Utils.hs

-- | Utilities for AI players.
module AI.Utils
  ( winOrPreventLoss
  , mapTree
  , prunedepth
  , prunebreadth
  , highfirst
  , lowfirst
  ) where

import Board
import Data.List (sortBy, minimumBy)

-- | Searches BoardTree to a depth of 1 looking for a 
-- | guaranteed win or a preventable loss.
winOrPreventLoss :: Strategy -> Strategy
winOrPreventLoss s (GameTree a branches) = s $ GameTree a branches2
  where
  winning = [ (t, b) | (t, b@(GameTree _ [])) <- branches ]
  losing  = [ t | (t, (GameTree _ branches')) <- branches, 
                           (_, (GameTree _ [])) <- branches' ]
  branches1 = if not $ null winning
    then [head winning]
    else if length branches<100 then [ (t, b) | (t, b) <- branches, notElem t losing ] else branches
  branches2 = if null branches1 then [head branches] else branches1


-- | some auxiliary functions over game trees
-- apply a function to each node
mapTree :: (a->b) -> GameTree a m -> GameTree b m
mapTree f (GameTree x branches) 
    = GameTree (f x) [(m,mapTree f t) | (m,t)<-branches]

-- apply a function to each edge
mapTree' :: (a->b) -> GameTree s a -> GameTree s b
mapTree' f (GameTree x branches) 
    = GameTree x [(f m,mapTree' f t) | (m,t)<-branches]


-- heuristic to order subtrees with highest values first
highfirst, lowfirst  :: 
    (Ord a) => GameTree a m -> GameTree a m
highfirst (GameTree x branches) 
    = GameTree x $ sortBy cmp [(m,lowfirst t) | (m,t)<-branches]
    where cmp (_,GameTree x _) (_,GameTree y _) = compare y x

lowfirst (GameTree x branches) 
    = GameTree x $ sortBy cmp [(m,highfirst t) | (m,t)<-branches]
    where cmp (_,GameTree x _) (_, GameTree y _) = compare x y


-- prune to a fixed depth
prunedepth :: Int -> GameTree a m -> GameTree a m
prunedepth n (GameTree x branches) 
    | n>0       = GameTree x [(m,prunedepth (n-1) t) | (m,t)<-branches]
    | otherwise = GameTree x []

-- prune to a fixed breadth
prunebreadth :: Int -> GameTree a m -> GameTree a m
prunebreadth n (GameTree x branches) 
    = GameTree x [(m, prunebreadth n t) | (m,t)<-take n branches]