#!/usr/bin/runhaskell

import Data.Maybe (fromMaybe)

-- given two points, returns the average of them
average :: (Int, Int) -> (Int, Int) -> (Int, Int)
average (x, y) (x_, y_) = ((x + x_) `div` 2, (y + y_) `div` 2)

-- given a maze and a tuple of position and wall position, returns
-- true if the wall position is not blocked (first position is unused)
notBlocked :: [String] -> ((Int, Int), (Int, Int)) -> Bool
notBlocked maze (_, (x, y)) = ' ' == (maze !! y) !! x

-- given a list, a position, and an element, returns a new list
-- with the new element substituted at the position
-- (it seems such a function should exist in the standard library;
-- I must be missing it)
substitute :: [a] -> Int -> a -> [a]
substitute orig pos el =
  let (before, after) = splitAt pos orig
  in before ++ [el] ++ tail after

-- given a maze and a position, draw a '*' at that position in the maze
draw :: [String] -> (Int, Int) -> [String]
draw maze (x, y) =
  let row = maze !! y
  in substitute maze y $ substitute row x '*'

-- given a maze, a previous position, and a list of tuples of potential
-- new positions and their wall positions, returns the solved maze, or
-- None if it cannot be solved
tryMoves :: [String]
         -> (Int, Int)
         -> [((Int, Int), (Int, Int))]
         -> Maybe [String]
tryMoves _ _ [] = Nothing
tryMoves maze prevPos ((newPos, wallPos):more) =
  case solve_ maze newPos prevPos of
    Nothing -> tryMoves maze prevPos more
    Just maze_ -> Just $ foldl draw maze_ [newPos, wallPos]

-- given a maze, a new position, and a previous position, returns
-- the solved maze, or None if it cannot be solved
-- (assumes goal is upper-left corner of maze)
solve_ :: [String] -> (Int, Int) -> (Int, Int) -> Maybe [String]
solve_ maze (2, 1) _ = Just maze
solve_ maze pos@(x, y) prevPos =
  let newPositions = [(x, y - 2), (x + 4, y), (x, y + 2), (x - 4, y)]
      notPrev pos_ = pos_ /= prevPos
      newPositions_ = filter notPrev newPositions
      wallPositions = map (average pos) newPositions_
      zipped = zip newPositions_ wallPositions
      legalMoves = filter (notBlocked maze) zipped
  in tryMoves maze pos legalMoves

-- given a maze, returns a solved maze, or None if it cannot be solved
-- (starts at lower right corner and goes to upper left corner)
solve :: [String] -> Maybe [String]
solve maze = solve_ (draw maze start) start (-1, -1)
  where
    startx = length (head maze) - 3
    starty = length maze - 2
    start = (startx, starty)

-- takes unsolved maze on standard input, prints solved maze on standard output
main =
  let main_ = unlines . fromMaybe ["can_t solve"] . solve . lines
  in interact main_