{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE ImportQualifiedPost #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TemplateHaskell #-}

module PPL.Internal
  ( uniform,
    Prob (..),
    Meas,
    score,
    scoreLog,
    sample,
    samples,
    newTree,
    HashMap,
    Tree (..),
  )
where

import Control.Monad
import Control.Monad.Trans.Class
import Control.Monad.Trans.Writer
import Data.Bifunctor
import Data.Bits
import Data.IORef
import Data.Monoid
import Data.Vector.Hashtables qualified as H
import Data.Vector.Mutable qualified as M
import Data.Vector.Unboxed.Mutable qualified as UM
import Data.Word
import Language.Haskell.TH.Syntax qualified as TH
import Numeric.Log
import System.IO.Unsafe
import System.Random hiding (split, uniform)
import System.Random qualified as R

type HashMap k v = H.Dictionary (H.PrimState IO) M.MVector k UM.MVector v

-- Reimplementation of the LazyPPL monads to avoid some dependencies

data Tree = Tree
  { draw :: !Double,
    split :: (Tree, Tree)
  }

{-# INLINE newTree #-}
newTree :: IORef (HashMap Integer Double, StdGen) -> Tree
newTree s = go 1
  where
    go :: Integer -> Tree
    go id =
      Tree
        ( unsafePerformIO $ do
            (m, g) <- readIORef s
            H.lookup m id >>= \case
              Nothing -> do
                let (x, g') = R.random g
                H.insert m id x
                writeIORef s (m, g')
                pure x
              Just x -> pure x
        )
        (go (2 * id), go (2 * id + 1))

newtype Prob a = Prob {runProb :: Tree -> a}

instance Monad Prob where
  Prob f >>= g = Prob $ \t ->
    let (t1, t2) = split t
        (Prob g') = g (f t1)
     in g' t2

instance Functor Prob where fmap = liftM

instance Applicative Prob where pure = Prob . const; (<*>) = ap

uniform :: Prob Double
uniform = Prob $ \(Tree r _) -> r

newtype Meas a = Meas (WriterT (Product (Log Double)) Prob a)
  deriving (Functor, Applicative, Monad)

{-# INLINE score #-}
score :: Double -> Meas ()
score = scoreLog . Exp . log . max eps
  where
    eps = $(TH.lift (2 * until ((== 1) . (1 +)) (/ 2) (1 :: Double))) -- machine epsilon, force compile time eval

{-# INLINE scoreLog #-}
scoreLog :: Log Double -> Meas ()
scoreLog = Meas . tell . Product

{-# INLINE sample #-}
sample :: Prob a -> Meas a
sample = Meas . lift

{-# INLINE samples #-}
samples :: forall a. Meas a -> Tree -> [(a, Log Double)]
samples (Meas m) = map (second getProduct) . runProb f
  where
    f = runWriterT m >>= \x -> (x :) <$> f