Implement ParticleRay class

[GuySten]

Description

This PR implement ParticleRay class to perform ray tracing and store time interval and mfp interval during tracing.
This class will be used in point detector estimator.

Fixes # (issue)

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

Summary by CodeRabbit

• New Features
  • Ray tracing can now be limited to a maximum distance, including travel through void regions. Rays track total and in-model travel separately.
  • Particle-based rays can track particle state and update time and physics data as they travel.
• Bug Fixes
  • Particle random-number generator seeds and stream now start with defined defaults, avoiding indeterminate initial values.

[vitmog]

Adding the references to #3833 and #3645 here for attribution.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: defaults
• Review profile: CHILL
• Plan: Advanced
• Run ID: b23b3cb3-d259-464c-b14b-a37c1530560a

📥 Commits

Reviewing files that changed from the base of the PR and between 15bd04b and 3574ce5.

📒 Files selected for processing: 1

• src/ray.cpp

🚧 Files skipped from review as they are similar to previous changes: 1

• src/ray.cpp

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📝 Walkthrough

Ray tracing now supports a maximum distance and uses a shared tracing loop for Ray and ParticleRay. ParticleRay adds particle initialization and updates particle physics as it advances.

Changes: Ray Tracing

Layer / File(s)	Summary
Ray state and particle-ray contracts include/openmc/particle_data.h, include/openmc/ray.h	ParticleData initializes RNG seeds and stream state and adds const RNG accessors. RayState holds shared flight state. Ray gains a distance limit, and ParticleRay adds a particle-based ray interface.
Shared distance-bounded tracing src/ray.cpp	A shared tracing loop handles cell searches and boundary intersections for both ray types. It accounts for traveled distance and marks tracing complete when the distance budget is exhausted.
Particle-ray physics updates src/ray.cpp	ParticleRay initializes particle state, updates time and cross sections during traversal, tracks mean-free-path distance, and handles lost rays.

Priority: ⬇️ Low

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant ParticleRay
  participant trace_ray
  participant Geometry
  Caller->>ParticleRay: trace(max_distance)
  ParticleRay->>trace_ray: invoke shared tracing loop
  trace_ray->>Geometry: search cells and process boundary intersections
  trace_ray->>ParticleRay: update_distance(distance)
  ParticleRay-->>trace_ray: update particle state and cross sections

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Merge Risk: ⚪ Minimal · up to 3574c

The change adds distance-bounded ray tracing and a ParticleRay class. No outstanding merge-blocking issue was found in the reviewed context.

Security architecture risk: 🔵 Low · up to 15bd0

The change remains within the simulation process, and existing plotting callers retain unlimited tracing by default. A failure-containment inconsistency remains in the new particle-ray interface: reporting a lost ray does not consistently prevent subsequent geometry and physics operations. No current in-tree caller of the new particle-ray interface was identified, which limits immediate exposure.

Retained concerns

• Low · reliability · observed: ParticleRay makes geometry loss nonfatal by setting stop_, but the consumers of that transition do not consistently terminate. After cross_lattice reports loss, trace_ray still updates flight physics and performs neighbor lookup before its conditional stop check. Geometry lookup can also continue internally after reporting a missing lattice outer universe. The new subtype can therefore operate on partially resolved geometry after its loss decision, weakening the intended containment of a failed ray. This is currently an opt-in API concern; no in-tree ParticleRay tracing caller was identified.

Security review details

Security Blast Radius

• inferred — The demonstrated affected scope is the simulation process and its loaded geometry and physics data. Existing plotting paths reach the shared flow through Ray-derived objects. Focused source inspection identified ParticleRay declarations and implementation but no in-tree consumer that launches it, so production exposure through the planned point-detector integration is not established.

Trust Boundaries and Controls

• observed — The new ParticleRay loss handler does not invoke Particle's restart-file writing or shared lost-particle accounting; it optionally warns and sets the ray's local stop flag. This narrows the handler's side effects, but does not by itself guarantee termination of the geometry operations that invoked it.

Resilience and Maintainability Implications

• inferred — Loss handling now spans geometry callbacks, shared traversal, and subtype-specific policy. The new nonfatal subtype exposes an inconsistent terminal-state contract relative to the previous fatal geometry-ray behavior. Existing Particle handling already permitted nonfatal returns, so this is not evidence that the PR introduced the underlying geometry continuation assumption or a new exploitable transport path.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 43.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 30 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title clearly identifies the main change: adding the ParticleRay class for ray tracing.
Description check		The description explains the ParticleRay purpose and motivation, and it includes the checklist. The AI Assistance section from the repository template is missing, and no issue number is provided after…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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[coderabbitai]

Actionable comments posted: 1

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🤖 Prompt to fix review comments

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Inline comments:
Review comments at @src/ray.cpp:
- Around line 187-189: Move the ray.update_distance(advance) call before
cross_lattice() so it accumulates the crossed segment using the current
material’s cross sections before cross_lattice() replaces the material state.
Keep the existing distance advance and subsequent cell-search flow unchanged.

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ℹ️ Review info

⚙️ Run configuration

• Configuration used: defaults
• Review profile: CHILL
• Plan: Advanced
• Run ID: 811147e9-568e-42f8-9897-15e1fa7f0b41

📥 Commits

Reviewing files that changed from the base of the PR and between 46c4c60 and 15bd04b.

📒 Files selected for processing: 3

• include/openmc/particle_data.h
• include/openmc/ray.h
• src/ray.cpp

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 4 remain after this review.