---
id: value-hash-detection
name: Value.Hash O(1) change detection
status: pending
depends_on: [value-clone-prevprops]
created: 2026-05-18T16:22:57.369878873Z
modified: 2026-05-18T16:22:57.369879316Z
scope: narrow
risk: medium
impact: component
level: implementation
---

# Description

Add `Value.Hash` as an O(1) change detection layer before `Value.Equal`. Hash comparison is faster than deep-equal for large subtrees because it's a single integer comparison.

**Critical constraint:** `Value.Hash` uses a global mutable accumulator (FNV-1a state). It is NOT re-entrant. You cannot call `Value.Hash` from within a `computed` or `effect` that is itself triggered by a hash comparison. Hashes must be computed outside reactive computations, during the commit phase.

The optimization strategy from the architecture doc says to add `Value.Hash` after confirming the global accumulator constraint is manageable. This task must verify that the reconciler never calls `Value.Hash` from within a `computed` or `effect`.

Approach:
1. Cache hash on fiber after each commit
2. On next reconciliation, compute hash of new node
3. If hashes differ, proceed to `Value.Equal` check
4. If hashes match, skip subtree (hash match is sufficient for "unchanged")

## Acceptance Criteria

- [ ] `Value.Hash` imported from `@alkdev/typebox/value`
- [ ] Fiber caches hash value after each commit (new `hash` field)
- [ ] On reconciliation: hash comparison before `Value.Equal` comparison
- [ ] Hash computed OUTSIDE reactive computations (commit phase only)
- [ ] Verified: no `Value.Hash` call happens inside `computed` or `effect` callbacks
- [ ] When hashes differ, falls through to `Value.Equal` (then to full reconciliation)
- [ ] When hashes match, skip subtree (fast path)
- [ ] Existing tests pass
- [ ] New test: unchanged subtree detected by hash match
- [ ] New test: hash mismatch falls through to Value.Equal / reconciliation
- [ ] New test: hash never called from within a computed/effect

## References

- docs/architecture/reconciler.md — TypeBox Optimization Layer, Value.Hash row, Value.Hash Constraint
- docs/architecture/reconciler.md — Optimization Strategy (step 2)
- docs/architecture/reconciler.md — Open Question 3 (should Value.Hash be used given the constraint)

## Notes

> To be filled by implementation agent

## Summary

> To be filled on completion