From 64876af98f0140e276243364d3a5dff62a7d91c6 Mon Sep 17 00:00:00 2001
From: DualFroz <me@dualfroz.com>
Date: Tue, 25 Aug 2026 19:15:00 +0200
Subject: [PATCH] feat(xor): add xor reasoning with gaussian elimination

---
 src/Sift/Xor.hs | 126 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 126 insertions(+)
 create mode 100644 src/Sift/Xor.hs

diff --git a/src/Sift/Xor.hs b/src/Sift/Xor.hs
new file mode 100644
index 0000000..0aa7552
--- /dev/null
+++ b/src/Sift/Xor.hs
@@ -0,0 +1,126 @@
+module Sift.Xor
+  ( XorClause(..)
+  , xorClause
+  , xorVarsOf
+  , xorSatisfies
+  , systemSatisfies
+  , xorToCnf
+  , systemToCnf
+  , GaussResult(..)
+  , gauss
+  , gaussSolve
+  ) where
+
+import Sift.Types
+import Data.List (foldl', sortOn)
+import Data.IntSet (IntSet)
+import qualified Data.IntSet as IntSet
+import qualified Data.Map.Strict as Map
+
+data XorClause = XorClause
+  { xorVars   :: IntSet
+  , xorParity :: Bool
+  } deriving (Eq, Ord, Show)
+
+xorClause :: [Var] -> Bool -> XorClause
+xorClause vs parity =
+  let raw = map varId vs
+      folded = foldl' (\acc v -> if IntSet.member v acc
+                                   then IntSet.delete v acc
+                                   else IntSet.insert v acc)
+                      IntSet.empty raw
+  in XorClause folded parity
+
+xorVarsOf :: XorClause -> [Var]
+xorVarsOf = map Var . IntSet.toList . xorVars
+
+xorSatisfies :: Assignment -> XorClause -> Bool
+xorSatisfies asn (XorClause vs parity) =
+  IntSet.foldr (\v acc -> acc /= Map.findWithDefault False (Var v) asn) False vs == parity
+
+systemSatisfies :: Assignment -> [XorClause] -> Bool
+systemSatisfies asn = all (xorSatisfies asn)
+
+xorToCnf :: XorClause -> [Clause]
+xorToCnf (XorClause vs parity) =
+  [ clauseFor signs
+  | signs <- assignmentsOver n
+  , violates signs ]
+  where
+    varList = map Var (IntSet.toList vs)
+    n = length varList
+    violates signs = odd (length (filter id signs)) /= parity
+    clauseFor signs =
+      [ if s then negLit v else posLit v
+      | (v, s) <- zip varList signs ]
+
+assignmentsOver :: Int -> [[Bool]]
+assignmentsOver 0 = [[]]
+assignmentsOver k = [ b : rest | b <- [False, True], rest <- assignmentsOver (k - 1) ]
+
+systemToCnf :: [XorClause] -> CNF
+systemToCnf = CNF . concatMap xorToCnf
+
+data Row = Row
+  { rowVars :: IntSet
+  , rowRhs  :: Bool
+  } deriving (Eq, Show)
+
+xorRows :: Row -> Row -> Row
+xorRows a b = Row (symDiff (rowVars a) (rowVars b)) (rowRhs a /= rowRhs b)
+
+symDiff :: IntSet -> IntSet -> IntSet
+symDiff a b = IntSet.union (IntSet.difference a b) (IntSet.difference b a)
+
+data GaussResult
+  = GaussUnsat
+  | GaussSat [(Int, Row)]
+  deriving (Eq, Show)
+
+gauss :: [XorClause] -> GaussResult
+gauss system = go [] (map toRow system)
+  where
+    toRow (XorClause vs parity) = Row vs parity
+    go pivots [] = GaussSat pivots
+    go pivots (r : rest) =
+      let reduced = reduce pivots r
+      in if IntSet.null (rowVars reduced)
+           then if rowRhs reduced then GaussUnsat else go pivots rest
+           else
+             let pivotVar = IntSet.findMin (rowVars reduced)
+                 pivots'  = insertPivot pivotVar reduced pivots
+             in go pivots' rest
+
+reduce :: [(Int, Row)] -> Row -> Row
+reduce pivots row =
+  foldl' (\acc (pv, prow) ->
+            if IntSet.member pv (rowVars acc) then xorRows acc prow else acc)
+         row
+         (sortOn fst pivots)
+
+insertPivot :: Int -> Row -> [(Int, Row)] -> [(Int, Row)]
+insertPivot pivotVar row pivots =
+  (pivotVar, row) : map back pivots
+  where
+    back (pv, prow) =
+      if IntSet.member pivotVar (rowVars prow)
+        then (pv, xorRows prow row)
+        else (pv, prow)
+
+gaussSolve :: [XorClause] -> Maybe Assignment
+gaussSolve system =
+  case gauss system of
+    GaussUnsat      -> Nothing
+    GaussSat pivots ->
+      let allVarsSet = IntSet.unions (map xorVars system)
+          pivotVars  = IntSet.fromList (map fst pivots)
+          freeVars   = IntSet.difference allVarsSet pivotVars
+          base       = Map.fromList [ (Var v, False) | v <- IntSet.toList freeVars ]
+          solved     = foldl' assignPivot base pivots
+      in Just solved
+  where
+    assignPivot acc (pv, row) =
+      let others = IntSet.delete pv (rowVars row)
+          othersXor = IntSet.foldr (\v a -> a /= Map.findWithDefault False (Var v) acc) False others
+          val = othersXor /= rowRhs row
+      in Map.insert (Var pv) val acc