combinations.js

function range(start, stop, step) {
    if (typeof stop == 'undefined') {
        stop = start;
        start = 0;
    }
    if (typeof step == 'undefined')
        step = 1;
    if ((step > 0 && start >= stop) || (step < 0 && start <= stop))
        return [];
    var result = [];
    for (var i = start; step > 0 ? i < stop : i > stop; i += step) {
        result.push(i);
    }
    return result;
}


function Combinations(n, r) {
    this.n = n;
    this.r = r;
    this.indices = null;
    this.getNext = function() {
        if (r > n || r <= 0)
            return;
        if (this.indices == null) {
            this.indices = range(r);
            return this.indices;
        } else {
            var i = r - 1;
            for (; i >= 0; i -= 1) {
                if (this.indices[i] != n - r + i)
                    break;
                if (i == 0)
                    return;
            }
            this.indices[i] += 1;
            for (var j = i + 1; j < r; j += 1)
                this.indices[j] = this.indices[j - 1] + 1;
            return this.indices;
        }
    };
}