groupBy.js

//#   groupBy :: (a -> b) -> Array a -> Array (Array a)
//.
//.   Group the given elements in accordance with the given comparator.
//.
//.   > groupBy (x => x % 3) ([1, 2, 3, 4, 5, 6, 7, 8, 9])
//.   [[1, 4, 7], [2, 5, 8], [3, 6, 9]]
const groupBy = f => xs => {
  const groups = [];
  const ys = [];
  xs.forEach (x => {
    const y = f (x);
    const idx = ys.indexOf (y);
    if (idx < 0) {
      ys.push (y);
      groups.push ([x]);
    } else {
      groups[idx].push (x);
    }
  });
  return groups;
};