cluster_analysis: raise on fractal dimension of degenerate cluster (bug-sweep #45)#5370
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…ug-sweep #45) Cluster::fractal_dimension fits a line through (log diameter, log particle count) points accumulated over radial shells. A 2-particle cluster (the normal minimum of pair-based clustering) yields at most one such point: with dr=0 the loop appends a single point, and with dr large enough that no second shell forms, no points are appended at all. Passing n<2 points to gsl_fit_linear, together with the residual division by log_diameters.size() (which is zero when the vector is empty), produced a silent [NaN, NaN] result with no diagnostic. The fractal dimension is mathematically undefined for fewer than two radial shells, so fabricating a value is wrong. Prevent the degenerate input by raising a clear std::runtime_error (surfacing to Python as RuntimeError via the script-interface dispatch, matching the existing sanity_checks() and no-GSL conventions) before calling gsl_fit_linear. Adds testsuite/python/cluster_analysis.py coverage that a 2-particle cluster raises RuntimeError for both dr=0 and a large dr. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Cluster::fractal_dimension fits a line through (log diameter, log particle
count) points accumulated over radial shells. A 2-particle cluster (the
normal minimum of pair-based clustering) yields at most one such point:
with dr=0 the loop appends a single point, and with dr large enough that
no second shell forms, no points are appended at all. Passing n<2 points
to gsl_fit_linear, together with the residual division by
log_diameters.size() (which is zero when the vector is empty), produced a
silent [NaN, NaN] result with no diagnostic.
The fractal dimension is mathematically undefined for fewer than two
radial shells, so fabricating a value is wrong. Prevent the degenerate
input by raising a clear std::runtime_error (surfacing to Python as
RuntimeError via the script-interface dispatch, matching the existing
sanity_checks() and no-GSL conventions) before calling gsl_fit_linear.
Adds testsuite/python/cluster_analysis.py coverage that a 2-particle
cluster raises RuntimeError for both dr=0 and a large dr.
Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com
🤖 Generated with Claude Code