wai-quantum 0.3.32: union-find decoding in near-linear time #155

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dcharlot merged 2 commits from wai-quantum-uf into main 2026-10-05 10:56:33 -04:00

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2026-10-05

Transaction Science

wai-quantum 0.3.32: close the optimisation pin's test function ...

The restack onto 0.3.31 left tests/bit_identity.rs with an unclosed function,
so the integration tests did not compile (the linux energy-receipts job). The
fix was staged after the commit by mistake. Also trims the union-find vs exact
matching comparison to 6,000 shots per distance (about 25 s in debug).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

79dd50d20b

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2026-10-05 09:07:52 -04:00

Transaction Science

wai-quantum 0.3.32: union-find decoding in near-linear time ...

A fault-tolerant machine must decode each syndrome round before the next
arrives. Exact matching (quantum_match) is the accuracy reference; quantum_uf
is the fast decoder over the same matching graph:

- Grow: every fired detector starts a cluster; odd clusters (not on the
  boundary) grow all frontier edges at once, on the graph's integer weights,
  an edge between two growing clusters at double speed.
- Merge: a filled edge joins its clusters; a cluster on the boundary stops.
- Peel: each cluster's filled edges as a spanning tree, rooted at the
  boundary when it holds it, peeled leaves-first into a correction.
- Growth is event-driven: each frontier edge carries a fill time, and a merge
  reschedules only the frontier of a side whose growth changed. Fill times
  never run backwards, so events sit in a radix heap at amortised O(1).
  Profiling put 45% of the time in a binary heap before; the radix heap
  halved the time per shot at d = 21. Time now grows linearly with fired
  detectors, about 1.1-1.5 us each from d = 9 to d = 21.
- A reusable workspace resets in time proportional to the last shot.

Checked:
- every single edge of d = 3 and d = 5 graphs is corrected exactly;
- the event-driven schedule agrees with a round-by-round one (kept in the
  tests) on 2,000 shots at each of three distances, up to tie order;
- on the same 20,000 shots, union-find fails 0.92-1.18x as often as exact
  matching (d = 3-9, p = 0.003-0.007), 77x faster at d = 11; its threshold sits
  a little lower (at p = 0.007 its failures stop falling with distance).

CLI: wai-quantum qec --decoder uf|exact. Example: uf_bench. Pinned bit for bit
natively and under wasm32-wasip2 (17 pins); CLI output byte-identical.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

e76acbe9e5

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2026-10-05 07:46:12 -04:00