define(['../internal/createAggregator'], function(createAggregator) {

  /** Used for native method references. */
  var objectProto = Object.prototype;

  /** Used to check objects for own properties. */
  var hasOwnProperty = objectProto.hasOwnProperty;

  /**
   * Creates an object composed of keys generated from the results of running
   * each element of `collection` through `iteratee`. The corresponding value
   * of each key is the number of times the key was returned by `iteratee`.
   * The `iteratee` is bound to `thisArg` and invoked with three arguments:
   * (value, index|key, collection).
   *
   * If a property name is provided for `iteratee` the created `_.property`
   * style callback returns the property value of the given element.
   *
   * If a value is also provided for `thisArg` the created `_.matchesProperty`
   * style callback returns `true` for elements that have a matching property
   * value, else `false`.
   *
   * If an object is provided for `iteratee` the created `_.matches` style
   * callback returns `true` for elements that have the properties of the given
   * object, else `false`.
   *
   * @static
   * @memberOf _
   * @category Collection
   * @param {Array|Object|string} collection The collection to iterate over.
   * @param {Function|Object|string} [iteratee=_.identity] The function invoked
   *  per iteration.
   * @param {*} [thisArg] The `this` binding of `iteratee`.
   * @returns {Object} Returns the composed aggregate object.
   * @example
   *
   * _.countBy([4.3, 6.1, 6.4], function(n) {
   *   return Math.floor(n);
   * });
   * // => { '4': 1, '6': 2 }
   *
   * _.countBy([4.3, 6.1, 6.4], function(n) {
   *   return this.floor(n);
   * }, Math);
   * // => { '4': 1, '6': 2 }
   *
   * _.countBy(['one', 'two', 'three'], 'length');
   * // => { '3': 2, '5': 1 }
   */
  var countBy = createAggregator(function(result, value, key) {
    hasOwnProperty.call(result, key) ? ++result[key] : (result[key] = 1);
  });

  return countBy;
});