d3.layout.hierarchy = function() {
  var sort = d3_layout_hierarchySort,
      children = d3_layout_hierarchyChildren,
      value = d3_layout_hierarchyValue;

  // Recursively compute the node depth and value.
  // Also converts to a standard hierarchy structure.
  function recurse(node, depth, nodes) {
    var childs = children.call(hierarchy, node, depth);
    node.depth = depth;
    nodes.push(node);
    if (childs && (n = childs.length)) {
      var i = -1,
          n,
          c = node.children = [],
          v = 0,
          j = depth + 1,
          d;
      while (++i < n) {
        d = recurse(childs[i], j, nodes);
        d.parent = node;
        c.push(d);
        v += d.value;
      }
      if (sort) c.sort(sort);
      if (value) node.value = v;
    } else if (value) {
      node.value = +value.call(hierarchy, node, depth) || 0;
    }
    return node;
  }

  // Recursively re-evaluates the node value.
  function revalue(node, depth) {
    var children = node.children,
        v = 0;
    if (children && (n = children.length)) {
      var i = -1,
          n,
          j = depth + 1;
      while (++i < n) v += revalue(children[i], j);
    } else if (value) {
      v = +value.call(hierarchy, node, depth) || 0;
    }
    if (value) node.value = v;
    return v;
  }

  function hierarchy(d) {
    var nodes = [];
    recurse(d, 0, nodes);
    return nodes;
  }

  hierarchy.sort = function(x) {
    if (!arguments.length) return sort;
    sort = x;
    return hierarchy;
  };

  hierarchy.children = function(x) {
    if (!arguments.length) return children;
    children = x;
    return hierarchy;
  };

  hierarchy.value = function(x) {
    if (!arguments.length) return value;
    value = x;
    return hierarchy;
  };

  // Re-evaluates the `value` property for the specified hierarchy.
  hierarchy.revalue = function(root) {
    revalue(root, 0);
    return root;
  };

  return hierarchy;
};

// A method assignment helper for hierarchy subclasses.
function d3_layout_hierarchyRebind(object, hierarchy) {
  d3.rebind(object, hierarchy, "sort", "children", "value");

  // Add an alias for nodes and links, for convenience.
  object.nodes = object;
  object.links = d3_layout_hierarchyLinks;

  return object;
}

function d3_layout_hierarchyChildren(d) {
  return d.children;
}

function d3_layout_hierarchyValue(d) {
  return d.value;
}

function d3_layout_hierarchySort(a, b) {
  return b.value - a.value;
}

// Returns an array source+target objects for the specified nodes.
function d3_layout_hierarchyLinks(nodes) {
  return d3.merge(nodes.map(function(parent) {
    return (parent.children || []).map(function(child) {
      return {source: parent, target: child};
    });
  }));
}
