var _append = require('./internal/_append');
var _curry2 = require('./internal/_curry2');
var _dispatchable = require('./internal/_dispatchable');
var _reduce = require('./internal/_reduce');
var _xgroupBy = require('./internal/_xgroupBy');


/**
 * Splits a list into sub-lists stored in an object, based on the result of calling a String-returning function
 * on each element, and grouping the results according to values returned.
 *
 * Acts as a transducer if a transformer is given in list position.
 * @see R.transduce
 *
 * @func
 * @memberOf R
 * @category List
 * @sig (a -> String) -> [a] -> {String: [a]}
 * @param {Function} fn Function :: a -> String
 * @param {Array} list The array to group
 * @return {Object} An object with the output of `fn` for keys, mapped to arrays of elements
 *         that produced that key when passed to `fn`.
 * @example
 *
 *      var byGrade = R.groupBy(function(student) {
 *        var score = student.score;
 *        return score < 65 ? 'F' :
 *               score < 70 ? 'D' :
 *               score < 80 ? 'C' :
 *               score < 90 ? 'B' : 'A';
 *      });
 *      var students = [{name: 'Abby', score: 84},
 *                      {name: 'Eddy', score: 58},
 *                      // ...
 *                      {name: 'Jack', score: 69}];
 *      byGrade(students);
 *      // {
 *      //   'A': [{name: 'Dianne', score: 99}],
 *      //   'B': [{name: 'Abby', score: 84}]
 *      //   // ...,
 *      //   'F': [{name: 'Eddy', score: 58}]
 *      // }
 */
module.exports = _curry2(_dispatchable('groupBy', _xgroupBy, function groupBy(fn, list) {
  return _reduce(function(acc, elt) {
    var key = fn(elt);
    acc[key] = _append(elt, acc[key] || (acc[key] = []));
    return acc;
  }, {}, list);
}));