randint.js

/**
 * Unfortunately ECMAScript is not precise enough with its definition of
 * Math.random(), so this parameter is implementation dependent. It should
 * definitely be a power of two otherwise I'm pretty sure the algorithm below
 * breaks while calculating the limit.
 */
var MAX_RANDOM = Math.pow(2, 30);

/**
 * CAVEAT: I knocked this together in a couple minutes. It could use some
 * review.
 *
 * Returns a pseudo-random, uniformly distributed integer between 0 (inclusive)
 * and the specified value (exclusive).
 */
Math.randomInt = function Math_randomInt(n) {
    if (n <= 0 || n > MAX_RANDOM) {
        throw new RangeError('Parameter must be between 0 and ' + MAX_RANDOM);
    }

    var limit = (MAX_RANDOM - (MAX_RANDOM % n)) / MAX_RANDOM;
    var rand = Math.random();
    while (rand >= limit) {
        rand = Math.random();
    }

    return Math.floor(rand * n);
}