function TreeBuilder(element_factory) {
  this._data = [];
  this._elem = [];
  this._last = null;
  this._tail = null;
  if (!element_factory) {
    /* evil circular dep */
    element_factory = require('./elementtree').Element;
  }
  this._factory = element_factory;
}

TreeBuilder.prototype.close = function() {
  return this._last;
};

TreeBuilder.prototype._flush = function() {
  if (this._data) {
    if (this._last !== null) {
      var text = this._data.join("");
      if (this._tail) {
        this._last.tail = text;
      }
      else {
        this._last.text = text;
      }
    }
    this._data = [];
  }
};

TreeBuilder.prototype.data = function(data) {
  this._data.push(data);
};

TreeBuilder.prototype.start = function(tag, attrs) {
  this._flush();
  var elem = this._factory(tag, attrs);
  this._last = elem;

  if (this._elem.length) {
    this._elem[this._elem.length - 1].append(elem);
  }

  this._elem.push(elem);

  this._tail = null;
};

TreeBuilder.prototype.end = function(tag) {
  this._flush();
  this._last = this._elem.pop();
  if (this._last.tag !== tag) {
    throw new Error("end tag mismatch");
  }
  this._tail = 1;
  return this._last;
};

exports.TreeBuilder = TreeBuilder;