{-# LANGUAGE TypeFamilies #-}

module Feldspar.Compiler.Plugins.ConstantFolding where

import Feldspar.Compiler.PluginArchitecture

data ConstantFolding = ConstantFolding

instance Plugin ConstantFolding where
  type ExternalInfo ConstantFolding = ()
  executePlugin ConstantFolding _ procedure =
    fst $ executeTransformationPhase ConstantFolding () procedure

instance TransformationPhase ConstantFolding where
  type From ConstantFolding = ()
  type To ConstantFolding = ()
  type Downwards ConstantFolding = ()
  type Upwards ConstantFolding = ()

  transformFunctionCall ConstantFolding _ _ (InfosFromFunctionCallParts funData _) =
    case roleOfFunctionToCall $ funData of
      InfixOp -> case nameOfFunctionToCall $ funData of
        "+"     -> elimParamIf (isConstIntN 0) True funCall
        "-"     -> elimParamIf (isConstIntN 0) False funCall
        "*"     -> elimParamIf (isConstIntN 1) True funCall
        _       -> FunctionCallExpression funCall
      _       -> FunctionCallExpression funCall

    where
      funCall = FunctionCall (funData) ()

      isConstIntN n (ConstantExpression (IntConstant (IntConstantType i _))) = n == i
      isConstIntN _ _ = False

      elimParamIf pred flippable funCall@(FunctionCall (FunctionCallData InfixOp _ _ (x:xs)) _)
        | pred (head xs)      = x
        | flippable && pred x = head xs
        | otherwise           = FunctionCallExpression funCall
      elimParamIf _ _ funCall = FunctionCallExpression funCall