{-# LANGUAGE TypeOperators, FlexibleContexts, TypeFamilies, GeneralizedNewtypeDeriving, StandaloneDeriving #-}
{-# OPTIONS_GHC -Wall -fno-warn-orphans #-}
----------------------------------------------------------------------
-- |
-- Module      :  Data.LinearMap
-- Copyright   :  (c) Conal Elliott 2008
-- License     :  BSD3
--
-- Maintainer  :  conal@conal.net
-- Stability   :  experimental
--
-- Linear maps
----------------------------------------------------------------------

module Data.LinearMap
( (:-*) , linear, lapply, atBasis, idL, (*.*)
, inLMap, inLMap2, inLMap3
, liftMS, liftMS2, liftMS3
, liftL, liftL2, liftL3
, firstL
)
where

import Control.Applicative (Applicative,liftA2,liftA3)
import Control.Arrow       (first)

import Data.MemoTrie      ((:->:)(..))
import Data.VectorSpace   (VectorSpace(..))
import Data.Basis         (HasBasis(..), linearCombo)

-- Linear maps are almost but not quite a Control.Category.  The type
-- class constraints interfere.  They're almost an Arrow also, but for the
-- constraints and the generality of arr.

-- | An optional additive value
type MSum a = Maybe (Sum a)

jsum :: a -> MSum a
jsum = Just . Sum

type LMap' u v = MSum (Basis u :->: v)

-- | Linear map, represented as an optional memo-trie from basis to
-- values, where 'Nothing' means the zero map (an optimization).
newtype u :-* v = LMap { unLMap :: LMap' u v }

deriving instance (HasTrie (Basis u), AdditiveGroup v) => AdditiveGroup (u :-* v)

instance (HasTrie (Basis u), VectorSpace v) =>
VectorSpace (u :-* v) where
type Scalar (u :-* v) = Scalar v
(*^) s = (inLMap.liftMS.fmap) (s *^)

firstL :: ( HasBasis u, HasBasis u', HasBasis v
, HasTrie (Basis u), HasTrie (Basis v)
, Scalar u ~ Scalar v, Scalar u ~ Scalar u'
) =>
(u :-* u') -> ((u,v) :-* (u',v))
firstL = linear.first.lapply

-- TODO: more efficient firstL

-- liftMS :: (AdditiveGroup a) => (a -> b) -> (MSum a -> MSum b)

-- (s *^) :: v -> v
-- fmap (s *^) :: (Basis u :->: v) -> (Basis u :->: v)
-- (liftMS.fmap) (s *^) :: LMap' u v -> LMap' u v
-- (inLMap.liftMS.fmap) (s *^) :: (u :-* v) -> (u :-* v)

-- Before version 0.7, u :-* v was a type synonym, resulting in a subtle
-- ambiguity: u:-*v == u':-*v' does not imply that u==u', since Basis
-- might map different types to the same basis (e.g., Float & Double).

-- TODO: Try a partial trie instead, excluding (known) zero elements.
-- Then 'lapply' could be much faster for sparse situations.  Make sure to
-- correctly sum them.  It'd be more like Jason Foutz's formulation

-- which uses in @IntMap@.

-- | Function (assumed linear) as linear map.
linear :: (HasBasis u, HasTrie (Basis u)) =>
(u -> v) -> (u :-* v)
linear f = LMap (jsum (trie (f . basisValue)))

atZ :: AdditiveGroup b => (a -> b) -> (MSum a -> b)
atZ f = maybe zeroV (f . getSum)

-- atZ :: AdditiveGroup b => (a -> b) -> (a -> b)
-- atZ = id

inLMap :: (LMap' r s -> LMap' t u) -> ((r :-* s) -> (t :-* u))
inLMap = unLMap ~> LMap

inLMap2 :: (LMap' r s -> LMap' t u -> LMap' v w)
-> ((r :-* s) -> (t :-* u) -> (v :-* w))
inLMap2 = unLMap ~> inLMap

inLMap3 :: (LMap' r s -> LMap' t u -> LMap' v w -> LMap' x y)
-> ((r :-* s) -> (t :-* u) -> (v :-* w) -> (x :-* y))
inLMap3 = unLMap ~> inLMap2

-- | Apply a linear map to a vector.
lapply :: ( VectorSpace v, Scalar u ~ Scalar v
, HasBasis u, HasTrie (Basis u) ) =>
(u :-* v) -> (u -> v)
lapply = atZ lapply' . unLMap

-- | Evaluate a linear map on a basis element.
atBasis :: (AdditiveGroup v, HasTrie (Basis u)) =>
(u :-* v) -> Basis u -> v
LMap m `atBasis` b = atZ (`untrie` b) m

-- | Handy for 'lapply' and '(*.*)'.
lapply' :: ( VectorSpace v, Scalar u ~ Scalar v
, HasBasis u, HasTrie (Basis u) ) =>
(Basis u :->: v) -> (u -> v)
lapply' tr = linearCombo . fmap (first (untrie tr)) . decompose

-- | Identity linear map
idL :: (HasBasis u, HasTrie (Basis u)) =>
u :-* u
idL = linear id

infixr 9 *.*
-- | Compose linear maps
(*.*) :: ( HasBasis u, HasTrie (Basis u)
, HasBasis v, HasTrie (Basis v)
, VectorSpace w
, Scalar v ~ Scalar w ) =>
(v :-* w) -> (u :-* v) -> (u :-* w)

-- Simple definition, but only optimizes out uv == zero

-- vw *.* uv = LMap ((fmap.fmap.fmap) (lapply vw) (unLMap uv))

(*.*) vw = (inLMap.fmap.fmap.fmap) (lapply vw)

-- Eep:
--     (*.*) = inLMap.fmap.fmap.fmap.lapply

-- Instead, use Nothing/zero if /either/ map is zeroV (exploiting linearity
-- when uv == zeroV.)

-- LMap Nothing         *.* _                    = LMap Nothing
-- _                    *.* LMap Nothing         = LMap Nothing
-- LMap (Just (Sum vw)) *.* LMap (Just (Sum uv)) = LMap (Just (Sum (lapply' vw <\$> uv)))

-- (*.*) = liftA2 (\ (LMap (Sum vw)) (LMap (Sum uv)) -> LMap (Sum (lapply' vw <\$> uv)))

-- (*.*) = (liftA2.inSum2.inLMap2) (\ vw uv -> lapply' vw <\$> uv)

-- (*.*) = (liftA2.inSum2.inLMap2) (\ vw -> fmap (lapply' vw))

-- (*.*) = (liftA2.inSum2.inLMap2) (fmap . lapply')

-- It may be helpful that @lapply vw@ is evaluated just once and not
-- once per uv.  'untrie' can strip off all of its trie constructors.

-- Less efficient definition:
--
--   vw `compL` uv = linear (lapply vw . lapply uv)
--
--   i.e., compL = inL2 (.)
--
-- The problem with these definitions is that basis elements get converted
-- to values and then decomposed, followed by recombination of the
-- results.

liftMS :: (AdditiveGroup a) =>
(a -> b)
-> (MSum a -> MSum b)
-- liftMS _ Nothing = Nothing
-- liftMS h ma = Just (Sum (h (z ma)))

liftMS = fmap.fmap

liftMS2 :: (AdditiveGroup a, AdditiveGroup b) =>
(a -> b -> c) ->
(MSum a -> MSum b -> MSum c)
liftMS2 _ Nothing Nothing = Nothing
liftMS2 h ma mb = Just (Sum (h (fromMS ma) (fromMS mb)))

(a -> b -> c -> d) ->
(MSum a -> MSum b -> MSum c -> MSum d)
liftMS3 _ Nothing Nothing Nothing = Nothing
liftMS3 h ma mb mc = Just (Sum (h (fromMS ma) (fromMS mb) (fromMS mc)))

fromMS :: AdditiveGroup u => MSum u -> u
fromMS Nothing        = zeroV
fromMS (Just (Sum u)) = u

-- | Apply a linear function to each element of a linear map.
-- @liftL f l == linear f *.* l@, but works more efficiently.
liftL :: (Functor f, AdditiveGroup (f a)) =>
(a -> b) -> MSum (f a) -> MSum (f b)
liftL = liftMS . fmap

-- | Apply a linear binary function (not to be confused with a bilinear
-- function) to each element of a linear map.
liftL2 :: (Applicative f, AdditiveGroup (f a), AdditiveGroup (f b)) =>
(a -> b -> c)
-> (MSum (f a) -> MSum (f b) -> MSum (f c))
liftL2 = liftMS2 . liftA2

-- | Apply a linear ternary function (not to be confused with a trilinear
-- function) to each element of a linear map.
liftL3 :: ( Applicative f
, AdditiveGroup (f a), AdditiveGroup (f b), AdditiveGroup (f c)) =>
(a -> b -> c -> d)
-> (MSum (f a) -> MSum (f b) -> MSum (f c) -> MSum (f d))
liftL3 = liftMS3 . liftA3

{-

infixr 9 *.*
-- | Compose linear maps
(*.*) :: ( HasBasis u, HasTrie (Basis u)
, HasBasis v, HasTrie (Basis v)
, VectorSpace w
, Scalar v ~ Scalar w ) =>
(v :-* w) -> (u :-* v) -> (u :-* w)

-- Simple definition, but only optimizes out uv == zero
--
-- (*.*) vw = (fmap.fmap) (lapply vw)

-- Instead, use Nothing/zero if /either/ map is zeroV (exploiting linearity
-- when uv == zeroV.)

-- Nothing       *.* _             = Nothing
-- _             *.* Nothing       = Nothing
-- Just (Sum vw) *.* Just (Sum uv) = Just (Sum (lapply' vw <\$> uv))

-- (*.*) = liftA2 (\ (Sum vw) (Sum uv) -> Sum (lapply' vw <\$> uv))

-- (*.*) = (liftA2.inSum2) (\ vw uv -> lapply' vw <\$> uv)
(*.*) = (liftA2.inSum2) (\ vw uv -> lapply' vw <\$> uv)

-- (*.*) = (liftA2.inSum2) (\ vw -> fmap (lapply' vw))

-- (*.*) = (liftA2.inSum2) (fmap . lapply')

-}

-----

(~>) :: (a' -> a) -> (b -> b') -> ((a -> b) -> (a' -> b'))
(f ~> h) g = h . g . f