import constant from "./constant.js";

export default function(radius, x, y) {
  var nodes,
      strength = constant(0.1),
      strengths,
      radiuses;

  if (typeof radius !== "function") radius = constant(+radius);
  if (x == null) x = 0;
  if (y == null) y = 0;

  function force(alpha) {
    for (var i = 0, n = nodes.length; i < n; ++i) {
      var node = nodes[i],
          dx = node.x - x || 1e-6,
          dy = node.y - y || 1e-6,
          r = Math.sqrt(dx * dx + dy * dy),
          k = (radiuses[i] - r) * strengths[i] * alpha / r;
      node.vx += dx * k;
      node.vy += dy * k;
    }
  }

  function initialize() {
    if (!nodes) return;
    var i, n = nodes.length;
    strengths = new Array(n);
    radiuses = new Array(n);
    for (i = 0; i < n; ++i) {
      radiuses[i] = +radius(nodes[i], i, nodes);
      strengths[i] = isNaN(radiuses[i]) ? 0 : +strength(nodes[i], i, nodes);
    }
  }

  force.initialize = function(_) {
    nodes = _, initialize();
  };

  force.strength = function(_) {
    return arguments.length ? (strength = typeof _ === "function" ? _ : constant(+_), initialize(), force) : strength;
  };

  force.radius = function(_) {
    return arguments.length ? (radius = typeof _ === "function" ? _ : constant(+_), initialize(), force) : radius;
  };

  force.x = function(_) {
    return arguments.length ? (x = +_, force) : x;
  };

  force.y = function(_) {
    return arguments.length ? (y = +_, force) : y;
  };

  return force;
}