{- |

Module : $Header$

Description : folding function for propositional formulas

Copyright : (c) Christian Maeder, DFKI GmbH 2009

License : similar to LGPL, see HetCATS/LICENSE.txt or LIZENZ.txt

Maintainer : Christian.Maeder@dfki.de

Stability : provisional

Portability : portable

folding and simplification of propositional formulas

-}

module Propositional.Fold where

import Common.Id

import Common.Utils (combine)

import Propositional.AS_BASIC_Propositional

import qualified Data.Set as Set

data FoldRecord a = FoldRecord

{ foldNegation :: a -> Range -> a

, foldConjunction :: [a] -> Range -> a

, foldDisjunction :: [a] -> Range -> a

, foldImplication :: a -> a -> Range -> a

, foldEquivalence :: a -> a -> Range -> a

, foldTrue_atom :: Range -> a

, foldFalse_atom :: Range -> a

, foldPredication :: Token -> a }

foldFormula :: FoldRecord a -> FORMULA -> a

foldFormula r frm = case frm of

Negation f n -> foldNegation r (foldFormula r f) n

Conjunction xs n -> foldConjunction r (map (foldFormula r) xs) n

Disjunction xs n -> foldDisjunction r (map (foldFormula r) xs) n

Implication x y n -> foldImplication r (foldFormula r x) (foldFormula r y) n

Equivalence x y n -> foldEquivalence r (foldFormula r x) (foldFormula r y) n

True_atom n -> foldTrue_atom r n

False_atom n -> foldFalse_atom r n

Predication x -> foldPredication r x

mapRecord :: FoldRecord FORMULA

mapRecord = FoldRecord

{ foldNegation = Negation

, foldConjunction = Conjunction

, foldDisjunction = Disjunction

, foldImplication = Implication

, foldEquivalence = Equivalence

, foldTrue_atom = True_atom

, foldFalse_atom = False_atom

, foldPredication = Predication }

isNeg :: FORMULA -> Bool

isNeg f = case f of

Negation _ _ -> True

_ -> False

getLits :: Set.Set FORMULA -> Set.Set FORMULA

getLits = Set.fold (\ f -> case f of

Negation x _ -> Set.insert x

_ -> Set.insert f) Set.empty

bothLits :: Set.Set FORMULA -> Bool

bothLits fs = let

(ns, ps) = Set.partition isNeg fs

in not $ Set.null $ Set.intersection (getLits ns) (getLits ps)

getConj :: FORMULA -> [FORMULA]

getConj f = case f of

Conjunction xs _ -> xs

_ -> [f]

getDisj :: FORMULA -> [FORMULA]

getDisj f = case f of

Disjunction xs _ -> xs

_ -> [f]

flatConj :: [FORMULA] -> Set.Set FORMULA

flatConj = Set.fromList

. concatMap (\ f -> case f of

True_atom _ -> []

Conjunction fs _ -> fs

_ -> [f])

mkConj :: [FORMULA] -> Range -> FORMULA

mkConj fs n = let s = flatConj fs in

if Set.member (False_atom nullRange) s || bothLits s then False_atom n else

case Set.toList s of

[] -> True_atom n

[x] -> x

ls -> Conjunction (map (flip mkDisj n . Set.toList)

$ foldr (\ l ll ->

if any (flip Set.isSubsetOf l) ll then ll else

l : filter (not . Set.isSubsetOf l) ll) []

$ map (Set.fromList . getDisj) ls) n

flatDisj :: [FORMULA] -> Set.Set FORMULA

flatDisj = Set.fromList

. concatMap (\ f -> case f of

False_atom _ -> []

Disjunction fs _ -> fs

_ -> [f])

mkDisj :: [FORMULA] -> Range -> FORMULA

mkDisj fs n = let s = flatDisj fs in

if Set.member (True_atom nullRange) s || bothLits s then True_atom n else

case Set.toList s of

[] -> False_atom n

[x] -> x

ls -> Disjunction (map (flip mkConj n . Set.toList)

$ foldr (\ l ll ->

if any (flip Set.isSubsetOf l) ll then ll else

l : filter (not . Set.isSubsetOf l) ll) []

$ map (Set.fromList . getConj) ls) n

simplify :: FORMULA -> FORMULA

simplify = foldFormula mapRecord

{ foldNegation = \ f n -> case f of

True_atom p -> False_atom p

False_atom p -> True_atom p

Negation g _ -> g

s -> Negation s n

, foldConjunction = mkConj

, foldDisjunction = mkDisj

, foldImplication = \ x y n -> case x of

False_atom p -> True_atom p

_ -> if x == y then True_atom n else case x of

True_atom _ -> y

False_atom _ -> True_atom n

Negation z _ | z == y -> x

_ -> case y of

Negation z _ | z == x -> x

_ -> Implication x y n

, foldEquivalence = \ x y n -> case compare x y of

LT -> case y of

Negation z _ | x == z -> False_atom n

_ -> Equivalence x y n

EQ -> True_atom n

GT -> case x of

Negation z _ | z == y -> False_atom n

_ -> Equivalence y x n }

elimEquiv :: FORMULA -> FORMULA

elimEquiv = foldFormula mapRecord

{ foldEquivalence = \ x y n ->

Conjunction [Implication x y n, Implication y x n] n }

elimImpl :: FORMULA -> FORMULA

elimImpl = foldFormula mapRecord

{ foldImplication = \ x y n ->

Disjunction [Negation x n, y] n }

negForm :: Range -> FORMULA -> FORMULA

negForm r frm = case frm of

Negation f _ -> f

Conjunction xs n -> Disjunction (map (negForm r) xs) n

Disjunction xs n -> Conjunction (map (negForm r) xs) n

True_atom n -> False_atom n

False_atom n -> True_atom n

_ -> Negation frm r

moveNegIn :: FORMULA -> FORMULA

moveNegIn frm = case frm of

Negation f n -> negForm n f

Conjunction xs n -> Conjunction (map moveNegIn xs) n

Disjunction xs n -> Disjunction (map moveNegIn xs) n

_ -> frm

distributeAndOverOr :: FORMULA -> FORMULA

distributeAndOverOr f = case f of

Conjunction xs n -> mkConj (map distributeAndOverOr xs) n

Disjunction xs n -> if all isPrimForm xs then mkDisj xs n else

distributeAndOverOr

$ mkConj (map (flip mkDisj n) . combine $ map getConj xs) n

_ -> f

cnf :: FORMULA -> FORMULA

cnf = distributeAndOverOr . moveNegIn . elimImpl . elimEquiv