Vervis/src/Data/List/Local.hs

{- This file is part of Vervis.
 -
 - Written in 2016, 2018 by fr33domlover <fr33domlover@riseup.net>.
 -
 - ♡ Copying is an act of love. Please copy, reuse and share.
 -
 - The author(s) have dedicated all copyright and related and neighboring
 - rights to this software to the public domain worldwide. This software is
 - distributed without any warranty.
 -
 - You should have received a copy of the CC0 Public Domain Dedication along
 - with this software. If not, see
 - <http://creativecommons.org/publicdomain/zero/1.0/>.
 -}

module Data.List.Local
    ( -- groupByFst
      groupJusts
    , groupEithers
    )
where

import Prelude

import Data.List.NonEmpty (NonEmpty (..), (<|))

-- | Takes a list of pairs and groups them by consecutive ranges with equal
-- first element. Returns a list of pairs, where each pair corresponds to one
-- such range.
--groupByFst :: Eq a => [(a, b)] -> [(a, [b])]
--groupByFst []          = []
--groupByFst ((x, y):ps) =
--    let (same, rest) = span ((== x) . fst) ps
--    in  (x, y : map snd same) : groupByFst rest

-- | Group together sublists of Just items, and drop the Nothing items.
--
-- >>> groupJusts [Nothing, Nothing, Just 1, Just 4, Nothing, Just 2]
-- [[1, 4], [2]]
groupJusts :: [Maybe a] -> [NonEmpty a]
groupJusts maybes = prepend $ foldr go (Nothing, []) maybes
    where
    prepend (Nothing, l) = l
    prepend (Just x , l) = x : l
    go Nothing  (Nothing, ls) = (Nothing       , ls)
    go Nothing  (Just l , ls) = (Nothing       , l : ls)
    go (Just x) (Nothing, ls) = (Just $ x :| [], ls)
    go (Just x) (Just l , ls) = (Just $ x <| l , ls)

groupEithers :: [Either a b] -> ([b], [(NonEmpty a, NonEmpty b)], [a])
groupEithers = foldr go ([], [], [])
    where
    go (Left x) ([]  , []         , as) = ([], []                     , x : as)
    go (Left x) ([]  , (xs, ys):ps, as) = ([], (x <| xs, ys) : ps     , as)
    go (Left x) (b:bs, ps         , as) = ([], (x :| [], b :| bs) : ps, as)
    go (Right y) (bs, ps, as)           = (y : bs, ps, as)