-- |
-- Module      : Data.Manifold.Riemannian
-- Copyright   : (c) Justus Sagemüller 2015
-- License     : GPL v3
-- 
-- Maintainer  : (@) jsag $ hvl.no
-- Stability   : experimental
-- Portability : portable
-- 
-- Riemannian manifolds are manifolds equipped with a 'Metric' at each point.
-- That means, these manifolds aren't merely topological objects anymore, but
-- have a geometry as well. This gives, in particular, a notion of distance
-- and shortest paths (geodesics) along which you can interpolate.
-- 
-- Keep in mind that the types in this library are
-- generally defined in an abstract-mathematical spirit, which may not always
-- match the intuition if you think about manifolds as embedded in ℝ³.
-- (For instance, the torus inherits its geometry from the decomposition as
-- @'S¹' × 'S¹'@, not from the “doughnut” embedding; the cone over @S¹@ is
-- simply treated as the unit disk, etc..)

{-# LANGUAGE FlexibleInstances          #-}
{-# LANGUAGE UndecidableInstances       #-}
{-# LANGUAGE StandaloneDeriving         #-}
{-# LANGUAGE DeriveGeneric              #-}
{-# LANGUAGE DeriveFunctor              #-}
{-# LANGUAGE DeriveFoldable             #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE TypeFamilies               #-}
{-# LANGUAGE FunctionalDependencies     #-}
{-# LANGUAGE FlexibleContexts           #-}
{-# LANGUAGE GADTs                      #-}
{-# LANGUAGE RankNTypes                 #-}
{-# LANGUAGE TupleSections              #-}
{-# LANGUAGE ParallelListComp           #-}
{-# LANGUAGE UnicodeSyntax              #-}
{-# LANGUAGE ConstraintKinds            #-}
{-# LANGUAGE PatternGuards              #-}
{-# LANGUAGE TypeOperators              #-}
{-# LANGUAGE ScopedTypeVariables        #-}
{-# LANGUAGE LiberalTypeSynonyms        #-}
{-# LANGUAGE CPP                        #-}
{-# LANGUAGE DataKinds                  #-}
{-# LANGUAGE DefaultSignatures          #-}


module Data.Manifold.Riemannian  where


import Data.Maybe
import Data.List.NonEmpty (NonEmpty(..))
import qualified Data.List.NonEmpty as NE
import qualified Data.Vector as Arr

import Data.VectorSpace
import Data.VectorSpace.Free
import Data.AffineSpace
import Math.LinearMap.Category
import Linear (V0(..), V1(..), V2(..), V3(..), V4(..))

import Data.Manifold.Types
import Data.Manifold.Types.Primitive ((^), empty, embed, coEmbed)
import Data.Manifold.Types.Stiefel
import Data.Manifold.PseudoAffine
import Data.Manifold.Atlas (AffineManifold)

import qualified Prelude as Hask hiding(foldl, sum, sequence)
import qualified Control.Applicative as Hask
import qualified Control.Monad       as Hask hiding(forM_, sequence)
import Data.Functor.Identity
import qualified Data.Foldable       as Hask
import qualified Data.Traversable as Hask

import Control.Category.Constrained.Prelude hiding
     ((^), all, elem, sum, forM, Foldable(..), Traversable)
import Control.Arrow.Constrained
import Control.Monad.Constrained hiding (forM)
import Data.Foldable.Constrained


data GeodesicWitness x where
  GeodesicWitness :: Geodesic (Interior x)
       => SemimanifoldWitness x -> GeodesicWitness x

class Semimanifold x => Geodesic x where
  geodesicBetween ::
          x -- ^ Starting point; the interpolation will yield this at -1.
       -> x -- ^ End point, for +1.
            -- 
            --   If the two points are actually connected by a path...
       -> Maybe (D¹ -> x) -- ^ ...then this is the interpolation function. Attention: 
                          --   the type will change to 'Differentiable' in the future.
  geodesicWitness :: GeodesicWitness x
  default geodesicWitness :: Geodesic (Interior x) => GeodesicWitness x
  geodesicWitness = GeodesicWitness semimanifoldWitness
  middleBetween :: x -> x -> Maybe x
  middleBetween p₀ p₁ = ($ D¹ 0) <$> geodesicBetween p₀ p₁


interpolate :: (Geodesic x, IntervalLike i) => x -> x -> Maybe (i -> x)
interpolate a b = (. toClosedInterval) <$> geodesicBetween a b




#define deriveAffineGD(x)                                         \
instance Geodesic x where {                                        \
  geodesicBetween a b = return $ alerp a b . (/2) . (+1) . xParamD¹; \
  middleBetween a b = return $ alerp a b (1/2) \
 }

deriveAffineGD (ℝ)

instance Geodesic (ZeroDim s) where
  geodesicBetween Origin Origin = return $ \_ -> Origin
  middleBetween Origin Origin = return Origin

instance ∀ a b . (Geodesic a, Geodesic b) => Geodesic (a,b) where
  geodesicBetween (a,b) (α,β) = liftA2 (&&&) (geodesicBetween a α) (geodesicBetween b β)
  geodesicWitness = case ( geodesicWitness :: GeodesicWitness a
                         , geodesicWitness :: GeodesicWitness b ) of
     (GeodesicWitness _, GeodesicWitness _) -> GeodesicWitness semimanifoldWitness
  middleBetween (a,b) (α,β) = fzip (middleBetween a α, middleBetween b β)

instance ∀ a b c . (Geodesic a, Geodesic b, Geodesic c) => Geodesic (a,b,c) where
  geodesicBetween (a,b,c) (α,β,γ)
      = liftA3 (\ia ib ic t -> (ia t, ib t, ic t))
           (geodesicBetween a α) (geodesicBetween b β) (geodesicBetween c γ)
  geodesicWitness = case ( geodesicWitness :: GeodesicWitness a
                         , geodesicWitness :: GeodesicWitness b
                         , geodesicWitness :: GeodesicWitness c ) of
     (GeodesicWitness _, GeodesicWitness _, GeodesicWitness _)
         -> GeodesicWitness semimanifoldWitness

-- instance (KnownNat n) => Geodesic (FreeVect n ℝ) where
--   geodesicBetween (FreeVect v) (FreeVect w)
--       = return $ \(D¹ t) -> let μv = (1-t)/2; μw = (t+1)/2
--                             in FreeVect $ Arr.zipWith (\vi wi -> μv*vi + μw*wi) v w

instance ∀ v . ( Geodesic v, FiniteFreeSpace v, FiniteFreeSpace (DualVector v)
               , LinearSpace v, Scalar v ~ ℝ, Geodesic (DualVector v)
               , InnerSpace (DualVector v) )
             => Geodesic (Stiefel1 v) where
  geodesicBetween = gb dualSpaceWitness
   where gb :: DualSpaceWitness v -> Stiefel1 v -> Stiefel1 v -> Maybe (D¹ -> Stiefel1 v)
         gb DualSpaceWitness (Stiefel1 p') (Stiefel1 q')
           = (\f -> \(D¹ t) -> Stiefel1 . f . D¹ $ g * tan (ϑ*t))
            <$> geodesicBetween p q
          where p = normalized p'; q = normalized q'
                l = magnitude $ p^-^q
                ϑ = asin $ l/2
                g = sqrt $ 4/l^2 - 1
  middleBetween = gb dualSpaceWitness
   where gb :: DualSpaceWitness v -> Stiefel1 v -> Stiefel1 v -> Maybe (Stiefel1 v)
         gb DualSpaceWitness  (Stiefel1 p) (Stiefel1 q)
             = Stiefel1 <$> middleBetween (normalized p) (normalized q)


instance Geodesic S⁰ where
  geodesicBetween PositiveHalfSphere PositiveHalfSphere = return $ const PositiveHalfSphere
  geodesicBetween NegativeHalfSphere NegativeHalfSphere = return $ const NegativeHalfSphere
  geodesicBetween _ _ = empty

instance Geodesic S¹ where
  geodesicBetween (S¹Polar φ) (S¹Polar ϕ)
    | abs (φ-ϕ) < pi  = (>>> S¹Polar) <$> geodesicBetween φ ϕ
    | φ > 0           = (>>> S¹Polar . \ψ -> signum ψ*pi - ψ)
                        <$> geodesicBetween (pi-φ) (-ϕ-pi)
    | otherwise       = (>>> S¹Polar . \ψ -> signum ψ*pi - ψ)
                        <$> geodesicBetween (-pi-φ) (pi-ϕ)
  middleBetween (S¹Polar φ) (S¹Polar ϕ)
    | abs (φ-ϕ) < pi  = S¹Polar <$> middleBetween φ ϕ
    | φ > 0           = S¹Polar <$> middleBetween (pi-φ) (-ϕ-pi)
    | otherwise       = S¹Polar <$> middleBetween (-pi-φ) (pi-ϕ)


-- instance Geodesic (Cℝay S⁰) where
--   geodesicBetween p q = (>>> fromℝ) <$> geodesicBetween (toℝ p) (toℝ q)
--    where toℝ (Cℝay h PositiveHalfSphere) = h
--          toℝ (Cℝay h NegativeHalfSphere) = -h
--          fromℝ x | x>0        = Cℝay x PositiveHalfSphere
--                  | otherwise  = Cℝay (-x) NegativeHalfSphere
-- 
-- instance Geodesic (CD¹ S⁰) where
--   geodesicBetween p q = (>>> fromI) <$> geodesicBetween (toI p) (toI q)
--    where toI (CD¹ h PositiveHalfSphere) = h
--          toI (CD¹ h NegativeHalfSphere) = -h
--          fromI x | x>0        = CD¹ x PositiveHalfSphere
--                  | otherwise  = CD¹ (-x) NegativeHalfSphere
-- 
-- instance Geodesic (Cℝay S¹) where
--   geodesicBetween p q = (>>> fromP) <$> geodesicBetween (toP p) (toP q)
--    where fromP = fromInterior
--          toP w = case toInterior w of {Just i -> i}
-- 
-- instance Geodesic (CD¹ S¹) where
--   geodesicBetween p q = (>>> fromI) <$> geodesicBetween (toI p) (toI q)
--    where toI (CD¹ h (S¹ φ)) = (h*cos φ, h*sin φ)
--          fromI (x,y) = CD¹ (sqrt $ x^2+y^2) (S¹ $ atan2 y x)
-- 
-- instance Geodesic (Cℝay S²) where
--   geodesicBetween p q = (>>> fromP) <$> geodesicBetween (toP p) (toP q)
--    where fromP = fromInterior
--          toP w = case toInterior w of {Just i -> i}
-- 
-- instance Geodesic (CD¹ S²) where
--   geodesicBetween p q = (>>> fromI) <$> geodesicBetween (toI p) (toI q :: ℝ³)
--    where toI (CD¹ h sph) = h *^ embed sph
--          fromI v = CD¹ (magnitude v) (coEmbed v)

#define geoVSpCone(c,t)                                               \
instance (c) => Geodesic (Cℝay (t)) where {                            \
  geodesicBetween p q = (>>> fromP) <$> geodesicBetween (toP p) (toP q) \
   where { fromP (x,0) = Cℝay 0 x                                        \
         ; fromP (x,h) = Cℝay h (x^/h)                                    \
         ; toP (Cℝay h w) = ( h*^w, h ) } } ;                              \
instance (c) => Geodesic (CD¹ (t)) where {                                  \
  geodesicBetween p q = (>>> fromP) <$> geodesicBetween (toP p) (toP q)      \
   where { fromP (x,0) = CD¹ 0 x                                              \
         ; fromP (x,h) = CD¹ h (x^/h)                                          \
         ; toP (CD¹ h w) = ( h*^w, h ) } }

-- geoVSpCone ((), ℝ)
-- geoVSpCone ((), ℝ⁰)
-- geoVSpCone ((WithField ℝ HilbertManifold a, WithField ℝ HilbertManifold b
--             , Geodesic (a,b)), (a,b))
-- geoVSpCone (KnownNat n, FreeVect n ℝ)

deriveAffineGD ((V0 ℝ))
deriveAffineGD (ℝ¹)
deriveAffineGD (ℝ²)
deriveAffineGD (ℝ³)
deriveAffineGD (ℝ⁴)

instance (TensorSpace v, Scalar v ~ ℝ, TensorSpace w, Scalar w ~ ℝ)
             => Geodesic (Tensor ℝ v w) where
  geodesicBetween a b = return $ alerp a b . (/2) . (+1) . xParamD¹
instance (LinearSpace v, Scalar v ~ ℝ, TensorSpace w, Scalar w ~ ℝ)
             => Geodesic (LinearMap ℝ v w) where
  geodesicBetween a b = return $ alerp a b . (/2) . (+1) . xParamD¹
instance (TensorSpace v, Scalar v ~ ℝ, TensorSpace w, Scalar w ~ ℝ)
             => Geodesic (LinearFunction ℝ v w) where
  geodesicBetween a b = return $ alerp a b . (/2) . (+1) . xParamD¹


-- | One-dimensional manifolds, whose closure is homeomorpic to the unit interval.
class WithField ℝ PseudoAffine i => IntervalLike i where
  toClosedInterval :: i -> D¹ -- Differentiable ℝ i D¹

instance IntervalLike D¹ where
  toClosedInterval = id
-- instance IntervalLike (CD¹ S⁰) where
--   toClosedInterval (CD¹ h PositiveHalfSphere) = D¹ h
--   toClosedInterval (CD¹ h NegativeHalfSphere) = D¹ (-h)
-- instance IntervalLike (Cℝay S⁰) where
--   toClosedInterval (Cℝay h PositiveHalfSphere) = D¹ $ tanh h
--   toClosedInterval (Cℝay h NegativeHalfSphere) = D¹ $ -tanh h
-- instance IntervalLike (CD¹ ℝ⁰) where
--   toClosedInterval (CD¹ h Origin) = D¹ $ h*2 - 1
-- instance IntervalLike (Cℝay ℝ⁰) where
--   toClosedInterval (Cℝay h Origin) = D¹ $ 1 - 2/(h+1)
instance IntervalLike ℝ where
  toClosedInterval x = D¹ $ tanh x





class Geodesic m => Riemannian m where
  rieMetric :: RieMetric m

instance Riemannian ℝ where
  rieMetric = const euclideanNorm





pointsBarycenter :: Geodesic m => NonEmpty m -> Maybe m
pointsBarycenter (p:|[]) = Just p
pointsBarycenter ps = case ( pointsBarycenter (NE.fromList group₀)
                           , pointsBarycenter (NE.fromList group₁) ) of
            (Just bc₀, Just bc₁)
                | δn == 0      -> middleBetween bc₀ bc₁
                | otherwise    -> ($ D¹ (fromIntegral δn/fromIntegral ntot))
                                    <$> geodesicBetween bc₀ bc₁
            _                  -> Nothing
 where psl = Hask.toList ps
       (group₀, group₁) = splitAt nl psl
       ntot = length psl
       nl = ntot`quot`2
       δn = ntot  - nl*2


type FlatSpace x = (AffineManifold x, Geodesic x, SimpleSpace x)