tidaldb/.sdlc/features/pg1-baseline-comparison/tasks.md

Baseline Comparison Study — Tasks

T1: Register chronological builtin profile

• Add chronological() function to tidal/src/ranking/builtins.rs
• Uses Sort::New for pure reverse-chronological ordering, no personalization signals
• Register in register_builtins()
• Add unit test verifying sort mode and no boosts
• Acceptance: cargo test --manifest-path tidal/Cargo.toml --lib passes; profile available via retrieve

T2: Create experiment types module

• New file tidal/src/experiment/mod.rs
• ExperimentConfig struct: experiment_id, treatment_fraction, treatment/control profile names, click/completion signal names, return_window
• ExperimentGroup enum: Treatment, Control (PartialEq, Eq, Debug, Clone)
• assign_group(user_id, experiment_id, treatment_fraction) -> ExperimentGroup using FNV-1a hash for deterministic assignment
• GroupMetrics struct: views, clicks, completions, ctr, completion_rate, return_rate
• MetricLift struct with compute() and relative_lift()
• ExperimentReport struct aggregating both groups
• Unit tests for hash determinism, balanced split, metric lift math
• Acceptance: cargo check --manifest-path tidal/Cargo.toml clean; 16 unit tests pass

T3: Build report aggregation

• New file tidal/src/experiment/report.rs
• ReportBuilder takes &Arc<SignalLedger> and &UserSignalIndex
• aggregate_group_metrics() resolves signal type IDs, iterates users calling user_signal_count() and user_activity_split()
• Add user_signal_count(user_id, type_id) -> u64 to UserSignalIndex
• Add user_activity_split(user_id, window_boundary_ns) -> (bool, bool) to UserSignalIndex
• Acceptance: Report builder compiles and aggregates correctly in unit tests

T4: Wire experiment API onto TidalDb

• New file tidal/src/db/experiment.rs
• experiment_group(user_id, &ExperimentConfig) -> Result<ExperimentGroup>
• experiment_profile(user_id, &ExperimentConfig) -> Result<&str>
• experiment_report(&ExperimentConfig, &[u64]) -> Result<ExperimentReport>
• Add mod experiment; to db/mod.rs
• Acceptance: Methods callable from integration tests

T5: Integration tests

• New file tidal/tests/pg1_baseline.rs
• Test chronological profile returns reverse created_at order
• Test chronological is deterministic across users
• Test experiment group assignment is deterministic
• Test experiment profile returns correct name
• Test balanced 50/50 split across 10k users
• Test experiment report with synthetic signals (treatment > control CTR)
• Test experiment report is reproducible
• Acceptance: All 7 integration tests pass

T6: Wire re-exports in lib.rs

• Add pub mod experiment; to tidal/src/lib.rs
• Re-export ExperimentConfig, ExperimentGroup, ExperimentReport, GroupMetrics, MetricLift
• Acceptance: Types accessible as tidaldb::ExperimentConfig etc.