module Language.Verilog.Preprocess
  ( uncomment
  , preprocess
  ) where

-- | Remove comments from code.
uncomment :: FilePath -> String -> String
uncomment file a = uncomment a
  where
  uncomment a = case a of
    ""               -> ""
    '/' : '/' : rest -> "  " ++ removeEOL rest
    '/' : '*' : rest -> "  " ++ remove rest
    '"'       : rest -> '"' : ignoreString rest
    a         : rest -> a   : uncomment rest

  removeEOL a = case a of
    ""          -> ""
    '\n' : rest -> '\n' : uncomment rest 
    '\t' : rest -> '\t' : removeEOL rest
    _    : rest -> ' '  : removeEOL rest

  remove a = case a of
    ""               -> error $ "File ended without closing comment (*/): " ++ file
    '"' : rest       -> removeString rest
    '\n' : rest      -> '\n' : remove rest
    '\t' : rest      -> '\t' : remove rest
    '*' : '/' : rest -> "  " ++ uncomment rest
    _ : rest         -> " "  ++ remove rest

  removeString a = case a of
    ""                -> error $ "File ended without closing string: " ++ file
    '"' : rest        -> " "  ++ remove       rest
    '\\' : '"' : rest -> "  " ++ removeString rest
    '\n' : rest       -> '\n' :  removeString rest
    '\t' : rest       -> '\t' :  removeString rest
    _    : rest       -> ' '  :  removeString rest

  ignoreString a = case a of
    ""                -> error $ "File ended without closing string: " ++ file
    '"' : rest        -> '"' : uncomment rest
    '\\' : '"' : rest -> "\\\"" ++ ignoreString rest
    a : rest          -> a : ignoreString rest

-- | A simple `define preprocessor.  
preprocess :: [(String, String)] -> FilePath -> String -> String
preprocess env file content = unlines $ pp True [] env $ lines $ uncomment file content
  where
  pp :: Bool -> [Bool] -> [(String, String)] -> [String] -> [String]
  pp _ _ _ [] = []
  pp on stack env (a : rest) = case words a of
    "`define" : name : value -> "" : pp on stack (if on then (name, ppLine env $ unwords value) : env else env) rest
    "`ifdef"  : name : _     -> "" : pp (on && (elem    name $ fst $ unzip env)) (on : stack) env rest 
    "`ifndef" : name : _     -> "" : pp (on && (notElem name $ fst $ unzip env)) (on : stack) env rest 
    "`else" : _
      | not $ null stack     -> "" : pp (head stack && not on) stack env rest
      | otherwise            -> error $ "`else  without associated `ifdef/`ifndef: " ++ file
    "`endif" : _
      | not $ null stack     -> "" : pp (head stack) (tail stack) env rest
      | otherwise            -> error $ "`endif  without associated `ifdef/`ifndef: " ++ file
    _                        -> (if on then ppLine env a else "") : pp on stack env rest

ppLine :: [(String, String)] -> String -> String
ppLine _ "" = ""
ppLine env ('`' : a) = case lookup name env of
  Just value -> value ++ ppLine env rest
  Nothing    -> error $ "Undefined macro: `" ++ name
  where
  name = takeWhile (flip notElem " \t\n\r") a
  rest = drop (length name) a
ppLine env (a : b) = a : ppLine env b