Packing rectangles...

pack.js

/**
 * Return an array of paired factors (arrays) of an integer.
 */
function factor(n) {
    var fact = [[1, n]],
        check = 2,
        root = Math.sqrt(n);
    while (check <= root) {
        if (n % check == 0) {
            fact.push([check, n / check]);
        }
        check++;
    }
    return fact;
}

/**
 * Given a number of rectangles to construct and dimensions
 * to fill, return the [width, height] of the "best fit"
 * rectangles.
 */
function find_best_size(n, width, height) {
    var f = factor(n),
        size = [0, height + width];

    for (var i = 0; i < f.length; i++) {
        // New browsers only, kthx.
        var [x, y] = f[i];
        var w = width / y,
            h = height / x; // Assuming wide screen/rectangles for now.
        // Aim for "most square." This could be replaced with a number of
        // properties to optimize, like getting close to the original aspect
        // ratio or closest to 4:3, etc.
        if (Math.abs(w - h) < Math.abs(size[0] - size[1])) {
            size[0] = ~~w, size[1] = ~~h;
        }
    }
    return size;
}