`importance_ranking` computed `100.0 * imp[j] / total` in f32, which associates
left: a lone importance above 3.4e36 overflowed on the multiply, before the
divide that would have brought it back into range, and the feature carrying all
of the importance was reported as `inf%`.
Dividing first — the fix the ticket's title proposes — is not enough, and that
is the half worth recording. `total` overflows too: several large importances
sum past f32::MAX, and the reassociated version then divides by `inf` and
reports a serene 0% for every feature, where the original reported NaN%. That is
improvise-pv3g's `cv_regress` shape again: an overflow that corrupts a reported
number with no panic to announce it.
So the sum and the ratio move to f64 — no sum of finite f32s can overflow it —
and only the resulting share, bounded by 100 and therefore always finite, is
narrowed back. The arithmetic stops being able to produce a non-finite share
rather than a call site checking for one afterwards; `fit_csv`'s existing clamp
stays as the bar-length bound it also is. The one remaining guard,
`!total.is_finite()`, answers only an importance that arrives non-finite: an
infinite total apportions nothing, so the ranking is empty rather than NaN.
Reachable end to end: both callers score importance in the target's own units,
so an importance is an MSE difference and grows as the square of the target.
That leaves a live window between pv3g's `standardize` refusal and this
overflow, and `a_large_measures_importance_shares_reach_the_report_finite` sits
inside it — before the fix, `fit_measure` reported `[("Cost", inf)]`.
The scoring bit patterns in tests/scoring_known_answers.rs are unchanged, which
is the evidence the fix is confined to the non-finite path: `importance_ranking`
is not on any path that file pins. gp-tree still prints nothing.
Evidence in context/test-evidence/gp.md; the argument in context/code/gp.md §8.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FC2jVn8Au7E9JPkw6bwXXH
Edward Langley
bbfdf992cd