fix: O(1) positional lookup in the web streaming reducers

[saucam]

Problem (#90)

The web streaming reducers scanned the message array with buf.findIndex(...) from index 0 on every message and every delta. The #73/#75 fix added an O(1) idsBySession existence Set, but only for the existence check — the positional lookup stayed linear, and it runs over store-proxied entries inside produce(), which is far more expensive per element than a plain array walk. Deltas almost always target the tail message, so a 5000-message session paid the full O(N) scan per streaming chunk: O(N²) over the session, compounding the markdown re-parse cost (#87, fixed separately in #98).

Fix

The per-session existence Set becomes a messageId → index Map (indexBySession):

• applyMessage uses it for the upsert position and records the index on append — valid forever, since the buffer is append-only.
• applyDelta replaces both the hasMessage pre-check and the findIndex with one map lookup.
• replaceScrollback already computes exactly this map during its dedupe pass (posById) — it's now installed wholesale instead of being collapsed into a Set.
• hasMessage / clearSessionMessages / the test-reset hook read the same structure; semantics unchanged.

Micro-bench

Same script (public reducer API), run against main and this branch — 2000 tail deltas on a 5000-message session:

OLD  findIndex through store proxies: 91 ms total, 46 µs/delta
NEW  map lookup:                       1 ms total,  1 µs/delta   (~46×)

The old per-delta cost also grew linearly with transcript length; the new one is constant.

Tests

All 17 existing reducer tests pass unchanged (upsert semantics, delta patching, dedupe-on-replay, epoch/version bumps, session clearing). Two new cases pin the positional index specifically: deltas land on the correct non-tail targets in a 2000-message buffer, and the index survives re-broadcast upserts plus replaceScrollback rebuilds with duplicate messageIds.

tsc -b ✓ · eslint identical warning set to main ✓ · vitest run 154/154 ✓

Note: web coverage is not uploaded to codecov, so codecov/patch is unaffected by this PR.

Fixes #90

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes

  • Improved message updates so edits and deltas apply to the correct message, even in long conversation histories.
  • Fixed issues where repeated or rebroadcast messages could cause later updates to land on the wrong entry.
  • Reduced the chance of stale message changes being applied after older messages are removed or replaced.

• Performance

  • Made message lookups and updates faster, especially in large sessions.

[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between f7487bc and 2a86a7e.

📒 Files selected for processing (2)

• web/src/state/messages.test.ts
• web/src/state/messages.ts

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📝 Walkthrough

Walkthrough

Replaces the per-session existence Set (idsBySession) with an O(1) positional index (indexBySession: messageId → array index) in the message store. applyMessage, applyDelta, replaceScrollback, hasMessage, clearSessionMessages, and the test reset helper are updated to use the new index; new tests validate positional lookup correctness.

Changes

Positional message index optimization

Layer / File(s)	Summary
Positional index data structure web/src/state/messages.ts	Introduces indexBySession (sessionId → messageId → array index) replacing idsBySession.
applyMessage/applyDelta positional lookups web/src/state/messages.ts	applyMessage maintains the index on insert/overwrite; applyDelta uses it for O(1) lookup instead of hasMessage + findIndex.
Index rebuild and cleanup web/src/state/messages.ts	replaceScrollback rebuilds the index from deduped positions; hasMessage checks the positional map; clearSessionMessages and the test reset helper clean up the new index and dependent version counters.
Positional indexing tests web/src/state/messages.test.ts	New tests validate correct positional delta targeting in large buffers and after upsert/replaceScrollback rebuilds with duplicate messageIds.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Possibly related PRs

• saucam/codeoid#73: Prior PR introduced the existence-based idsBySession Set in the same file/functions that this PR replaces with a positional index.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: switching streaming reducers to O(1) positional lookup.
Linked Issues check	✅ Passed	The PR implements the requested messageId→index map and uses it in applyMessage and applyDelta, matching #90.
Out of Scope Changes check	✅ Passed	The changes stay focused on the lookup optimization and related tests, with no obvious unrelated code paths added.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch perf/web-reducer-index

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.06%. Comparing base (f7487bc) to head (2a86a7e).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #99   +/-   ##
=======================================
  Coverage   73.06%   73.06%           
=======================================
  Files          65       65           
  Lines       11032    11032           
=======================================
  Hits         8060     8060           
  Misses       2972     2972           

Flag	Coverage Δ
daemon	73.06% <ø> (ø)

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