{-# LANGUAGE
    UnicodeSyntax
  #-}
-- | FIXME
module Codec.Audio.WavPack.Entropy
    ( EntropyData(..)

    , clearMedian

    , getMedian0
    , getMedian1
    , getMedian2

    , incMedian0
    , decMedian0
    , incMedian1
    , decMedian1
    , incMedian2
    , decMedian2
    )
    where
import Control.Monad.ST
import Control.Monad.Unicode
import Data.Bits
import Data.STRef
import Data.Word
import Prelude.Unicode

-- | FIXME
data EntropyData s
    = EntropyData {
        -- | Median log2 values for a channel.
        edMedian0    ∷ !(STRef s Word32)
      , edMedian1    ∷ !(STRef s Word32)
      , edMedian2    ∷ !(STRef s Word32)
        -- | FIXME
      , edSlowLevel  ∷ !(STRef s Word32)
        -- | FIXME
      , edErrorLimit ∷ !(STRef s Word32)
      }

clearMedian ∷ EntropyData s → ST s ()
clearMedian e
    = writeSTRef (edMedian0 e) 0 ≫
      writeSTRef (edMedian1 e) 0 ≫
      writeSTRef (edMedian2 e) 0

-- | The time constant of the 3 median level breakpoints
div0, div1, div2 ∷ Word32
div0 = 128 --  5/  7 of samples
div1 =  64 -- 10/ 49 of samples
div2 =  32 -- 20/343 of samples

-- | Retrieve the specified median breakpoint (without frac; min = 1)
getMedian0, getMedian1, getMedian2 ∷ EntropyData s → ST s Word32
getMedian0 = fmap ((`shiftR` 4) ∘ (+ 1)) ∘ readSTRef ∘ edMedian0
getMedian1 = fmap ((`shiftR` 4) ∘ (+ 1)) ∘ readSTRef ∘ edMedian1
getMedian2 = fmap ((`shiftR` 4) ∘ (+ 1)) ∘ readSTRef ∘ edMedian2

-- | Update the specified median breakpoints. Note that the median is
-- incremented when the sample is higher than the median, else
-- decremented.  They are designed so that the median will never drop
-- below 1 and the value is essentially stationary if there are 2
-- increments for every 5 decrements.
incMedian0, decMedian0, incMedian1, decMedian1, incMedian2, decMedian2 ∷ EntropyData s → ST s ()
incMedian0 = flip modifySTRef (\x → x + ((x +  div0   ) `div` div0) ⋅ 5) ∘ edMedian0
decMedian0 = flip modifySTRef (\x → x - ((x + (div0-2)) `div` div0) ⋅ 2) ∘ edMedian0
incMedian1 = flip modifySTRef (\x → x + ((x +  div1   ) `div` div1) ⋅ 5) ∘ edMedian1
decMedian1 = flip modifySTRef (\x → x - ((x + (div1-2)) `div` div1) ⋅ 2) ∘ edMedian1
incMedian2 = flip modifySTRef (\x → x + ((x +  div2   ) `div` div2) ⋅ 5) ∘ edMedian2
decMedian2 = flip modifySTRef (\x → x - ((x + (div2-2)) `div` div2) ⋅ 2) ∘ edMedian2