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ADR-016 — Bitmap tail zeroing and output determinism

  • Identifier: ADR-016
  • Title: Bitmap tail zeroing and output determinism
  • Status: Accepted
  • Source: extracted verbatim from the Engineering PRD, docs/prd/06-memory-model.md (REQ-DOC-004). From M0 onward this file is the canonical, living ADR record; status changes happen here and in the index, with the PRD text remaining the historical record.

ADR-016 — Bitmap tail zeroing and output determinism. Status: Accepted. Context: differential testing and cross-backend determinism depend on byte-comparable bitmap outputs; the Arrow spec leaves tail bits unspecified, so a choice is required. Constraints: single-pass zero-allocation kernels; Arrow compatibility (REQ-MEM-006). Problem: leave tail bits unspecified (cheaper by one mask op) or force them to zero. Alternatives: (a) unspecified tails — rejected: breaks byte-comparability, leaks nondeterminism across backends, makes memcmp-based differential testing impossible; (b) zero tails (selected). Decision: producers zero tails; consumers ignore input tails (robustness against foreign bitmaps). Consequences: one extra mask per batch tail (negligible); deterministic outputs (REQ-API-006). Reconsideration: none foreseen. Related: REQ-MEM-006/008, REQ-TEST differential oracle.