perf(lindblad): skip the second hop and corrector when hop 2 admits nothing - #230
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…othing With admit_basis set, once the basis has reached max_basis the first leakage admission fills the whole room admit - |basis|, so the second leakage call ran with room = 0 and admitted nothing in every capped step while still evaluating all its candidates, and the corrector then repeated the predictor exactly on the same basis and input. pc_step and pc_step_orbit_rep now skip the second leakage pass when room = 0 and reuse the predicted state whenever the second hop admitted no string. Results are bit-identical; skipped phases report 0 in PcStepTimings. Default behaviour is otherwise unchanged: capped steps still carry no second-order admission, as before. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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david-pl
approved these changes
Sep 28, 2026
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Stacked on #222 (
split/5-kossakowski).Summary
With
admit_basisset, once the basis has reachedmax_basisthe first leakage admission fills the whole roomadmit − |basis|. The second leakage call then ran with room = 0 and admitted nothing in every capped step, while still evaluating all its candidates, and the corrector repeated the predictor exactly on the same basis and input.pc_stepandpc_step_orbit_repnow skip the second leakage pass when room = 0 and reuse the predicted state whenever the second hop admitted no string. Results are bit-identical; skipped phases report 0 inPcStepTimings. Default behaviour is otherwise unchanged: capped steps still carry no second-order admission, as before.Single commit,
crates/ppvm-lindblad/src/step.rsonly.Measurements
pc_stepon the two-leg XY ladder with a local probe (not part of this PR): L = 41 rungs (N = 82),max_basis = 2^18,admit_basis = 3·2^18, dt = 0.1, 25 steps, 4 threads,drop_tol = 0. Apple M4 MacBook Air (fanless, so wall times are only comparable within a rep).In the base, 21 of the 25 steps are capped and the second hop (leakage2 + expm2) accounts for 53% of the wall time (34.5 of 65.2 s).
Tests:
cargo test -p ppvm-lindbladpasses (8/8) with this change stacked together with the blocked-CSC PR; not yet run on this branch alone (CI will).🤖 Generated with Claude Code