Eliminate mjContact copying during collision detection.

This results in approximately 4-5% speedup in collision detection for particle.xml.

PiperOrigin-RevId: 562526634
Change-Id: Ia6487b1908618064d858bac484fe6861ed37a402

src/engine/engine_io.h

@@ -18,6 +18,7 @@
 #include <mujoco/mjdata.h>
 #include <mujoco/mjexport.h>
 #include <mujoco/mjmodel.h>
+#include <mujoco/mjxmacro.h>
 
 #ifdef __cplusplus
 #include <cstddef>
@@ -117,6 +118,16 @@ MJAPI int* mj_stackAllocInt(mjData* d, int size);
 // de-allocate data
 MJAPI void mj_deleteData(mjData* d);
 
+// clear arena pointers in mjData
+static inline void mj_clearEfc(mjData* d) {
+#define X(type, name, nr, nc) d->name = NULL;
+  MJDATA_ARENA_POINTERS
+#undef X
+  d->nefc = 0;
+  d->nisland = 0;
+  d->contact = (mjContact*) d->arena;
+}
+
 #ifdef __cplusplus
 }
 #endif