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Fix MT random seed and SPLIT KISS random state restore #2620

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Fix MT random seed and SPLIT KISS random state restore #2620

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c10ac59
Fix RNG state handling for SPLIT
mads-bertelsen-agentic Sep 1, 2026
933ec9b
Make SPLIT RNG backup length configurable
mads-bertelsen-agentic Sep 1, 2026
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8 changes: 6 additions & 2 deletions common/lib/share/mccode-r.h.in
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Original file line number Diff line number Diff line change
Expand Up @@ -619,7 +619,9 @@ void mcdis_sphere(double x, double y, double z, double r);

#if RNG_ALG == _RNG_ALG_MT // MT (currently not functional for GPU)
# define MC_RAND_MAX ((uint32_t)0xffffffffUL)
# define RANDSTATE_LEN 1
# ifndef RANDSTATE_LEN
# define RANDSTATE_LEN 1
# endif
# define srandom(seed) mt_srandom_empty()
# define random() mt_random()
# define _random() mt_random()
Expand All @@ -628,7 +630,9 @@ void mcdis_sphere(double x, double y, double z, double r);
# define UINT64_MAX ((uint64_t)0xffffffffffffffffULL)
# endif
# define MC_RAND_MAX UINT64_MAX
# define RANDSTATE_LEN 7
# ifndef RANDSTATE_LEN
# define RANDSTATE_LEN 7
# endif
# define srandom(seed) kiss_srandom(_particle->randstate, seed)
# define random() kiss_random(_particle->randstate)
# define _random() kiss_random(state)
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2 changes: 1 addition & 1 deletion common/lib/share/mccode_main.c
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Expand Up @@ -150,7 +150,7 @@ int mccode_main(int argc, char *argv[])
#endif

// MT specific init, note that per-ray init is empty
#if RNG_ALG == 2
#if RNG_ALG == 1
mt_srandom(mcseed);
#endif

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11 changes: 9 additions & 2 deletions mccode/src/cogen.c.in
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Expand Up @@ -1863,9 +1863,14 @@ int cogen_raytrace(struct instr_def *instr)
"Split_%s_counter< SplitS_%s; "
"Split_%s_counter++) {",
comp->name, comp->name, comp->name, comp->name);
coutf(" randstate_t randbackup = *_particle->randstate;");
coutf(" randstate_t randbackup[RANDSTATE_LEN];");
coutf(" for (int Split_%s_randstate_idx = 0; Split_%s_randstate_idx < RANDSTATE_LEN; Split_%s_randstate_idx++) {", comp->name, comp->name, comp->name);
coutf(" randbackup[Split_%s_randstate_idx] = _particle->randstate[Split_%s_randstate_idx];", comp->name, comp->name);
coutf(" }");
coutf(" *_particle=Split_%s_particle;", comp->name); // restore particle for SPLIT loop
coutf(" *_particle->randstate = randbackup;");
coutf(" for (int Split_%s_randstate_idx = 0; Split_%s_randstate_idx < RANDSTATE_LEN; Split_%s_randstate_idx++) {", comp->name, comp->name, comp->name);
coutf(" _particle->randstate[Split_%s_randstate_idx] = randbackup[Split_%s_randstate_idx];", comp->name, comp->name);
coutf(" }");
// Experimentally taking out the following line of code following discussions with Milan Klausz:
// It seems that especially for the _uid=0 particle we get strange repetitions in the rng.
// coutf(" srandom(_hash(_particle->_uid* %lu *(1+Split_%s_counter)));", comp->index, comp->name);
Expand Down Expand Up @@ -2423,8 +2428,10 @@ cogen_header(struct instr_def *instr, char *output_name)
cout("#endif");
cout("#if RNG_ALG == _RNG_ALG_MT // MT ");
cout("#define randstate_t uint32_t");
cout("#define RANDSTATE_LEN 1");
cout("#elif RNG_ALG == _RNG_ALG_KISS // KISS");
cout("#define randstate_t uint64_t");
cout("#define RANDSTATE_LEN 7");
cout("#endif");
cout("");

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