internal/binaryIndexBy.js

define([], function() {

  /** Native method references. */
  var floor = Math.floor;

  /* Native method references for those with the same name as other `lodash` methods. */
  var nativeMin = Math.min;

  /** Used as references for the maximum length and index of an array. */
  var MAX_ARRAY_LENGTH = Math.pow(2, 32) - 1,
      MAX_ARRAY_INDEX =  MAX_ARRAY_LENGTH - 1;

  /**
   * This function is like `binaryIndex` except that it invokes `iteratee` for
   * `value` and each element of `array` to compute their sort ranking. The
   * iteratee is invoked with one argument; (value).
   *
   * @private
   * @param {Array} array The sorted array to inspect.
   * @param {*} value The value to evaluate.
   * @param {Function} iteratee The function invoked per iteration.
   * @param {boolean} [retHighest] Specify returning the highest, instead
   *  of the lowest, index at which a value should be inserted into `array`.
   * @returns {number} Returns the index at which `value` should be inserted
   *  into `array`.
   */
  function binaryIndexBy(array, value, iteratee, retHighest) {
    value = iteratee(value);

    var low = 0,
        high = array ? array.length : 0,
        valIsNaN = value !== value,
        valIsUndef = typeof value == 'undefined';

    while (low < high) {
      var mid = floor((low + high) / 2),
          computed = iteratee(array[mid]),
          isReflexive = computed === computed;

      if (valIsNaN) {
        var setLow = isReflexive || retHighest;
      } else if (valIsUndef) {
        setLow = isReflexive && (retHighest || typeof computed != 'undefined');
      } else {
        setLow = retHighest ? (computed <= value) : (computed < value);
      }
      if (setLow) {
        low = mid + 1;
      } else {
        high = mid;
      }
    }
    return nativeMin(high, MAX_ARRAY_INDEX);
  }

  return binaryIndexBy;
});