source: node_modules/d3-force/src/link.js@ e4c61dd

import constant from "./constant.js";
import jiggle from "./jiggle.js";

function index(d) {
  return d.index;
}

function find(nodeById, nodeId) {
  var node = nodeById.get(nodeId);
  if (!node) throw new Error("node not found: " + nodeId);
  return node;
}

export default function(links) {
  var id = index,
      strength = defaultStrength,
      strengths,
      distance = constant(30),
      distances,
      nodes,
      count,
      bias,
      random,
      iterations = 1;

  if (links == null) links = [];

  function defaultStrength(link) {
    return 1 / Math.min(count[link.source.index], count[link.target.index]);
  }

  function force(alpha) {
    for (var k = 0, n = links.length; k < iterations; ++k) {
      for (var i = 0, link, source, target, x, y, l, b; i < n; ++i) {
        link = links[i], source = link.source, target = link.target;
        x = target.x + target.vx - source.x - source.vx || jiggle(random);
        y = target.y + target.vy - source.y - source.vy || jiggle(random);
        l = Math.sqrt(x * x + y * y);
        l = (l - distances[i]) / l * alpha * strengths[i];
        x *= l, y *= l;
        target.vx -= x * (b = bias[i]);
        target.vy -= y * b;
        source.vx += x * (b = 1 - b);
        source.vy += y * b;
      }
    }
  }

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

    var i,
        n = nodes.length,
        m = links.length,
        nodeById = new Map(nodes.map((d, i) => [id(d, i, nodes), d])),
        link;

    for (i = 0, count = new Array(n); i < m; ++i) {
      link = links[i], link.index = i;
      if (typeof link.source !== "object") link.source = find(nodeById, link.source);
      if (typeof link.target !== "object") link.target = find(nodeById, link.target);
      count[link.source.index] = (count[link.source.index] || 0) + 1;
      count[link.target.index] = (count[link.target.index] || 0) + 1;
    }

    for (i = 0, bias = new Array(m); i < m; ++i) {
      link = links[i], bias[i] = count[link.source.index] / (count[link.source.index] + count[link.target.index]);
    }

    strengths = new Array(m), initializeStrength();
    distances = new Array(m), initializeDistance();
  }

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

    for (var i = 0, n = links.length; i < n; ++i) {
      strengths[i] = +strength(links[i], i, links);
    }
  }

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

    for (var i = 0, n = links.length; i < n; ++i) {
      distances[i] = +distance(links[i], i, links);
    }
  }

  force.initialize = function(_nodes, _random) {
    nodes = _nodes;
    random = _random;
    initialize();
  };

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

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

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

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

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

  return force;
}