module Token (
    Symbol(..), unparse,
    Token(..), unparseTokenS,
    collapse,
  ) where

-- | A symbol in the input stream of a parser. Sizes are used to determine positions within the stream.
class Symbol a where
  -- | Yields the size of this symbol measured in number of characters.
  symbolSize      :: a -> Int
  -- | Yields the size of the whitespace preceding this symbol.
  spaceBeforeSize :: a -> Int
  -- | Yields the size of the whitespace succeeding this token.
  spaceAfterSize  :: a -> Int
  -- | Unparses a list of symbols back to a string.
  unparseS        :: [a] -> ShowS

-- | Variant of 'unparseS' that directly yields a 'String'.
unparse :: Symbol a => [a] -> String
unparse = ($ "") . unparseS

instance Symbol Char where
  symbolSize      = const 1
  spaceBeforeSize = const 0  
  spaceAfterSize  = const 0
  unparseS        = showString

-- | A token is a symbol in an input stream accompanied by whitespace on both sides.
data Token s
  = Token
    { tSpaceBefore  :: [s]  -- ^ Yields the whitespace preceding this token.
    , tImage        :: s    -- ^ Yields the meaningful symbol of this token.
    , tSpaceAfter   :: [s]  -- ^ Yields the whitespace succeeding this token.
    }
  deriving (Eq, Show)

instance (Symbol s) => Symbol (Token s) where
  symbolSize (Token _ im _)       = symbolSize im
  spaceBeforeSize (Token sb _ _)  = sum (map symbolSize sb)
  spaceAfterSize (Token _ _ sa)   = sum (map symbolSize sa)
  unparseS = unparseTokenS

-- | Unparses a list of tokens, handling whitespace in a clever way.
unparseTokenS :: Symbol s => [Token s] -> ShowS
unparseTokenS [] = id
unparseTokenS [Token sb im sa] = unparseS sb . unparseS [im] . unparseS sa
unparseTokenS ((Token sb im _):ts) = unparseS sb . unparseS [im] . unparseS ts

-- | Given a predicate that tells when a symbol is whitespace, converts a list of symbols to a list of tokens.
collapse :: (s -> Bool) -> [s] -> [Token s]
collapse space input = collapse' space xs yzs
  where
    (xs, yzs) = span space input

collapse' :: (s -> Bool) -> [s] -> [s] -> [Token s]
collapse' space spaceBefore input = output where
  output = if null input then [] else token : rest
  token = Token
    { tImage = head input
    , tSpaceBefore = spaceBefore
    , tSpaceAfter = spaceAfter
    }
  (spaceAfter, toProcess) = span space (tail input)
  rest = collapse' space spaceAfter toProcess