function defaultSeparation(a, b) {
  return a.parent === b.parent ? 1 : 2;
}

function meanX(children) {
  return children.reduce(meanXReduce, 0) / children.length;
}

function meanXReduce(x, c) {
  return x + c.x;
}

function maxY(children) {
  return 1 + children.reduce(maxYReduce, 0);
}

function maxYReduce(y, c) {
  return Math.max(y, c.y);
}

function leafLeft(node) {
  var children;
  while (children = node.children) node = children[0];
  return node;
}

function leafRight(node) {
  var children;
  while (children = node.children) node = children[children.length - 1];
  return node;
}

export default function() {
  var separation = defaultSeparation,
      dx = 1,
      dy = 1,
      nodeSize = false;

  function cluster(root) {
    var previousNode,
        x = 0;

    // First walk, computing the initial x & y values.
    root.eachAfter(function(node) {
      var children = node.children;
      if (children) {
        node.x = meanX(children);
        node.y = maxY(children);
      } else {
        node.x = previousNode ? x += separation(node, previousNode) : 0;
        node.y = 0;
        previousNode = node;
      }
    });

    var left = leafLeft(root),
        right = leafRight(root),
        x0 = left.x - separation(left, right) / 2,
        x1 = right.x + separation(right, left) / 2;

    // Second walk, normalizing x & y to the desired size.
    return root.eachAfter(nodeSize ? function(node) {
      node.x = (node.x - root.x) * dx;
      node.y = (root.y - node.y) * dy;
    } : function(node) {
      node.x = (node.x - x0) / (x1 - x0) * dx;
      node.y = (1 - (root.y ? node.y / root.y : 1)) * dy;
    });
  }

  cluster.separation = function(x) {
    return arguments.length ? (separation = x, cluster) : separation;
  };

  cluster.size = function(x) {
    return arguments.length ? (nodeSize = false, dx = +x[0], dy = +x[1], cluster) : (nodeSize ? null : [dx, dy]);
  };

  cluster.nodeSize = function(x) {
    return arguments.length ? (nodeSize = true, dx = +x[0], dy = +x[1], cluster) : (nodeSize ? [dx, dy] : null);
  };

  return cluster;
}