d3.layout.pie = function() {

  var value = Number,

      sort = d3_layout_pieSortByValue,

      startAngle = 0,

      endAngle = 2 * Math.PI;

  function pie(data, i) {

    // Compute the numeric values for each data element.

    var values = data.map(function(d, i) { return +value.call(pie, d, i); });

    // Compute the start angle.

    var a = +(typeof startAngle === "function"

        ? startAngle.apply(this, arguments)

        : startAngle);

    // Compute the angular scale factor: from value to radians.

    var k = ((typeof endAngle === "function"

        ? endAngle.apply(this, arguments)

        : endAngle) - startAngle)

        / d3.sum(values);

    // Optionally sort the data.

    var index = d3.range(data.length);

    if (sort != null) index.sort(sort === d3_layout_pieSortByValue

        ? function(i, j) { return values[j] - values[i]; }

        : function(i, j) { return sort(data[i], data[j]); });

    // Compute the arcs!

    var arcs = index.map(function(i) {

      return {

        data: data[i],

        value: d = values[i],

        startAngle: a,

        endAngle: a += d * k

      };

    });

    // Return the arcs in the original data's order.

    return data.map(function(d, i) {

      return arcs[index[i]];

    });

  }

  /**

   * Specifies the value function *x*, which returns a nonnegative numeric value

   * for each datum. The default value function is `Number`. The value function

   * is passed two arguments: the current datum and the current index.

   */

  pie.value = function(x) {

    if (!arguments.length) return value;

    value = x;

    return pie;

  };

  /**

   * Specifies a sort comparison operator *x*. The comparator is passed two data

   * elements from the data array, a and b; it returns a negative value if a is

   * less than b, a positive value if a is greater than b, and zero if a equals

   * b.

   */

  pie.sort = function(x) {

    if (!arguments.length) return sort;

    sort = x;

    return pie;

  };

  /**

   * Specifies the overall start angle of the pie chart. Defaults to 0. The

   * start angle can be specified either as a constant or as a function; in the

   * case of a function, it is evaluated once per array (as opposed to per

   * element).

   */

  pie.startAngle = function(x) {

    if (!arguments.length) return startAngle;

    startAngle = x;

    return pie;

  };

  /**

   * Specifies the overall end angle of the pie chart. Defaults to 2π. The

   * end angle can be specified either as a constant or as a function; in the

   * case of a function, it is evaluated once per array (as opposed to per

   * element).

   */

  pie.endAngle = function(x) {

    if (!arguments.length) return endAngle;

    endAngle = x;

    return pie;

  };

  return pie;

};

var d3_layout_pieSortByValue = {};