[willend]

Draft: depends on McCode's TinyMT64 runtime, which is not merged in McCode yet. It lives on branch rng-gpu-mt-design (commit 69b18b803, "Add TinyMT64 as GPU-capable RNG_ALG=3"); the design and results are in RNG_GPU_DESIGN.md.

What

This adds TinyMT64 (Saito & Matsumoto, BSD-3) as an opt-in random number generator, RNG_ALG=3. It mirrors the header block in McCode's cogen.c.in on that branch: four lines in c_header.py, namely #define _RNG_ALG_TINYMT 3 plus an #elif branch with randstate_t uint64_t and RANDSTATE_LEN 2.

The generator functions (tinymt64_srandom, tinymt64_random, ...) live in McCode's shared runtime mccode-r.c / mccode-r.h.in. mccode-antlr embeds that runtime from the libc registry, so it needs no copy of its own. Building with -DRNG_ALG=3 needs a McCode runtime that includes TinyMT64.

The default stays KISS. Results don't change unless RNG_ALG=3 is chosen.

Relation to #392

#392 (porting McCode #2808's per-ray seeding) adds RANDSTATE_LEN for MT/KISS and changes the particle struct to randstate[RANDSTATE_LEN]. This PR works without #392, because the current hard-coded randstate[7] has room for TinyMT's 2 words. The two PRs touch neighbouring lines of the same #if block, so whichever lands second will hit a trivial merge conflict.

Checks

• pytest passes.
• I generated C for McStas Test_RNG_rand01 with this branch, using the runtime from rng-gpu-mt-design (swapped in for the registry's runtime by a scratch script), then built it with -DRNG_ALG=3 and ran -s 1000 Ncount=1000:
  • With mccode-antlr's current per-ray seed _hash((pidx+1)*(seed+1)), the output differs from classic McCode, as it should: that branch uses #2808's seed.
  • After replacing the seed line in the generated C with #2808's _hash(_hash(seed) + cloop*gpu_innerloop + pidx) (what Port per-ray RNG seeding and _uid fix from McCode cogen (mccode-dev/McCode#2807) #392 ports), the md5 of the non-# lines of rngout.dat is 753159792665dde568a61d3afd8e42ae. That is identical to classic McCode on rng-gpu-mt-design.
• Default build (KISS): the C generated by this branch differs from upstream main's only in the four header lines above, and the KISS rngout.dat md5 is identical for both.

🤖 Generated with Claude Code

[willend]

Similar to what I have on https://github.com/mccode-dev/McCode/tree/rng-gpu-mt-design.

@g5t keeping this in draft, no reason to add until we've agreed IRL that this makes sense. (See slack chat on same topic.)