var reduceBy = require('./reduceBy');


/**
 * Counts the elements of a list according to how many match each value of a
 * key generated by the supplied function. Returns an object mapping the keys
 * produced by `fn` to the number of occurrences in the list. Note that all
 * keys are coerced to strings because of how JavaScript objects work.
 *
 * Acts as a transducer if a transformer is given in list position.
 *
 * @func
 * @memberOf R
 * @since v0.1.0
 * @category Relation
 * @sig (a -> String) -> [a] -> {*}
 * @param {Function} fn The function used to map values to keys.
 * @param {Array} list The list to count elements from.
 * @return {Object} An object mapping keys to number of occurrences in the list.
 * @example
 *
 *      var numbers = [1.0, 1.1, 1.2, 2.0, 3.0, 2.2];
 *      R.countBy(Math.floor)(numbers);    //=> {'1': 3, '2': 2, '3': 1}
 *
 *      var letters = ['a', 'b', 'A', 'a', 'B', 'c'];
 *      R.countBy(R.toLower)(letters);   //=> {'a': 3, 'b': 2, 'c': 1}
 */
module.exports = reduceBy(function(acc, elem) { return acc + 1; }, 0);