module Algo where

import Data.List
import qualified Data.Map as M

-- Group data from a list of (key,value) pairs
-- to a Map of key->[value] lists.
groupFirst :: (Ord k) => [(k, v)] -> M.Map k [v]
groupFirst = M.map reverse . groupFirst'

-- Group data from a list of (key,value) pairs
-- to a Map of key->[value] lists.
-- The values are present in reverse order.
groupFirst' :: (Ord k) => [(k, v)] -> M.Map k [v]
groupFirst' = foldl' add M.empty
  where
    add map_ (key, value) = M.insertWith prepend key [value] map_
    prepend (x:_) xs = x:xs
--    prepend _ _ = impossible