{-# LANGUAGE BangPatterns #-}
-- |
-- Module      : Data.Massiv.Core.Iterator
-- Copyright   : (c) Alexey Kuleshevich 2018-2019
-- License     : BSD3
-- Maintainer  : Alexey Kuleshevich <lehins@yandex.ru>
-- Stability   : experimental
-- Portability : non-portable
--
module Data.Massiv.Core.Iterator
  ( loop
  , loopA_
  , loopM
  , loopM_
  , loopDeepM
  , splitLinearly
  , splitLinearlyWith_
  , splitLinearlyWithM_
  , splitLinearlyWithStartAtM_
  ) where

import Control.Scheduler (Scheduler(..))

-- | Efficient loop with an accumulator
--
-- @since 0.1.0
loop :: Int -> (Int -> Bool) -> (Int -> Int) -> a -> (Int -> a -> a) -> a
loop !init' condition increment !initAcc f = go init' initAcc
  where
    go !step !acc
      | condition step = go (increment step) (f step acc)
      | otherwise = acc
{-# INLINE loop #-}


-- | Efficient monadic loop with an accumulator
--
-- >>> loopM 1 (< 20) (+ 2) [] (\i a -> Just (i:a))
-- Just [19,17,15,13,11,9,7,5,3,1]
--
-- @since 0.1.0
loopM :: Monad m => Int -> (Int -> Bool) -> (Int -> Int) -> a -> (Int -> a -> m a) -> m a
loopM !init' condition increment !initAcc f = go init' initAcc
  where
    go !step !acc
      | condition step = f step acc >>= go (increment step)
      | otherwise = return acc
{-# INLINE loopM #-}


-- | Efficient monadic loop. Result of each iteration is discarded.
--
-- @since 0.1.0
loopM_ :: Monad m => Int -> (Int -> Bool) -> (Int -> Int) -> (Int -> m a) -> m ()
loopM_ !init' condition increment f = go init'
  where
    go !step
      | condition step = f step >> go (increment step)
      | otherwise = pure ()

{-# INLINE loopM_ #-}


-- | Efficient Applicative loop. Result of each iteration is discarded.
--
-- @since 0.3.0
loopA_ :: Applicative f => Int -> (Int -> Bool) -> (Int -> Int) -> (Int -> f a) -> f ()
loopA_ !init' condition increment f = go init'
  where
    go !step
      | condition step = f step *> go (increment step)
      | otherwise = pure ()
{-# INLINE loopA_ #-}


-- | Similar to `loopM`, but slightly less efficient monadic loop with an accumulator that reverses
-- the direction of action application. eg:
--
-- >>> loopDeepM 1 (< 20) (+ 2) [] (\i a -> Just (i:a))
-- Just [1,3,5,7,9,11,13,15,17,19]
--
-- Equivalent to:
--
-- >>> loopM 19 (>= 1) (subtract 2) [] (\i a -> Just (i:a))
-- Just [1,3,5,7,9,11,13,15,17,19]
--
-- @since 0.1.0
loopDeepM :: Monad m => Int -> (Int -> Bool) -> (Int -> Int) -> a -> (Int -> a -> m a) -> m a
loopDeepM !init' condition increment !initAcc f = go init' initAcc
  where
    go !step !acc
      | condition step = go (increment step) acc >>= f step
      | otherwise = return acc
{-# INLINE loopDeepM #-}


-- | Divide length in chunks and apply a function to the computed results
--
-- @since 0.2.1
splitLinearly :: Int -- ^ Number of chunks
              -> Int -- ^ Total length
              -> (Int -> Int -> a) -- ^ Function that accepts a chunk length and slack start index
              -> a
splitLinearly numChunks totalLength action = action chunkLength slackStart
  where
    !chunkLength = totalLength `quot` numChunks
    !slackStart = chunkLength * numChunks
{-# INLINE splitLinearly #-}


-- | Interator that can be used to split computation amongst different workers. For monadic
-- generator see `splitLinearlyWithM_`.
--
-- @since 0.2.1
splitLinearlyWith_ ::
     Monad m => Scheduler m () -> Int -> (Int -> b) -> (Int -> b -> m ()) -> m ()
splitLinearlyWith_ scheduler totalLength index =
  splitLinearlyWithM_ scheduler totalLength (pure . index)
{-# INLINE splitLinearlyWith_ #-}


-- | Interator that can be used to split computation jobs
--
-- @since 0.2.6.0
splitLinearlyWithM_ ::
     Monad m => Scheduler m () -> Int -> (Int -> m b) -> (Int -> b -> m c) -> m ()
splitLinearlyWithM_ scheduler totalLength make write =
  splitLinearly (numWorkers scheduler) totalLength $ \chunkLength slackStart -> do
    loopM_ 0 (< slackStart) (+ chunkLength) $ \ !start ->
      scheduleWork scheduler $ loopM_ start (< (start + chunkLength)) (+ 1) $ \ !k -> make k >>= write k
    scheduleWork scheduler $ loopM_ slackStart (< totalLength) (+ 1) $ \ !k -> make k >>= write k
{-# INLINE splitLinearlyWithM_ #-}


-- | Interator that can be used to split computation jobs
--
-- @since 0.2.6.0
splitLinearlyWithStartAtM_ ::
     Monad m => Scheduler m () -> Int -> Int -> (Int -> m b) -> (Int -> b -> m c) -> m ()
splitLinearlyWithStartAtM_ scheduler startAt totalLength make write =
  splitLinearly (numWorkers scheduler) totalLength $ \chunkLength slackStart -> do
    loopM_ startAt (< (slackStart + startAt)) (+ chunkLength) $ \ !start ->
      scheduleWork scheduler $
      loopM_ start (< (start + chunkLength)) (+ 1) $ \ !k -> make k >>= write k
    scheduleWork scheduler $
      loopM_ (slackStart + startAt) (< (totalLength + startAt)) (+ 1) $ \ !k -> make k >>= write k
{-# INLINE splitLinearlyWithStartAtM_ #-}