-- ---------------------------------------------------------------------------
-- |
-- Module      : Data.Vector.Algorithms.Insertion
-- Copyright   : (c) 2008-2010 Dan Doel
-- Maintainer  : Dan Doel
-- Stability   : Experimental
-- Portability : Portable
--
-- A simple insertion sort. Though it's O(n^2), its iterative nature can be
-- beneficial for small arrays. It is used to sort small segments of an array
-- by some of the more heavy-duty, recursive algorithms.

module Data.Vector.Algorithms.Insertion
( sort
, sortBy
, sortByBounds
, sortByBounds'
, Comparison
) where

import Data.Vector.Generic.Mutable

import Data.Vector.Algorithms.Common (Comparison)

import qualified Data.Vector.Algorithms.Optimal as O

-- | Sorts an entire array using the default comparison for the type
sort :: (PrimMonad m, MVector v e, Ord e) => v (PrimState m) e -> m ()
sort = sortBy compare
{-# INLINABLE sort #-}

-- | Sorts an entire array using a given comparison
sortBy :: (PrimMonad m, MVector v e) => Comparison e -> v (PrimState m) e -> m ()
sortBy cmp a = sortByBounds cmp a 0 (length a)
{-# INLINE sortBy #-}

-- | Sorts the portion of an array delimited by [l,u)
sortByBounds :: (PrimMonad m, MVector v e)
=> Comparison e -> v (PrimState m) e -> Int -> Int -> m ()
sortByBounds cmp a l u
| len < 2   = return ()
| len == 2  = O.sort2ByOffset cmp a l
| len == 3  = O.sort3ByOffset cmp a l
| len == 4  = O.sort4ByOffset cmp a l
| otherwise = O.sort4ByOffset cmp a l >> sortByBounds' cmp a l (l + 4) u
where
len = u - l
{-# INLINE sortByBounds #-}

-- | Sorts the portion of the array delimited by [l,u) under the assumption
-- that [l,m) is already sorted.
sortByBounds' :: (PrimMonad m, MVector v e)
=> Comparison e -> v (PrimState m) e -> Int -> Int -> Int -> m ()
sortByBounds' cmp a l m u = sort m
where
sort i
| i < u     = do v <- unsafeRead a i
insert cmp a l v i
sort (i+1)
| otherwise = return ()
{-# INLINE sortByBounds' #-}

-- Given a sorted array in [l,u), inserts val into its proper position,
-- yielding a sorted [l,u]
insert :: (PrimMonad m, MVector v e)
=> Comparison e -> v (PrimState m) e -> Int -> e -> Int -> m ()
insert cmp a l = loop
where
loop val j
| j <= l    = unsafeWrite a l val
| otherwise = do e <- unsafeRead a (j - 1)
case cmp val e of
LT -> unsafeWrite a j e >> loop val (j - 1)
_  -> unsafeWrite a j val
{-# INLINE insert #-}