/**
 * lodash 4.3.0 (Custom Build) <https://lodash.com/>
 * Build: `lodash modularize exports="npm" -o ./`
 * Copyright jQuery Foundation and other contributors <https://jquery.org/>
 * Released under MIT license <https://lodash.com/license>
 * Based on Underscore.js 1.8.3 <http://underscorejs.org/LICENSE>
 * Copyright Jeremy Ashkenas, DocumentCloud and Investigative Reporters & Editors
 */
var baseIteratee = require('lodash._baseiteratee');

/**
 * The base implementation of `_.sum` and `_.sumBy` without support for
 * iteratee shorthands.
 *
 * @private
 * @param {Array} array The array to iterate over.
 * @param {Function} iteratee The function invoked per iteration.
 * @returns {number} Returns the sum.
 */
function baseSum(array, iteratee) {
  var result,
      index = -1,
      length = array.length;

  while (++index < length) {
    var current = iteratee(array[index]);
    if (current !== undefined) {
      result = result === undefined ? current : (result + current);
    }
  }
  return result;
}

/**
 * This method is like `_.sum` except that it accepts `iteratee` which is
 * invoked for each element in `array` to generate the value to be summed.
 * The iteratee is invoked with one argument: (value).
 *
 * @static
 * @memberOf _
 * @since 4.0.0
 * @category Math
 * @param {Array} array The array to iterate over.
 * @param {Array|Function|Object|string} [iteratee=_.identity]
 *  The iteratee invoked per element.
 * @returns {number} Returns the sum.
 * @example
 *
 * var objects = [{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }];
 *
 * _.sumBy(objects, function(o) { return o.n; });
 * // => 20
 *
 * // The `_.property` iteratee shorthand.
 * _.sumBy(objects, 'n');
 * // => 20
 */
function sumBy(array, iteratee) {
  return (array && array.length)
    ? baseSum(array, baseIteratee(iteratee))
    : 0;
}

module.exports = sumBy;
