Fix buildPatchContactMapping to compare normalized (min,max) pairs

The function now compares (min(A,B), max(A,B)) instead of (A,B) directly
to correctly match the same physical contact across timesteps regardless
of ID ordering. This fixes the issue where contacts were incorrectly
mapped to the first patch contact when IDs were in different order.

Co-authored-by: Ruochun <24469442+Ruochun@users.noreply.github.com>

Author: Copilot

src/algorithms/DEMKinematicMisc.cu

@@ -306,18 +306,24 @@ __global__ void buildPatchContactMapping(deme::bodyID_t* curr_idPatchA,
         size_t type_end = left;
 
         // Within this type segment, use binary search to find the matching A/B pair
-        // The segment is sorted by the combined patch ID pair (A in high bits, B in low bits)
-        // The encoding ensures that (smaller_A, larger_B) pattern creates a sortable value
+        // The segment is sorted by the encoded patch ID pair (A in high bits, B in low bits)
+        // Note: encodeType preserves the order, so arrays are sorted by (A, B) lexicographically
+        // For matching, we need to normalize to (min, max) since the same physical contact
+        // could have different A/B ordering between timesteps
+        deme::bodyID_t curr_min = (curr_A < curr_B) ? curr_A : curr_B;
+        deme::bodyID_t curr_max = (curr_A < curr_B) ? curr_B : curr_A;
+        
         left = type_start;
         right = type_end;
         while (left < right) {
             size_t mid = left + (right - left) / 2;
             deme::bodyID_t prev_A = prev_idPatchA[mid];
             deme::bodyID_t prev_B = prev_idPatchB[mid];
+            deme::bodyID_t prev_min = (prev_A < prev_B) ? prev_A : prev_B;
+            deme::bodyID_t prev_max = (prev_A < prev_B) ? prev_B : prev_A;
 
-            // Compare (A, B) pairs lexicographically
-            // Since they're sorted by patch ID pair where smaller ID is in high bits
-            if (prev_A < curr_A || (prev_A == curr_A && prev_B < curr_B)) {
+            // Compare (min, max) pairs lexicographically to find matching physical contact
+            if (prev_min < curr_min || (prev_min == curr_min && prev_max < curr_max)) {
                 left = mid + 1;
             } else {
                 right = mid;
@@ -328,7 +334,9 @@ __global__ void buildPatchContactMapping(deme::bodyID_t* curr_idPatchA,
         if (left < type_end) {
             deme::bodyID_t prev_A = prev_idPatchA[left];
             deme::bodyID_t prev_B = prev_idPatchB[left];
-            if (prev_A == curr_A && prev_B == curr_B) {
+            deme::bodyID_t prev_min = (prev_A < prev_B) ? prev_A : prev_B;
+            deme::bodyID_t prev_max = (prev_A < prev_B) ? prev_B : prev_A;
+            if (prev_min == curr_min && prev_max == curr_max) {
                 my_partner = left;
             }
         }