gistfile1.js

// find the largest value, no more than start, where fn(val) === True
function maximize(start, fn) {
    var val = start;
    var incr = val;
    var best = 0;
    var fudge = 1;

    do {
        incr >>= 1;
        if (fn(val)) {
            best = Math.max(val, best);
            if (val != start) val += incr + 1;
        } else {
            val -= incr + 1;
        }
    } while (incr || fudge--);

    return best;
}