{-# LANGUAGE InstanceSigs #-}
{-# LANGUAGE DerivingStrategies #-}
module Arbitrary.ValidBrainfuckProgram (ValidBrainfuckProgram(..)) where
import Data.Text (Text)
import Test.QuickCheck.Arbitrary (Arbitrary (arbitrary))
import Test.QuickCheck.Gen (Gen, sized, frequency, oneof, resize)
import Data.Coerce (coerce)

import qualified Data.Text as Text

newtype ValidBrainfuckProgram = ValidBrainfuckProgram { get :: Text }
  deriving stock Show

instance Arbitrary ValidBrainfuckProgram where
  arbitrary :: Gen ValidBrainfuckProgram
  arbitrary = coerce . fmap Text.pack $ character []
    where
      character rest = do
        body <- frequency
          [ (1, comment)
          , (9, operator)
          ]

        sized $ \ size -> frequency
          [ (1   , pure body)
          , (size, character body)
          ]

        where
          operator = let

            simpleOperator = oneof $ map (pure . (:rest)) "+-<>,."
            loopOperator = fmap ('[':) . sized $ \ s -> resize (s `div` 8) (character (']':rest))

            in frequency 
              [ (3, simpleOperator)
              , (1, loopOperator)
              ]

          comment = do
            commentChar <- oneof $ map pure "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
            pure $ commentChar : rest

-- >>> import Test.QuickCheck.Gen (generate)
-- >>> generate $ (arbitrary :: Gen ValidBrainfuckProgram)
-- ValidBrainfuckProgram {get = "<-"}