON-CYPTO hashing algorithms family.
 * @param {String} str a string to digest
 * @param {Integer=20} reduceToXBits a number of bytes the hash will be reduced to. must be between 16 to 32
 * @return {String} a 1-7 characters digest of str in base 36. (the safety to length minimax is max 4 characters for 20 bit reduction)
 */
var fnvHash = function (str, reduceToXBits) {
    var FNV_PRIME = 0x01000193,
        FNV_BASE_OFFSET = 0x811C9DC5,
        i,
        l = str.length,
        hash = FNV_BASE_OFFSET;


    // FNV1a implementation
    for (i = 0; i < l; i++) {
        var charCode = str.charCodeAt(i) & 0xff;
        hash = (hash ^ charCode) * FNV_PRIME;
    }

    return fnvHash.reduceHash(hash, reduceToXBits).toString(36);
};

/**
 * 32 bit hash reduction to 16-32 bits
 * @param {Integer} hash
 * @param {Integer=20} reduceToXBits
 * @return {Integer} a <i>reduceToXBits</i> reduction of <i>hash</i>
 */
fnvHash.reduceHash = function (hash, reduceToXBits) {
    reduceToXBits = reduceToXBits || 20;
    if (reduceToXBits < 16 || reduceToXBits > 32) {
        throw new Error('Invalid reduceToXBits value: must be in the 16-32 range');
    }
    if (reduceToXBits === 32) {
        return hash;
    }

    // xor folding to reduce the hash size to 20bit
    var shift = 32 - reduceToXBits;
    var mask = (1 << shift) - 1;
    var rightBits = hash & mask;
    var leftBits = hash >>> shift;

    return (rightBits ^ leftBits);
};

try {
    module.exports = fnvHash;
} catch (e) {}