{-# LANGUAGE CPP #-}
{-# LANGUAGE BangPatterns #-}
{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE UndecidableInstances #-}
{-# OPTIONS_HADDOCK not-home #-}

-----------------------------------------------------------------------------
-- |
-- Copyright   :  (c) Edward Kmett 2010-2015
-- License     :  BSD3
-- Maintainer  :  ekmett@gmail.com
-- Stability   :  experimental
-- Portability :  GHC only
--
-- Unsafe and often partial combinators intended for internal usage.
--
-- Handle with care.
-----------------------------------------------------------------------------

module Numeric.AD.Internal.Forward
  ( Forward(..)
  , primal
  , tangent
  , bundle
  , unbundle
  , apply
  , bind
  , bind'
  , bindWith
  , bindWith'
  , transposeWith
  ) where


import Control.Monad (join)
#if __GLASGOW_HASKELL__ < 710
import Control.Applicative hiding ((<**>))
import Data.Foldable (Foldable, toList)
import Data.Traversable (Traversable, mapAccumL)
#else
import Data.Foldable (toList)
import Data.Traversable (mapAccumL)
#endif
import Data.Data
import Data.Number.Erf
import Numeric.AD.Internal.Combinators
import Numeric.AD.Internal.Identity
import Numeric.AD.Jacobian
import Numeric.AD.Mode

#ifdef HLINT
{-# ANN module "HLint: ignore Reduce duplication" #-}
#endif

-- | 'Forward' mode AD
data Forward a
  = Forward !a a
  | Lift !a
  | Zero
  deriving (Show, Data, Typeable)

-- | Calculate the 'tangent' using forward mode AD.
tangent :: Num a => Forward a -> a
tangent (Forward _ da) = da
tangent _ = 0
{-# INLINE tangent #-}

unbundle :: Num a => Forward a -> (a, a)
unbundle (Forward a da) = (a, da)
unbundle Zero = (0,0)
unbundle (Lift a) = (a, 0)
{-# INLINE unbundle #-}

bundle :: a -> a -> Forward a
bundle = Forward
{-# INLINE bundle #-}

apply :: Num a => (Forward a -> b) -> a -> b
apply f a = f (bundle a 1)
{-# INLINE apply #-}

primal :: Num a => Forward a -> a
primal (Forward a _) = a
primal (Lift a) = a
primal Zero = 0

instance Num a => Mode (Forward a) where
  type Scalar (Forward a) = a

  auto = Lift
  zero = Zero

  isKnownZero Zero = True
  isKnownZero _    = False

  isKnownConstant Forward{} = False
  isKnownConstant _ = True

  a *^ Forward b db = Forward (a * b) (a * db)
  a *^ Lift b = Lift (a * b)
  _ *^ Zero = Zero

  Forward a da ^* b = Forward (a * b) (da * b)
  Lift a ^* b = Lift (a * b)
  Zero ^* _ = Zero

  Forward a da ^/ b = Forward (a / b) (da / b)
  Lift a ^/ b = Lift (a / b)
  Zero ^/ _ = Zero

(<+>) :: Num a => Forward a -> Forward a -> Forward a
Zero         <+> a            = a
a            <+> Zero         = a
Forward a da <+> Forward b db = Forward (a + b) (da + db)
Forward a da <+> Lift b       = Forward (a + b) da
Lift a       <+> Forward b db = Forward (a + b) db
Lift a       <+> Lift b       = Lift (a + b)

(<**>) :: Floating a => Forward a -> Forward a -> Forward a
Zero <**> y      = auto (0 ** primal y)
_    <**> Zero   = auto 1
x    <**> Lift y = lift1 (**y) (\z -> y *^ z ** Id (y - 1)) x
x    <**> y      = lift2_ (**) (\z xi yi -> (yi * z / xi, z * log xi)) x y

instance Num a => Jacobian (Forward a) where
  type D (Forward a) = Id a

  unary f (Id dadb) (Forward b db) = Forward (f b) (dadb * db)
  unary f _         (Lift b)       = Lift (f b)
  unary f _         Zero           = Lift (f 0)

  lift1 f _ Zero            = Lift (f 0)
  lift1 f _  (Lift b)       = Lift (f b)
  lift1 f df (Forward b db) = Forward (f b) (dadb * db) where
    Id dadb = df (Id b)

  lift1_ f _  Zero           = Lift (f 0)
  lift1_ f _  (Lift b)       = Lift (f b)
  lift1_ f df (Forward b db) = Forward a da where
    a = f b
    Id da = df (Id a) (Id b) ^* db

  binary f _         _         Zero           Zero           = Lift (f 0 0)
  binary f _         _         Zero           (Lift c)       = Lift (f 0 c)
  binary f _         _         (Lift b)       Zero           = Lift (f b 0)
  binary f _         _         (Lift b)       (Lift c)       = Lift (f b c)
  binary f _         (Id dadc) Zero           (Forward c dc) = Forward (f 0 c) $ dc * dadc
  binary f _         (Id dadc) (Lift b)       (Forward c dc) = Forward (f b c) $ dc * dadc
  binary f (Id dadb) _         (Forward b db) Zero           = Forward (f b 0) $ dadb * db
  binary f (Id dadb) _         (Forward b db) (Lift c)       = Forward (f b c) $ dadb * db
  binary f (Id dadb) (Id dadc) (Forward b db) (Forward c dc) = Forward (f b c) $ dadb * db + dc * dadc

  lift2 f _  Zero           Zero           = Lift (f 0 0)
  lift2 f _  Zero           (Lift c)       = Lift (f 0 c)
  lift2 f _  (Lift b)       Zero           = Lift (f b 0)
  lift2 f _  (Lift b)       (Lift c)       = Lift (f b c)
  lift2 f df Zero           (Forward c dc) = Forward (f 0 c) $ dc * runId (snd (df (Id 0) (Id c)))
  lift2 f df (Lift b)       (Forward c dc) = Forward (f b c) $ dc * runId (snd (df (Id b) (Id c)))
  lift2 f df (Forward b db) Zero           = Forward (f b 0) $ runId (fst (df (Id b) (Id 0))) * db
  lift2 f df (Forward b db) (Lift c)       = Forward (f b c) $ runId (fst (df (Id b) (Id c))) * db
  lift2 f df (Forward b db) (Forward c dc) = Forward a da where
    a = f b c
    (Id dadb, Id dadc) = df (Id b) (Id c)
    da = dadb * db + dc * dadc

  lift2_ f _  Zero           Zero           = Lift (f 0 0)
  lift2_ f _  Zero           (Lift c)       = Lift (f 0 c)
  lift2_ f _  (Lift b)       Zero           = Lift (f b 0)
  lift2_ f _  (Lift b)       (Lift c)       = Lift (f b c)
  lift2_ f df Zero           (Forward c dc) = Forward a $ dc * runId (snd (df (Id a) (Id 0) (Id c))) where a = f 0 c
  lift2_ f df (Lift b)       (Forward c dc) = Forward a $ dc * runId (snd (df (Id a) (Id b) (Id c))) where a = f b c
  lift2_ f df (Forward b db) Zero           = Forward a $ runId (fst (df (Id a) (Id b) (Id 0))) * db where a = f b 0
  lift2_ f df (Forward b db) (Lift c)       = Forward a $ runId (fst (df (Id a) (Id b) (Id c))) * db where a = f b c
  lift2_ f df (Forward b db) (Forward c dc) = Forward a da where
    a = f b c
    (Id dadb, Id dadc) = df (Id a) (Id b) (Id c)
    da = dadb * db + dc * dadc

#define HEAD Forward a
#include "instances.h"

bind :: (Traversable f, Num a) => (f (Forward a) -> b) -> f a -> f b
bind f as = snd $ mapAccumL outer (0 :: Int) as where
  outer !i _ = (i + 1, f $ snd $ mapAccumL (inner i) 0 as)
  inner !i !j a = (j + 1, if i == j then bundle a 1 else auto a)

bind' :: (Traversable f, Num a) => (f (Forward a) -> b) -> f a -> (b, f b)
bind' f as = dropIx $ mapAccumL outer (0 :: Int, b0) as where
  outer (!i, _) _ = let b = f $ snd $ mapAccumL (inner i) (0 :: Int) as in ((i + 1, b), b)
  inner !i !j a = (j + 1, if i == j then bundle a 1 else auto a)
  b0 = f (auto <$> as)
  dropIx ((_,b),bs) = (b,bs)

bindWith :: (Traversable f, Num a) => (a -> b -> c) -> (f (Forward a) -> b) -> f a -> f c
bindWith g f as = snd $ mapAccumL outer (0 :: Int) as where
  outer !i a = (i + 1, g a $ f $ snd $ mapAccumL (inner i) 0 as)
  inner !i !j a = (j + 1, if i == j then bundle a 1 else auto a)

bindWith' :: (Traversable f, Num a) => (a -> b -> c) -> (f (Forward a) -> b) -> f a -> (b, f c)
bindWith' g f as = dropIx $ mapAccumL outer (0 :: Int, b0) as where
  outer (!i, _) a = let b = f $ snd $ mapAccumL (inner i) (0 :: Int) as in ((i + 1, b), g a b)
  inner !i !j a = (j + 1, if i == j then bundle a 1 else auto a)
  b0 = f (auto <$> as)
  dropIx ((_,b),bs) = (b,bs)

-- we can't transpose arbitrary traversables, since we can't construct one out of whole cloth, and the outer
-- traversable could be empty. So instead we use one as a 'skeleton'
transposeWith :: (Functor f, Foldable f, Traversable g) => (b -> f a -> c) -> f (g a) -> g b -> g c
transposeWith f as = snd . mapAccumL go xss0 where
  go xss b = (tail <$> xss, f b (head <$> xss))
  xss0 = toList <$> as

mul :: (Num a) => Forward a -> Forward a -> Forward a
mul = lift2 (*) (\x y -> (y, x))