{-# LANGUAGE BangPatterns, DeriveDataTypeable #-}
module Control.Concurrent.Speculation
    ( 
    -- * Speculative application
      spec
    , spec'
    , specBy
    , specBy'
    , specOn
    , specOn'
    -- * Speculative application with transactional rollback
    , specSTM
    , specSTM'
    , specOnSTM
    , specOnSTM'
    , specBySTM
    , specBySTM'
    ) where

import Control.Concurrent.STM
import Control.Concurrent.Speculation.Internal (evaluated)
import Control.Exception (Exception, throw, fromException)
import Control.Parallel (par)
import Data.Typeable (Typeable)
import Data.Function (on)

-- * Basic speculation

-- | @'spec' g f a@ evaluates @f g@ while forcing @a@, if @g == a@ then @f g@ is returned. Otherwise @f a@ is evaluated.
--
-- Furthermore, if the argument has already been evaluated, we avoid sparking the parallel computation at all.
--
-- If a good guess at the value of @a@ is available, this is one way to induce parallelism in an otherwise sequential task. 
--
-- However, if the guess isn\'t available more cheaply than the actual answer, then this saves no work and if the guess is
-- wrong, you risk evaluating the function twice.
--
-- > spec a f a = f $! a
--
-- The best-case timeline looks like:
--
-- > [---- f g ----]
-- >    [----- a -----]
-- > [-- spec g f a --]
-- 
-- The worst-case timeline looks like:
--
-- > [---- f g ----]
-- >    [----- a -----]
-- >                  [---- f a ----]
-- > [------- spec g f a -----------]
--
-- Compare these to the timeline of @f $! a@:
--
-- > [---- a -----]
-- >              [---- f a ----]

spec :: Eq a => a -> (a -> b) -> a -> b
spec = specBy (==) 
{-# INLINE spec #-}

-- | Unlike 'spec', this version does not check to see if the argument has already been evaluated. This can save
-- a small amount of work when you know the argument will always require computation.

spec' :: Eq a => a -> (a -> b) -> a -> b
spec' = specBy' (==)
{-# INLINE spec' #-}

-- | 'spec' with a user defined comparison function
specBy :: (a -> a -> Bool) -> a -> (a -> b) -> a -> b
specBy cmp g f a
    | evaluated a = f a
    | otherwise = specBy' cmp g f a
{-# INLINE specBy #-}

-- | 'spec'' with a user defined comparison function
specBy' :: (a -> a -> Bool) -> a -> (a -> b) -> a -> b
specBy' cmp guess f a = 
    speculation `par` 
        if cmp guess a
        then speculation
        else f a
    where 
        speculation = f guess
{-# INLINE specBy' #-}

-- | 'spec' comparing by projection onto another type
specOn :: Eq c => (a -> c) -> a -> (a -> b) -> a -> b
specOn = specBy . on (==)
{-# INLINE specOn #-}

-- | 'spec'' comparing by projection onto another type
specOn' :: Eq c => (a -> c) -> a -> (a -> b) -> a -> b
specOn' = specBy' . on (==)
{-# INLINE specOn' #-}

-- * STM-based speculation

-- | @'specSTM' g f a@ evaluates @f g@ while forcing @a@, if @g == a@ then @f g@ is returned. Otherwise the side-effects 
-- of the current STM transaction are rolled back and @f a@ is evaluated.
--   
-- If the argument @a@ is already evaluated, we don\'t bother to perform @f g@ at all.
--
-- If a good guess at the value of @a@ is available, this is one way to induce parallelism in an otherwise sequential task. 
--
-- However, if the guess isn\'t available more cheaply than the actual answer then this saves no work, and if the guess is
-- wrong, you risk evaluating the function twice.
--
-- > specSTM a f a = f $! a
--
-- The best-case timeline looks like:
--
-- > [------ f g ------]
-- >     [------- a -------]
-- > [--- specSTM g f a ---]
--
-- The worst-case timeline looks like:
--
-- > [------ f g ------] 
-- >     [------- a -------]
-- >                       [-- rollback --]
-- >                                      [------ f a ------]     
-- > [------------------ spec g f a ------------------------]
--
-- Compare these to the timeline of @f $! a@:
--
-- > [------- a -------]
-- >                   [------ f a ------]

specSTM :: Eq a => STM a -> (a -> STM b) -> a -> STM b
specSTM = specBySTM (==)
{-# INLINE specSTM #-}

-- | Unlike 'specSTM', 'specSTM'' doesn't check if the argument has already been evaluated.

specSTM' :: Eq a => STM a -> (a -> STM b) -> a -> STM b
specSTM' = specBySTM' (==)
{-# INLINE specSTM' #-}

-- | 'specSTM' using a user defined comparison function
specBySTM :: (a -> a -> Bool) -> STM a -> (a -> STM b) -> a -> STM b
specBySTM cmp g f a 
    | evaluated a = f a 
    | otherwise   = specBySTM' cmp g f a
{-# INLINE specBySTM #-}

-- | 'specSTM'' using a user defined comparison function
specBySTM' :: (a -> a -> Bool) -> STM a -> (a -> STM b) -> a -> STM b
specBySTM' cmp g f a = a `par` 
    let 
      try = do
        g' <- g
        result <- f g'
        if cmp g' a
          then return result
          else throw Speculation
    in 
      try `catchSTM` \e -> case fromException e of
        Just Speculation -> f a -- rerun with alternative input
        _ -> throw e            -- this is a bigger problem
{-# INLINE specBySTM' #-}

-- | 'specBySTM' . 'on' (==)'
specOnSTM :: Eq c => (a -> c) -> STM a -> (a -> STM b) -> a -> STM b
specOnSTM = specBySTM . on (==)
{-# INLINE specOnSTM #-}

-- | 'specBySTM'' . 'on' (==)'
specOnSTM' :: Eq c => (a -> c) -> STM a -> (a -> STM b) -> a -> STM b
specOnSTM' = specBySTM' . on (==)
{-# INLINE specOnSTM' #-}

data Speculation = Speculation deriving (Show,Eq,Typeable)
instance Exception Speculation