/**
 * Pulls nodes toward a set of cluster center nodes / points.
 * Adapted from Mike Bostock's Clustered Force Layout III:
 * https://bl.ocks.org/mbostock/7881887
 */
function forceCluster(centers) {

    var nodes,
        centerpoints = [],
        strength = 0.1,
        centerInertia = 0.0;

    // coerce centers accessor into a function
    //if (typeof centers !== 'function') centers = () => centers;

    function force (alpha) {
        // scale + curve alpha value
        alpha *= strength * alpha;

        var c, x, y, l, r;
        nodes.forEach(function(d, i) {
            c = centerpoints[i];
        if (!c || c === d) return;

        x = d.x - c.x,
            y = d.y - c.y,
            l = Math.sqrt(x * x + y * y),
            r = d.radius + (c.radius || 0);

        if (l && l != r) {
            l = (l - r) / l * alpha;
            d.x -= x *= l;
            d.y -= y *= l;
            c.x += (1 - centerInertia) * x;
            c.y += (1 - centerInertia) * y;
        }
    });
      
    } 

    function initialize () {
        if (!nodes) return;

        // populate local `centerpoints` using `centers` accessor
        var i, n = nodes.length;
        centerpoints = new Array(n);
        for (i = 0; i < n; i++) centerpoints[i] = centers(nodes[i], i, nodes);
    }

    /**
     * Reinitialize the force with the specified nodes.
     */
    force.initialize = function(_) {
        nodes = _;
        initialize();
    };

    /**
     * An array of objects representing the centerpoint of each cluster,
     * or a function that returns such an array.
     * Each object must have `x` and `y` values, and optionally `radius`.
     */
    force.centers = function(_) {
        // return existing value if no value passed
        if (_ == null) return centers;

        // coerce centers accessor into a function
        centers = typeof _ === 'function' ? _ : function (n, i) {return _[i]};

        // reinitialize
        initialize();

        // allow chaining
        return force;
    };

    /**
     * Strength of attraction to the cluster center node/position.
     */
    force.strength = function(_) {
        return _ == null ? strength : (strength = +_, force);
    };

    /**
     * Inertia of cluster center nodes/positions.
     * Higher values mean the cluster center moves less;
     * lower values mean the cluster center is more easily
     * pulled around by other nodes in the cluster.
     * Typical values range from 0.0 (cluster centers move as much as all other nodes)
     * to 1.0 (cluster centers are not moved at all by the clustering force).
     */
    force.centerInertia = function(_) {
        return _ == null ? centerInertia : (centerInertia = +_, force);
    };

    return force;

}