// Sort list on x axis, h = 10px interaction radius

var b1:Ball = head;
while (b1 != null) {
    var b2 = b1.next;
    while (b2 != null && b1.x+h > b2.x-h) {
        var dy = b2.y-b1.y;
        if (dy > h || dy < -h) {
            b2 = b2.next;
            continue;
        }

        var dx = b2.x-b1.x;
        var d = dx*dx+dy*dy;
        if (d < h*h) {
            // colliding
        }

        b2 = b2.next;
    }
    b1 = b1.next;
}