// TODO data(null) for clearing data?

d3_selectionPrototype.data = function(data, join) {

  var enter = [],

      update = [],

      exit = [];

  function bind(group, groupData) {

    var i,

        n = group.length,

        m = groupData.length,

        n0 = Math.min(n, m),

        n1 = Math.max(n, m),

        updateNodes = [],

        enterNodes = [],

        exitNodes = [],

        node,

        nodeData;

    if (join) {

      var nodeByKey = {},

          keys = [],

          key,

          j = groupData.length;

      for (i = -1; ++i < n;) {

        key = join.call(node = group[i], node.__data__, i);

        if (key in nodeByKey) {

          exitNodes[j++] = node; // duplicate key

        } else {

          nodeByKey[key] = node;

        }

        keys.push(key);

      }

      for (i = -1; ++i < m;) {

        node = nodeByKey[key = join.call(groupData, nodeData = groupData[i], i)];

        if (node) {

          node.__data__ = nodeData;

          updateNodes[i] = node;

          enterNodes[i] = exitNodes[i] = null;

        } else {

          enterNodes[i] = d3_selection_dataNode(nodeData);

          updateNodes[i] = exitNodes[i] = null;

        }

        delete nodeByKey[key];

      }

      for (i = -1; ++i < n;) {

        if (keys[i] in nodeByKey) {

          exitNodes[i] = group[i];

        }

      }

    } else {

      for (i = -1; ++i < n0;) {

        node = group[i];

        nodeData = groupData[i];

        if (node) {

          node.__data__ = nodeData;

          updateNodes[i] = node;

          enterNodes[i] = exitNodes[i] = null;

        } else {

          enterNodes[i] = d3_selection_dataNode(nodeData);

          updateNodes[i] = exitNodes[i] = null;

        }

      }

      for (; i < m; ++i) {

        enterNodes[i] = d3_selection_dataNode(groupData[i]);

        updateNodes[i] = exitNodes[i] = null;

      }

      for (; i < n1; ++i) {

        exitNodes[i] = group[i];

        enterNodes[i] = updateNodes[i] = null;

      }

    }

    enterNodes.update

        = updateNodes;

    enterNodes.parentNode

        = updateNodes.parentNode

        = exitNodes.parentNode

        = group.parentNode;

    enter.push(enterNodes);

    update.push(updateNodes);

    exit.push(exitNodes);

  }

  var i = -1,

      n = this.length,

      group;

  if (typeof data === "function") {

    while (++i < n) {

      bind(group = this[i], data.call(group, group.parentNode.__data__, i));

    }

  } else {

    while (++i < n) {

      bind(group = this[i], data);

    }

  }

  var selection = d3_selection(update);

  selection.enter = function() { return d3_selection_enter(enter); };

  selection.exit = function() { return d3_selection(exit); };

  return selection;

};

function d3_selection_dataNode(data) {

  return {__data__: data};

}