miroir/notes/miroir-cdo.4.md

P1.4 Result Merger - Verification Summary

Status: ✅ COMPLETE

The result merger implementation in crates/miroir-core/src/merger.rs is fully implemented and tested.

Acceptance Criteria Verification

All acceptance criteria from plan §8 "Result merger" are met:

✅ Global sort by _rankingScore descending across shards

• Implementation: Lines 82-166 in merger.rs
• Test: test_global_sort_by_ranking_score
• Uses binary min-heap optimization for large result sets

✅ offset + limit applied after merge (not per-shard)

• Implementation: Lines 170-191 in merger.rs
• Tests: test_offset_and_limit_applied_after_merge, test_offset_limit_pagination_reconstruction
• Includes pagination reconstruction test (50 docs with known scores)

✅ _rankingScore stripped when client_requested_score=false

• Implementation: Lines 183-187 in merger.rs
• Tests: test_ranking_score_stripped_when_not_requested, test_ranking_score_included_when_requested

✅ _miroir_shard always stripped

• Implementation: Lines 178-180 in merger.rs
• Test: test_miroir_shard_always_stripped, test_strip_all_miroir_reserved_fields
• Strips all _miroir_* prefix fields (not just _miroir_shard)

✅ Facet counts sum correctly including keys unique to one shard

• Implementation: Lines 364-406 in merger.rs
• Tests: test_facet_counts_summed_across_shards, test_facet_keys_unique_to_one_shard_preserved
• Uses BTreeMap for stable, deterministic serialization

✅ estimatedTotalHits summed across shards

• Implementation: Lines 197-200 in merger.rs
• Test: test_estimated_total_hits_summed

✅ processingTimeMs = max across covering set

• Implementation: Lines 203-207 in merger.rs
• Test: test_processing_time_max_across_shards

✅ Stable serialization: merge on same input produces byte-identical JSON

• Test: test_stable_serialization_same_input_same_json
• Achieved via BTreeMap for facet aggregation

Additional Features Implemented

Binary Min-Heap Optimization

• Implementation: Lines 109-162 in merger.rs
• Test: test_binary_heap_efficiency_large_fan_out
• Avoids keeping all hits in RAM when fan-out is large
• Activates when all_hits.len() > heap_size * 2

Tie-Breaking by Primary Key

• Implementation: Lines 241-252 in merger.rs
• Test: test_tie_breaking_by_primary_key
• On equal _rankingScore, falls back to lexicographic primary_key for deterministic ordering

Degraded Mode

• Implementation: Lines 72-79 in merger.rs
• Tests: test_degraded_flag_when_shard_fails, test_not_degraded_when_all_succeed
• Sets degraded: true when some shards fail

Facet Filtering

• Implementation: Lines 374-379 in merger.rs
• Test: test_facet_filter_only_merges_requested_facets
• Supports optional facet filter to only merge requested facets

Test Results

All 21 tests pass:

running 21 tests
test merger::tests::test_binary_heap_efficiency_large_fan_out ... ok
test merger::tests::test_facet_keys_unique_to_one_shard_preserved ... ok
test merger::tests::test_limit_exceeds_available_hits ... ok
test merger::tests::test_degraded_flag_when_shard_fails ... ok
test merger::tests::test_empty_shards_returns_empty_result ... ok
test merger::tests::test_estimated_total_hits_summed ... ok
test merger::tests::test_facet_filter_only_merges_requested_facets ... ok
test merger::tests::test_global_sort_by_ranking_score ... ok
test merger::tests::test_facet_counts_summed_across_shards ... ok
test merger::tests::test_offset_and_limit_applied_after_merge ... ok
test merger::tests::test_ranking_score_stripped_when_not_requested ... ok
test merger::tests::test_offset_exceeds_total_hits ... ok
test merger::tests::test_miroir_shard_always_stripped ... ok
test merger::tests::test_missing_facet_distribution_handled_gracefully ... ok
test merger::tests::test_not_degraded_when_all_succeed ... ok
test merger::tests::test_stable_serialization_same_input_same_json ... ok
test merger::tests::test_strip_all_miroir_reserved_fields ... ok
test merger::tests::test_processing_time_max_across_shards ... ok
test merger::tests::test_ranking_score_included_when_requested ... ok
test merger::tests::test_tie_breaking_by_primary_key ... ok
test merger::tests::test_offset_limit_pagination_reconstruction ... ok

test result: ok. 21 passed; 0 failed; 0 ignored; 0 measured

Code Quality

• Pure function: The merge() function is deterministic and testable without network
• Memory efficient: Binary heap optimization for large result sets
• Stable serialization: BTreeMap ensures deterministic JSON output
• Well-documented: Comprehensive inline comments and docstrings
• Tested: 21 tests covering all acceptance criteria and edge cases

Conclusion

The result merger implementation is complete, correct, and production-ready. No changes are required.