Fix bug in engine_derivative.c

- The inner loop was reusing and modifying an outer loop variable.

PiperOrigin-RevId: 518843275
Change-Id: I922f8768c4e8ef74c2fb2b1fd897de8234336acf

doc/changelog.rst

@@ -2,6 +2,16 @@
 Changelog
 =========
 
+Upcoming version (not yet released)
+-----------------------------------
+
+Bug fixes
+^^^^^^^^^
+
+- Fixed bug in the handling of ellipsoid-based fluid model forces in the new implicitfast integrator. If using the
+  (as-yet undocumented) ellipsoid-based fluid model, please use a different integrator until the next release.
+
+
 Version 2.3.3 (March 20, 2023)
 ------------------------------
 

src/engine/engine_derivative.c

@@ -1210,10 +1210,10 @@ void mjd_ellipsoidFluid(const mjModel* m, mjData* d, int bodyid) {
     // make B symmetric if integrator is IMPLICITFAST
     if (m->opt.integrator == mjINT_IMPLICITFAST) {
       for (int i=0; i<5; i++) {
-        for (j=i+1; j<6; j++) {
-          mjtNum tmp = 0.5 * (B[6 * i + j] + B[6 * j + i]);
-          B[6 * i + j] = tmp;
-          B[6 * j + i] = tmp;
+        for (int k=i+1; k<6; k++) {
+          mjtNum tmp = 0.5 * (B[6 * i + k] + B[6 * k + i]);
+          B[6 * i + k] = tmp;
+          B[6 * k + i] = tmp;
         }
       }
     }