Make PGS solver constraint visitation order time-independent.

PiperOrigin-RevId: 914741347
Change-Id: I7889a42543e9a9f3ff883e25ee9eafaceedc29e1

doc/changelog.rst

@@ -5,6 +5,12 @@ Changelog
 Upcoming version (not yet released)
 -----------------------------------
 
+General
+^^^^^^^
+- The pseudo-random constraint visitation order in the :ref:`PGS solver<soAlgorithms>`, introduced in the previous
+  release, now uses a fixed seed. The previous implementation seeded with ``mjData.time``, which introduced subtle yet
+  undesirable time dependence.
+
 .. admonition:: Breaking API changes
    :class: attention
 

src/engine/engine_solver.c

@@ -444,15 +444,10 @@ static void solPGS(const mjModel* m, mjData* d, int island,
     }
   }
 
-  // seed PCG32 RNG from simulation time
+  // seed PCG32 RNG with a fixed seed
   pcg32_state rng;
-  uint64_t seed = 0;
-  memcpy(&seed, &d->time, sizeof(d->time));
   rng.state = 0;
   rng.inc = 1;
-  rng.state = seed;
-  pcg32_next(&rng);
-  rng.state += seed;
   pcg32_next(&rng);
 
   // main iteration