Beagle SCM

PACK-002: JS pack writer emits OFS_DELTA (offset-only) — jab post feeds raw today

The be/ JS pack writer feeds every object RAW: fold-commit.js::writePack calls pk.feed(type, body, -1, null) (base -1 = no delta), and ingest lands packs verbatim. The native keeper is OFS-ALWAYS — it deltifies against an earlier object in the SAME pack (PackLog). So JS-authored packs are bigger than native's and diverge from the on-disk format. Implement OFS_DELTA emit in the JS writer. Offset-only either way: NEVER a REF_DELTA — a base outside the current pack stays RAW (reclaimed at GIT-006 recompaction). Follow-up to PACK-001. Method Issues.

Input

Context

• be/verbs/post/fold-commit.js::writePack and shared/ingest.js write raw

  records; a JS commit stores a full blob even when a near-identical prior version sits earlier in the same pack.

• The binding _pack_feed(buf, off, type, content, base, baseOff, …) already

  accepts a base+offset (js/pack.hpp); git.delta.encode landed (JS-036) but its reconcile/wiring was left incomplete (GET-031).

• OFS-ALWAYS invariant (keeper/KEEP.c:1579): only OFS_DELTA against an earlier

  same-pack base; a cross-pack base → store RAW. NO REF_DELTA in a native log. (JS-037 is the REF_DELTA-emit path — distinct; this ticket is OFS-only.)

Goals

• writePack (and the commit-path ingest) emits OFS_DELTA against a chosen

  earlier same-pack object (by offset); readers (PACKResolveOfs, PIDXScan) already handle it — no reader change.

• Base selection: an earlier object in THIS pack (e.g. the prior version of the

  same path); no in-pack base → RAW. Never REF_DELTA, never a cross-pack base.

• Size-parity (ideally byte-parity) with a native OFS pack on a fixture;

  index + resolve round-trip byte-identical objects.

Constraints

• be/ JS writer + the git.delta binding only. Offset-only — NO REF_DELTA

  (that stays JS-037). OFS chain depth ≤ KEEP_WRITE_CHAIN_MAX (50).

• Repro-first (CLAUDE.md §17); be/ post/get parity suites stay green.

WIP

Design decisions

• Reuse git.delta.encode (JS-036) + _pack_feed(base, baseOff); pick the

  base among objects ALREADY fed to this pack (offset known at feed time).

• Start simple: deltify a blob against the same-path prior object present

  in-pack, else RAW. An OFS base may itself be a delta (chains, GIT-008) — cap the chain depth; a base outside the pack → RAW (never cross-file, never hash).

TODOs

• Wire git.delta.encode into writePack; feed OFS_DELTA with the base offset. (Audit 07Jul26: fold-commit.js ~176-186 still feeds every record pk.feed(type, body, -1, null); no git.delta use anywhere in be/.)
• Base-pick heuristic (same-path earlier in-pack object); RAW fallback.
• Chain-depth cap (≤50); verify PACKResolveOfs/PIDXScan round-trip.
• Parity test vs a native OFS pack (size/bytes); be/ suites green.
• NEW consumer since filing: GIT-018's pure-JS push pack (shared/wire.js emitPack) also feeds RAW (// GIT-018: RAW per this pass … OFS_DELTA is [PACK-002]) — deltify the wire pack here too, or factor one shared OFS-emitting feed both writers use.

Blockers and bummers

• Sequenced after the writer/append story settles (JS-099 pack-write path);

  may need to finish the JS-036/GET-031 delta-encode wiring first.

Outcome

• (pending)