-- tic-tac-toe.hs
import List

-- place a character in a particular place and return the new board
place char index board =
  (take index board) ++ [char] ++ (drop (index + 1) board)

-- returns winner: X, O, -, C
winner [a, b, c, _, _, _, _, _, _] | (a == b) && (b == c) && (a /= '-') = a
winner [_, _, _, a, b, c, _, _, _] | (a == b) && (b == c) && (a /= '-') = a
winner [_, _, _, _, _, _, a, b, c] | (a == b) && (b == c) && (a /= '-') = a
winner [a, _, _, b, _, _, c, _, _] | (a == b) && (b == c) && (a /= '-') = a
winner [_, a, _, _, b, _, _, c, _] | (a == b) && (b == c) && (a /= '-') = a
winner [_, _, a, _, _, b, _, _, c] | (a == b) && (b == c) && (a /= '-') = a
winner [a, _, _, _, b, _, _, _, c] | (a == b) && (b == c) && (a /= '-') = a
winner [_, _, a, _, b, _, c, _, _] | (a == b) && (b == c) && (a /= '-') = a
winner board =
  if elem '-' board then '-'
  else 'C'

-- transform a numpad key to coordinates
numberToCoords key = case key of
  "7" -> 0; "8" -> 1; "9" -> 2
  "4" -> 3; "5" -> 4; "6" -> 5
  "1" -> 6; "2" -> 7; "3" -> 8
  _ -> error "Invalid Input"

-- possible scores assigned to games
data Score = Great | Ok | Awful | Impossible deriving Eq

-- score the move and return it
scoreMove board player index =
  if (board !! index) /= '-' then Impossible
  else
    -- place the move
    let new_board = place player index board in
    -- we need to know the other player
    let other = if player == 'O' then 'X' else 'O' in
    -- check if we won or tied
    if (winner new_board) == player then Great
    else if (winner new_board) == 'C' then Ok
    else
      -- find best score other player can get now
      let other_score = scoreMove new_board other (bestMove other new_board) in
      if other_score == Great then Awful
      else if other_score == Awful then Great
      else Ok

-- return best move player can get
bestMove player board =
  -- score all 9 moves (uses currying on scoreMove)
  let scores = map (scoreMove board player) [0 .. 8] in
  -- find index of the best move
  case findIndex ((==) Great) scores of
    Just i -> i
    Nothing -> case findIndex ((==) Ok) scores of
      Just i -> i
      Nothing -> case findIndex ((==) Awful) scores of
        Just i -> i
        Nothing -> 0

-- get the AI move
getAi board =
  bestMove 'O' board

-- print the board line by line
printBoard "" = do
  putStrLn "\n"
printBoard (a : b : c : rest) = do
  putStrLn (a : b : [c])
  printBoard rest

-- report the end of the game
endgame w = do
  if w == 'C'
    then putStrLn "Cats Game!"
    else putStrLn (w : " has won!")

-- main game loop
game board = do
  if (winner board) /= '-'
    then endgame (winner board)
    else do
      -- print the board
      printBoard board
      -- get their input
      user <- getLine
      -- convert it to a coord, place the X and get the new board
      let board2 = place 'X' (numberToCoords user) board
      -- check for win again
      if (winner board) /= '-'
        then endgame (winner board)
        else do
          -- place the AI move and get the new board
          let board3 = place 'O' (getAi board2) board2
          -- recurse on the new board
          game board3

-- main function
main = do
  -- print a message and start with the empty board
  putStrLn "You are X, use number pad to play!"
  game "---------"