DDMC Bugfix?

[adamdempsey90]

From GPT

Background

Fix a device-side assertion in Jaybenne IMC/DDMC runs with static mesh
refinement:

Invalid transition from DDMC to IMC.

Problem

A DDMC particle crossing a mesh-block boundary is marked for communication by
zeroing its velocity. On receipt, Jaybenne identifies the DDMC entry face from
the particle position and either samples its fine-block face location or
resamples it as an IMC particle.

The DDMC position offset is eps_ddmc_offset(), but the transition checks use
only parthenon::robust::EPS() as their matching tolerance. Source and
destination cell faces are calculated independently, so valid coarse-to-fine
transitions can miss this machine-epsilon-sized window due to roundoff. The
particle remains zero-velocity and subsequently reaches the
Invalid transition from DDMC to IMC. device assertion.

The issue is exposed by static refinement, including the stratified
rotating-frame IMC/DDMC case on AMD GPUs. Rotating-frame physics can change
where DDMC/IMC interfaces occur but does not directly move photon positions.

Description of Changes

• Add eps_ddmc_match_tolerance() in transport_utils.hpp.

  It returns 0.1 * eps_ddmc_offset(), providing a tolerance that absorbs
  face-construction roundoff while remaining well below the separation between
  the valid 0.5x, 1x, and 2x DDMC entry offsets.

• Use this tolerance for DDMC transition detection in:

  • ptcl_ddmc_to_imc() in src/jaybenne/transport_utils.hpp
  • SampleDDMCBlockFace() in src/jaybenne/sample_ddmc_bface.cpp

• Add the missing break after successfully matching the lower-X2 face in
  ptcl_ddmc_to_imc(). This matches the control flow of the other five face
  branches and avoids a needless second iteration through the offset loop.

Checklist

• New features are documented
• Tests added for bug fixes and new features
• (@lanl.gov employees) Update copyright on changed files

[RyanWollaeger]

Ah - sorry for this - should've anticipated the possible slight block mesh discrepancy - LGTM (assuming it gets past the error you hit).