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// A rudimentary force layout using Gauss-Seidel.
d3.layout.force = function() {
  var force = {},
      event = d3.dispatch("start", "tick", "end"),
      size = [1, 1],
      drag,
      alpha,
      friction = .9,
      linkDistance = d3_layout_forceLinkDistance,
      linkStrength = d3_layout_forceLinkStrength,
      charge = -30,
      gravity = .1,
      theta = .8,
      nodes = [],
      links = [],
      distances,
      strengths,
      charges;

  function repulse(node) {
    return function(quad, x1, _, x2) {
      if (quad.point !== node) {
        var dx = quad.cx - node.x,
            dy = quad.cy - node.y,
            dn = 1 / Math.sqrt(dx * dx + dy * dy);

        /* Barnes-Hut criterion. */
        if ((x2 - x1) * dn < theta) {
          var k = quad.charge * dn * dn;
          node.px -= dx * k;
          node.py -= dy * k;
          return true;
        }

        if (quad.point && isFinite(dn)) {
          var k = quad.pointCharge * dn * dn;
          node.px -= dx * k;
          node.py -= dy * k;
        }
      }
      return !quad.charge;
    };
  }

  force.tick = function() {
    // simulated annealing, basically
    if ((alpha *= .99) < .005) {
      event.end({type: "end", alpha: alpha = 0});
      return true;
    }

    var n = nodes.length,
        m = links.length,
        q,
        i, // current index
        o, // current object
        s, // current source
        t, // current target
        l, // current distance
        k, // current force
        x, // x-distance
        y; // y-distance

    // gauss-seidel relaxation for links
    for (i = 0; i < m; ++i) {
      o = links[i];
      s = o.source;
      t = o.target;
      x = t.x - s.x;
      y = t.y - s.y;
      if (l = (x * x + y * y)) {
        l = alpha * strengths[i] * ((l = Math.sqrt(l)) - distances[i]) / l;
        x *= l;
        y *= l;
        t.x -= x * (k = s.weight / (t.weight + s.weight));
        t.y -= y * k;
        s.x += x * (k = 1 - k);
        s.y += y * k;
      }
    }

    // apply gravity forces
    if (k = alpha * gravity) {
      x = size[0] / 2;
      y = size[1] / 2;
      i = -1; if (k) while (++i < n) {
        o = nodes[i];
        o.x += (x - o.x) * k;
        o.y += (y - o.y) * k;
      }
    }

    // compute quadtree center of mass and apply charge forces
    if (charge) {
      d3_layout_forceAccumulate(q = d3.geom.quadtree(nodes), alpha, charges);
      i = -1; while (++i < n) {
        if (!(o = nodes[i]).fixed) {
          q.visit(repulse(o));
        }
      }
    }

    // position verlet integration
    i = -1; while (++i < n) {
      o = nodes[i];
      if (o.fixed) {
        o.x = o.px;
        o.y = o.py;
      } else {
        o.x -= (o.px - (o.px = o.x)) * friction;
        o.y -= (o.py - (o.py = o.y)) * friction;
      }
    }

    event.tick({type: "tick", alpha: alpha});
  };

  force.nodes = function(x) {
    if (!arguments.length) return nodes;
    nodes = x;
    return force;
  };

  force.links = function(x) {
    if (!arguments.length) return links;
    links = x;
    return force;
  };

  force.size = function(x) {
    if (!arguments.length) return size;
    size = x;
    return force;
  };

  force.linkDistance = function(x) {
    if (!arguments.length) return linkDistance;
    linkDistance = d3_functor(x);
    return force;
  };

  // For backwards-compatibility.
  force.distance = force.linkDistance;

  force.linkStrength = function(x) {
    if (!arguments.length) return linkStrength;
    linkStrength = d3_functor(x);
    return force;
  };

  force.friction = function(x) {
    if (!arguments.length) return friction;
    friction = x;
    return force;
  };

  force.charge = function(x) {
    if (!arguments.length) return charge;
    charge = typeof x === "function" ? x : +x;
    return force;
  };

  force.gravity = function(x) {
    if (!arguments.length) return gravity;
    gravity = x;
    return force;
  };

  force.theta = function(x) {
    if (!arguments.length) return theta;
    theta = x;
    return force;
  };

  force.alpha = function(x) {
    if (!arguments.length) return alpha;

    if (alpha) { // if we're already running
      if (x > 0) alpha = x; // we might keep it hot
      else alpha = 0; // or, next tick will dispatch "end"
    } else if (x > 0) { // otherwise, fire it up!
      event.start({type: "start", alpha: alpha = x});
      d3.timer(force.tick);
    }

    return force;
  };

  force.start = function() {
    var i,
        j,
        n = nodes.length,
        m = links.length,
        w = size[0],
        h = size[1],
        neighbors,
        o;

    for (i = 0; i < n; ++i) {
      (o = nodes[i]).index = i;
      o.weight = 0;
    }

    distances = [];
    strengths = [];
    for (i = 0; i < m; ++i) {
      o = links[i];
      if (typeof o.source == "number") o.source = nodes[o.source];
      if (typeof o.target == "number") o.target = nodes[o.target];
      distances[i] = linkDistance.call(this, o, i);
      strengths[i] = linkStrength.call(this, o, i);
      ++o.source.weight;
      ++o.target.weight;
    }

    for (i = 0; i < n; ++i) {
      o = nodes[i];
      if (isNaN(o.x)) o.x = position("x", w);
      if (isNaN(o.y)) o.y = position("y", h);
      if (isNaN(o.px)) o.px = o.x;
      if (isNaN(o.py)) o.py = o.y;
    }

    charges = [];
    if (typeof charge === "function") {
      for (i = 0; i < n; ++i) {
        charges[i] = +charge.call(this, nodes[i], i);
      }
    } else {
      for (i = 0; i < n; ++i) {
        charges[i] = charge;
      }
    }

    // initialize node position based on first neighbor
    function position(dimension, size) {
      var neighbors = neighbor(i),
          j = -1,
          m = neighbors.length,
          x;
      while (++j < m) if (!isNaN(x = neighbors[j][dimension])) return x;
      return Math.random() * size;
    }

    // initialize neighbors lazily
    function neighbor() {
      if (!neighbors) {
        neighbors = [];
        for (j = 0; j < n; ++j) {
          neighbors[j] = [];
        }
        for (j = 0; j < m; ++j) {
          var o = links[j];
          neighbors[o.source.index].push(o.target);
          neighbors[o.target.index].push(o.source);
        }
      }
      return neighbors[i];
    }

    return force.resume();
  };

  force.resume = function() {
    return force.alpha(.1);
  };

  force.stop = function() {
    return force.alpha(0);
  };

  // use `node.call(force.drag)` to make nodes draggable
  force.drag = function() {
    if (!drag) drag = d3.behavior.drag()
        .origin(d3_identity)
        .on("dragstart", d3_layout_forceDragstart)
        .on("drag", dragmove)
        .on("dragend", d3_layout_forceDragend);

    this.on("mouseover.force", d3_layout_forceMouseover)
        .on("mouseout.force", d3_layout_forceMouseout)
        .call(drag);
  };

  function dragmove(d) {
    d.px = d3.event.x, d.py = d3.event.y;
    force.resume(); // restart annealing
  }

  return d3.rebind(force, event, "on");
};

// The fixed property has three bits:
// Bit 1 can be set externally (e.g., d.fixed = true) and show persist.
// Bit 2 stores the dragging state, from mousedown to mouseup.
// Bit 3 stores the hover state, from mouseover to mouseout.
// Dragend is a special case: it also clears the hover state.

function d3_layout_forceDragstart(d) {
  d.fixed |= 2; // set bit 2
}

function d3_layout_forceDragend(d) {
  d.fixed &= 1; // unset bits 2 and 3
}

function d3_layout_forceMouseover(d) {
  d.fixed |= 4; // set bit 3
  d.px = d.x, d.py = d.y; // set velocity to zero
}

function d3_layout_forceMouseout(d) {
  d.fixed &= 3; // unset bit 3
}

function d3_layout_forceAccumulate(quad, alpha, charges) {
  var cx = 0,
      cy = 0;
  quad.charge = 0;
  if (!quad.leaf) {
    var nodes = quad.nodes,
        n = nodes.length,
        i = -1,
        c;
    while (++i < n) {
      c = nodes[i];
      if (c == null) continue;
      d3_layout_forceAccumulate(c, alpha, charges);
      quad.charge += c.charge;
      cx += c.charge * c.cx;
      cy += c.charge * c.cy;
    }
  }
  if (quad.point) {
    // jitter internal nodes that are coincident
    if (!quad.leaf) {
      quad.point.x += Math.random() - .5;
      quad.point.y += Math.random() - .5;
    }
    var k = alpha * charges[quad.point.index];
    quad.charge += quad.pointCharge = k;
    cx += k * quad.point.x;
    cy += k * quad.point.y;
  }
  quad.cx = cx / quad.charge;
  quad.cy = cy / quad.charge;
}

function d3_layout_forceLinkDistance() {
  return 20;
}

function d3_layout_forceLinkStrength() {
  return 1;
}
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