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The position in Region::distance_complex is recomputed from the accumulated distance. When a candidate step is smaller than the resolution of that distance, the position does not move. Two coincident surfaces in one region then make the search alternate between them forever. This occurs after about 100 m of flight inside one cell, when leaving a union and when entering an intersection. Take the smallest representable step when the accumulated distance does not change. Add a unit test with two coincident planes that covers both cases.
The position change in Region::distance_complex moves track end points by roundoff. Only the spherical mesh tallies of this test change, because SphericalMesh::find_theta_crossing detects the crossing of the theta = pi/2 grid surface depending on roundoff. All other regression results are unchanged. Generated with GCC and OPENMC_ENABLE_STRICT_FP=on.
Assert the returned distance, state that the test coordinates are deliberate, and scope the comment in Region::distance_complex to the position at the true boundary.
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LGTM.
Co-authored-by: GuySten <62616591+GuySten@users.noreply.github.com>
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This reverts commit f240a12.
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Description
This fixes a lost-particle failure that can occur at complex CSG boundaries. The issue was exposed by weight-window surface checkpoints.
I have a large model, in it, when weight window surface checkpoints are on, I would get the following failures:
WARNING: Could not find the cell containing particle 2488140WARNING: Could not find the cell containing particle 2771912This was previously working in 0.15.4, but these errors appear with 0.16.1-dev.
I narrowed it down to PR #3934
Issue #4140 documents the problem and provides a MWE to reproduce it.
per Codex:
During a complex-region distance search, the candidate position was previously advanced incrementally across virtual surface crossings. The particle was later transported by applying the returned total distance to its original position. Roundoff could therefore make the position used to identify the boundary differ from the particle's actual transported position. In affected cases, the returned surface was inconsistent with the final position and the subsequent cell search failed.
per Codex
Checklist