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fix(hash): spread high-bit keys across buckets in tableau and Pauli-word maps - #267
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The peripheral Lindblad test change is because it accidentally relied on insertion order. |
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I don't see any public user interface breaking in this PR, so I think it's totally fine.
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| /// Fast hasher for internal maps and word hashes, with a fixed seed. | ||
| /// Iteration is reproducible for the same build and insertion order. | ||
| pub type FastBuildHasher = foldhash::fast::FixedState; |
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should we just call it FoldBuildHasher? Fast is quite general, the convention here should be transparent about what downstream hasher we are using.
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`GeneralizedTableau::rotate_2` (RXX / RYY / RZZ) accumulated the new coefficients in a `std::collections::HashMap`. Its SipHash hasher is seeded randomly per process, so the order in which coefficients were written back into the tableau differed from run to run. This switches it to `ppvm_traits::FoldHashMap`, the fixed-seed hasher the other tableau coefficient maps have used since #267. That makes the post-rotation coefficient order reproducible within a build, and foldhash is also cheaper than SipHash for integer keys. `cargo test -p ppvm-tableau` passes with and without `--features rayon`, and clippy is clean. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
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## 🤖 New release * `ppvm-traits`: 0.1.0 -> 0.1.1 (✓ API compatible changes) * `ppvm-pauli-word`: 0.1.0 -> 0.1.1 (✓ API compatible changes) * `ppvm-pauli-sum`: 0.1.0 -> 0.1.1 (✓ API compatible changes) * `ppvm-sym`: 0.1.0 -> 0.1.1 (✓ API compatible changes) * `ppvm-tableau`: 0.1.0 -> 0.1.1 (✓ API compatible changes) * `stim-parser`: 0.1.0 -> 0.1.1 (✓ API compatible changes) * `ppvm-stim`: 0.1.0 -> 0.1.1 (✓ API compatible changes) * `ppvm-tableau-sum`: 0.1.0 -> 0.1.1 (✓ API compatible changes) * `vihaco-circuit-isa`: 0.1.0 -> 0.1.1 (✓ API compatible changes) * `ppvm-vihaco`: 0.1.0 -> 0.1.1 (✓ API compatible changes) * `ppvm-tui`: 0.1.0 -> 0.1.1 (✓ API compatible changes) * `ppvm-cli`: 0.1.0 -> 0.1.1 <details><summary><i><b>Changelog</b></i></summary><p> ## `ppvm-cli` <blockquote> ## [0.1.1](v0.1.0...v0.1.1) - 2026-10-23 ### Fixed - *(hash)* spread high-bit keys across buckets in tableau and Pauli-word maps ([#267](#267)) - *(tableau)* use FoldHashMap for two-qubit rotation coefficients ([#268](#268)) ### Other - add crates.io READMEs for the published crates ([#265](#265)) </blockquote> </p></details> --- This PR was generated with [release-plz](https://github.com/release-plz/release-plz/). Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
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FxHash clusters branch indices and Pauli words that differ mainly in their high bits. This made T gates much slower once extended-stabilizer runs crossed 16,384 coefficients.
This PR uses foldhash for branch-index maps, cached Pauli-word hashes, and the tableau fingerprint fallback on builds without AES. It keeps existing configuration types and uses a fixed seed for reproducible iteration within a build. It also initializes identity-word hashes correctly.
Regression tests cover high-bit key distribution and identity-word map lookups. One Lindblad test now tolerates tiny rounding differences caused by the changed summation order.
Representative timings on an Apple M4 Pro, release builds, medians:
Just the hash map
usize)u128)U256)U2048)Full simulations
rayon)TFIM term counts and noisy-distillation branch probabilities match
mainexactly. Workspace tests, tableau tests withrayon, Clippy, and formatting checks pass.