Review — gram-greedy

• Sources: src/placecell_gram_greedy.cpp, src/placecell_gram_greedy.h (namespace placecell::gram_greedy: State, Proposal, the two constants, the declarations), src/placecell_cull_method.h (CullScope, CullExecutor, shared by every method)
• Reviewer: Alejandro Fontan
• Last reviewed at: 74acd51 (2026-09-26) — 1 of 4 functions read; the file is the culler split out of src/placecell.cpp

Checklist

Function	Source	State	Notes
gram_greedy::cull	placecell_gram_greedy.cpp#L145	unread
gram_greedy::seed	placecell_gram_greedy.cpp#L23	question	unchecked LDLT (#3), jitter gap, see 2026-09-26
gram_greedy::propose	placecell_gram_greedy.cpp#L49	unread
gram_greedy::downdate	placecell_gram_greedy.cpp#L95	unread
gram_greedy::report	placecell_gram_greedy.cpp#L118	unread

CullExecutor::operator() (header-only, src/placecell_cull_method.h) is not listed; note it under the driver if it matters. The functions were renamed on 2026-09-29 (PlaceCell::gram_greedy_<step> → gram_greedy::<step>, cull_gram_greedy → gram_greedy::cull); the dated notes below keep the old names. report is new (split out of the driver).

Notes

2026-09-26 — gram_greedy_seed (at 74acd51)

• M is the LDLT solve of the jittered alive block against the identity and the result is never inspected: on an indefinite kernel (an unclipped set_kernel matrix) some M_ii come out non-positive and gram_greedy_propose drops those views without a word, so the culler goes quiet with an empty report. The only signal is set_kernel's PSD WARN at load time, far from the symptom. Already the proposal of the kernel-contract issue (a check on the pivots here). → #3
• The seeded v_h = K_hh − w_h · k_hA uses the unjittered K_hh and k_hA against the jittered M, so it differs from the Schur identity 1/M_ii that gram_greedy_downdate assigns to views culled in the same call by a gap of order ε. Harmless at 1e-6; the reference entry states it. → reference
• The history rows are seeded in count-driven mode too, where tau is infinite and the price test cannot fail, so their only consumer is worst_history in the report: O(|H||A|²) for a diagnostic. Costs nothing today (offline runs start with no history); skip them when tau is infinite if an online count-driven use ever appears. → reference
• Two spellings of one operation: the jitter is added inside the copy loop here and to the diagonal after the copy in solve_information, and the two constants are separate (gram_greedy_jitter vs a literal in solve_information) with a comment promising they are equal. One named constant in the header would make the promise a fact. <cmath> in this file is unused. → #6

2026-09-26 — checklist created (at 74acd51)

No findings yet.