-- | Common utilities.

module Serokell.Util.Common
       ( enumerate
       , indexModulo
       , indexModuloMay
       , indexedSubList
       , subList
       , allDistinct
       , chunksOf
       ) where

import Universum

import Data.List (genericDrop, genericIndex, genericLength, genericTake, sort)
import GHC.Exts (build)

-- | Enumerate function is analogous to python's enumerate. It
-- takes sequences of values and returns sequence of pairs where the
-- first element is index and the second one is corresponding value.
-- It's roughly equivalent to `zip [0..]`.
-- > enumerate "Hello" = [(0,'H'),(1,'e'),(2,'l'),(3,'l'),(4,'o')]
enumerate
    :: (Num i, Enum i, Traversable t)
    => t a -> t (i, a)
enumerate values = evalState action 0
  where
    action = mapM step values
    step v = do
        i <- get
        modify succ
        return (i, v)

-- | Returns element of a list with given index modulo length of
-- list. Raises error if list is empty.
-- Examples:
-- indexModulo [1, 2, 3] 10 = 2
-- indexModulo [1, 0] 2 = 1
-- indexModulo [] 199 = error
indexModulo :: Integral i => [a] -> i -> a
indexModulo xs =
    fromMaybe (error "Serokell.Util.Common.indexModulo: empty list") .
    indexModuloMay xs

-- | Behaves like `indexModulo` but uses Maybe to report error
-- (i. e. empty list).
indexModuloMay :: Integral i => [a] -> i -> Maybe a
indexModuloMay xs i = genericIndex xs <$> indexModuloIndex xs i

indexModuloIndex :: Integral i => [a] -> i -> Maybe i
indexModuloIndex [] _ = Nothing
indexModuloIndex xs i = Just $ i `mod` genericLength xs

-- | indexedSubList (lo, hi) returns sublist of given list with
-- indices in [max lo 0, hi). If the lower bound is negative,
-- 0 will in its place. If both indices are negative, the empty list
-- is returned.
-- Examples:
-- indexedSubList (2, 3) [0, 5, 10] = [(2, 10)]
-- indexedSubList (0, 2) [0, 5, 10] = [(0, 0), (1, 5)]
-- indexedSubList (0, 0) [0, 1, 11, 111] = []
-- indexedSubList (2000, 1000) [55, 47, 0, 1, 11, 111] = []
-- indexedSubList (-3, 3) [10, 11, 12, 13, 14] = [(0,10),(1,11),(2,12)]
-- indexedSubList (-6, -1) [1,2,3] = []
indexedSubList
    :: Integral i
    => (i, i) -> [a] -> [(i, a)]
indexedSubList (lo, hi)
    | hi <= lo = const []
    | otherwise = zip [max 0 lo .. hi - 1] . genericTake (hi - lo) . genericDrop lo

-- | Like indexedSubList, but not indexed :)
subList :: Integral i => (i, i) -> [a] -> [a]
subList range = map snd . indexedSubList range

-- | Determine whether all elements in a list are distinct.
--
-- >>> allDistinct [1,2,3]
-- True
-- >>> allDistinct [1,3,3]
-- False
--
-- Naturally, all elements in an empty list are distinct:
--
-- >>> allDistinct []
-- True
allDistinct :: Ord a => [a] -> Bool
allDistinct xs = and $ zipWith (/=) sorted (drop 1 sorted)
  where
    sorted = sort xs

-- | @'chunksOf' n@ splits a list into length-n pieces.  The last
--   piece will be shorter if @n@ does not evenly divide the length of
--   the list.  If @n <= 0@, @'chunksOf' n l@ returns an infinite list
--   of empty lists.  For example:
--
-- >>> chunksOf 3 [1..7]
-- [[1,2,3],[4,5,6],[7]]
--
-- >>> chunksOf 3 []
-- []
chunksOf :: Int -> [e] -> [[e]]
chunksOf i ls = map (take i) (build (splitter ls))
  where
    splitter :: [e] -> ([e] -> a -> a) -> a -> a
    splitter [] _ n = n
    splitter l c n  = l `c` splitter (drop i l) c n