From 8af1fa439a5a6b6f8a1f7ea1a4c7f2ea4d9e1a7a Mon Sep 17 00:00:00 2001
From: DualFroz <me@dualfroz.com>
Date: Wed, 26 Aug 2026 22:40:00 +0200
Subject: [PATCH] feat(encode): add hamiltonian cycle encoding

---
 src/Sift/Encode/Hamiltonian.hs | 82 ++++++++++++++++++++++++++++++++++
 1 file changed, 82 insertions(+)
 create mode 100644 src/Sift/Encode/Hamiltonian.hs

diff --git a/src/Sift/Encode/Hamiltonian.hs b/src/Sift/Encode/Hamiltonian.hs
new file mode 100644
index 0000000..896d0bd
--- /dev/null
+++ b/src/Sift/Encode/Hamiltonian.hs
@@ -0,0 +1,82 @@
+module Sift.Encode.Hamiltonian
+  ( hamCell
+  , hamiltonianPath
+  , hamiltonianCycle
+  , decodeHamiltonian
+  , validHamiltonianPath
+  , validHamiltonianCycle
+  ) where
+
+import Sift.Types
+import Sift.Encode.Cardinality
+import Sift.Encode.Coloring (Graph(..))
+import Data.List (sortOn)
+import Data.Set (Set)
+import qualified Data.Set as Set
+import qualified Data.Map.Strict as Map
+
+hamCell :: Int -> Int -> Int -> Var
+hamCell n vertex position = Var ((vertex - 1) * n + position)
+
+adjacency :: Graph -> Set (Int, Int)
+adjacency (Graph _ edges) =
+  Set.fromList (concat [ [(u, w), (w, u)] | (u, w) <- edges ])
+
+positionConstraints :: Int -> [Clause]
+positionConstraints n =
+  concat [ exactlyOne [ posLit (hamCell n vertex position) | vertex <- [1 .. n] ]
+         | position <- [1 .. n] ]
+  ++
+  concat [ exactlyOne [ posLit (hamCell n vertex position) | position <- [1 .. n] ]
+         | vertex <- [1 .. n] ]
+
+forbidNonAdjacent :: Int -> Set (Int, Int) -> Int -> Int -> [Clause]
+forbidNonAdjacent n adj fromPos toPos =
+  [ [ neg (posLit (hamCell n u fromPos)), neg (posLit (hamCell n w toPos)) ]
+  | u <- [1 .. n], w <- [1 .. n], u /= w
+  , not (Set.member (u, w) adj) ]
+
+hamiltonianPath :: Graph -> CNF
+hamiltonianPath g@(Graph n _) =
+  CNF (positionConstraints n ++ stepClauses)
+  where
+    adj = adjacency g
+    stepClauses = concat [ forbidNonAdjacent n adj i (i + 1) | i <- [1 .. n - 1] ]
+
+hamiltonianCycle :: Graph -> CNF
+hamiltonianCycle g@(Graph n _) =
+  CNF (positionConstraints n ++ stepClauses ++ wrapClauses)
+  where
+    adj = adjacency g
+    stepClauses = concat [ forbidNonAdjacent n adj i (i + 1) | i <- [1 .. n - 1] ]
+    wrapClauses = forbidNonAdjacent n adj n 1
+
+decodeHamiltonian :: Int -> Assignment -> [Int]
+decodeHamiltonian n asn =
+  map snd (sortOn fst
+    [ (position, vertex)
+    | position <- [1 .. n], vertex <- [1 .. n]
+    , Map.findWithDefault False (hamCell n vertex position) asn ])
+
+isPermutation :: Int -> [Int] -> Bool
+isPermutation n xs = Set.fromList xs == Set.fromList [1 .. n] && length xs == n
+
+edgesOfPath :: [Int] -> [(Int, Int)]
+edgesOfPath xs = zip xs (drop 1 xs)
+
+validHamiltonianPath :: Graph -> Assignment -> Bool
+validHamiltonianPath g@(Graph n _) asn =
+  isPermutation n route
+    && all (`Set.member` adj) (edgesOfPath route)
+  where
+    route = decodeHamiltonian n asn
+    adj = adjacency g
+
+validHamiltonianCycle :: Graph -> Assignment -> Bool
+validHamiltonianCycle g@(Graph n _) asn =
+  isPermutation n route
+    && all (`Set.member` adj) (edgesOfPath route)
+    && (n < 2 || Set.member (last route, head route) adj)
+  where
+    route = decodeHamiltonian n asn
+    adj = adjacency g