d3.extent = function(array, f) {

  var i = -1,

      n = array.length,

      a,

      b,

      c;

  if (arguments.length === 1) {

    while (++i < n && ((a = c = array[i]) == null || a != a)) a = c = undefined;

    while (++i < n) if ((b = array[i]) != null) {

      if (a > b) a = b;

      if (c < b) c = b;

    }

  } else {

    while (++i < n && ((a = c = f.call(array, array[i], i)) == null || a != a)) a = undefined;

    while (++i < n) if ((b = f.call(array, array[i], i)) != null) {

      if (a > b) a = b;

      if (c < b) c = b;

    }

  }

  return [a, c];

};