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Added some more comments to the fmElimination function

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Neil Brown 2008-01-09 12:46:50 +00:00
parent 8ec4832a03
commit 4ee25b62ca
1 changed files with 14 additions and 0 deletions
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14
transformations/ArrayUsageCheck.hs
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@ -586,6 +586,7 @@ fmElimination vm ineq = fmElimination' vm (presentItems ineq) ineq
presentItems :: InequalityProblem -> [Int]
presentItems = nub . map fst . filter ((/= 0) . snd) . concatMap (tail . assocs)
-- The real body of the function:
fmElimination' :: VariableMapping -> [Int] -> InequalityProblem -> Maybe VariableMapping
fmElimination' vm [] ineqs = pruneAndHandleIneq (vm,ineqs) >>* fst
-- We have to prune the ineqs when they have no variables to
@ -593,14 +594,25 @@ fmElimination vm ineq = fmElimination' vm (presentItems ineq) ineq
fmElimination' vm indexes@(ix:ixs) ineqs
= do (vm',ineqsPruned) <- pruneAndHandleIneq (vm,ineqs)
case listToMaybe $ filter (flip isExactProjection ineqsPruned) indexes of
-- If there is an exact projection (real shadow = dark shadow), eliminate that
-- variable, and therefore just recurse to process this shadow:
Just ex -> fmElimination' vm' (indexes \\ [ex]) (getRealShadow ex ineqs)
Nothing ->
-- Otherwise, check the real shadow first:
case fmElimination' vm' ixs (getRealShadow ix ineqs) of
-- No solution to the real shadow means no solution to the problem:
Nothing -> Nothing
-- Check the dark shadow:
Just vm'' -> case fmElimination' vm'' ixs (getDarkShadow ix ineqs) of
-- Solution to the dark shadow means there is a solution to the problem:
Just vm''' -> return vm'''
-- Solution to real but not to dark; we must brute force the problem.
-- If we find any solutions during the brute-forcing, we have our solution.
-- Otherwise there is no solution
Nothing -> listToMaybe $ mapMaybe (uncurry $ solveProblem' vm'') $ getBruteForceProblems ix ineqs
-- Prunes the inequalities. If any equalities arise, those are processed, so
-- that the return is only inequalities
pruneAndHandleIneq :: (VariableMapping, InequalityProblem) -> Maybe (VariableMapping, InequalityProblem)
pruneAndHandleIneq (vm,ineq)
= do (eq,ineq') <- pruneIneq ineq
@ -639,9 +651,11 @@ fmElimination vm ineq = fmElimination' vm (presentItems ineq) ineq
pairIneqsDark :: (Integer, InequalityConstraintEquation) -> (Integer, InequalityConstraintEquation) -> InequalityConstraintEquation
pairIneqsDark (x,ex) (y,ey) = addConstant (-1*(y-1)*(x-1)) (arrayZipWith (+) (amap (* y) ex) (amap (* x) ey))
-- Adds a constant value to an equation:
addConstant :: Integer -> InequalityConstraintEquation -> InequalityConstraintEquation
addConstant x e = e // [(0,(e ! 0) + x)]
-- Checks if eliminating the specified variable would yield an exact projection (real shadow = dark shadow):
isExactProjection :: Int -> InequalityProblem -> Bool
isExactProjection n ineqs = (all (== 1) $ pos n ineqs) || (all (== (-1)) $ neg n ineqs)
where