Filtered OSD-0

The post-processor has three components:

NeighbourSorter filters and orders soft values;
FilteredOsd0 constructs reduced systems and controls progressive attempts; and
BpFilteredOsd0 connects fixed-budget BP to the OSD fallback.

Filtering and order

NeighbourSorter accepts one parallel soft frame and an optional runtime scope. Eligibility uses a strict threshold:

SignedAscending: include soft < threshold, then order by signed value;
MagnitudeAscending: include abs(soft) < threshold, then order by magnitude.

Equal values retain ascending variable index. For \(N\) inputs and \(K\) eligible outputs, an unstalled run takes \(N+K\) active clocks; sorting all inputs takes \(2N\).

Progressive solve

FilteredOsd0Config.prefixes is a strictly increasing list of selected-column capacities. The sorter runs once. The controller keeps at most the largest prefix, constructs each selected parity-check column from the elaborated graph, and streams active rows to the matching GF(2) solver.

An active row has either a selected coefficient or an asserted syndrome bit. If a prefix is inconsistent and more selected columns exist, the controller tries the next prefix without sorting again. It does not perform a separate linear-independence pass; rank deficiency is handled by the solver. When rejectOverflow is true, more eligible values than the largest prefix produce Overflow without a solve.

The correction stream contains (index, value, last) records in selected-slot order. The result reports Solved, Inconsistent, or Overflow, plus selected columns, active rows, total controller clocks, and accumulated solver clocks. Correction or result backpressure can extend interface latency.

BP composition

BpFilteredOsd0 runs up to its configured BP iteration count and checks convergence after each complete two-clock iteration. A converged BP result is streamed directly. Otherwise, the final marginals, stored syndrome, and optional scope launch FilteredOsd0 without a host round trip.

Its result distinguishes BpConverged, OsdSolved, OsdInconsistent, and OsdOverflow, and separates BP, OSD, and solver cycle counts. The maintained BB144 BP artifacts do not instantiate this post-processor.

Inspect and test

./scripts/emit.sh bp-filtered-osd0-steane
./mill chipsLDPC.test.testOnly chipsldpc.sort.NeighbourSorterSpec
./mill chipsLDPC.test.testOnly chipsldpc.osd.FilteredOsd0Spec
./mill chipsLDPC.test.testOnly chipsldpc.osd.BpFilteredOsd0Spec