{-# LANGUAGE UndecidableInstances #-}

module Graphics.Glyph.ArrayGenerator where

import Data.Array
import qualified Data.Map as M
import Data.Maybe

data ArrayTransaction ix val b = ArrayBuilderM_ (M.Map ix val) b

instance (Ord ix) => Functor (ArrayTransaction ix a) where
  fmap f bb = bb >>= (return . f)

instance (Ord ix) => Applicative (ArrayTransaction ix a) where
  (<*>) afn aa = do
    fn <- afn
    a <- aa
    return (fn a)

  pure = return

instance (Ord ix) => Monad (ArrayTransaction ix a) where
  return = ArrayBuilderM_ M.empty
  (ArrayBuilderM_ map1 val) >>= f =
    ArrayBuilderM_ (map1 `M.union` map2) val2
    where
      (ArrayBuilderM_ map2 val2) = f val

class HasDefault a where
  theDefault :: a

instance (Num a) => HasDefault a where
  theDefault = 0

instance (HasDefault a, HasDefault b) => HasDefault (a, b) where
  theDefault = (theDefault, theDefault)

instance (HasDefault a, HasDefault b, HasDefault c) => HasDefault (a, b, c) where
  theDefault = (theDefault, theDefault, theDefault)

writeArray :: ix -> a -> ArrayTransaction ix a ()
writeArray index' val = ArrayBuilderM_ (M.singleton index' val) ()

buildArray :: (Ix ix) => (ix, ix) -> e -> ArrayTransaction ix e () -> Array ix e
buildArray bounds' def (ArrayBuilderM_ map' _) =
  listArray bounds' [maybeLookup map' bound | bound <- range bounds']
  where
    maybeLookup map_ key = fromMaybe def (M.lookup key map_)