# Baseline Comparison Study — Design

## Architecture

Pure backend/library feature. No UI. Three components added:

1. **`experiment` module** (`tidal/src/experiment/`) — types, assignment function, report builder
2. **`db/experiment.rs`** — `TidalDb` impl block wiring experiment methods to existing infrastructure
3. **`chronological` builtin profile** — registered in `ranking/builtins.rs`

## Module Layout

```
tidal/src/
├── experiment/
│   ├── mod.rs          # ExperimentConfig, ExperimentGroup, ExperimentReport,
│   │                   # GroupMetrics, MetricLift, assign_group()
│   └── report.rs       # ReportBuilder: scans UserSignalIndex per group
├── db/
│   └── experiment.rs   # TidalDb::experiment_group/profile/report
└── ranking/
    └── builtins.rs     # + chronological() profile
```

## Assignment

FNV-1a hash over `experiment_id.as_bytes()` concatenated with `user_id.to_le_bytes()`. Bucket = hash % 10,000. Treatment if bucket < threshold. Pure function, no I/O, deterministic across platforms.

## Report

`ReportBuilder` takes `&Arc<SignalLedger>` and `&UserSignalIndex`. Partitions users via `assign_group`, then for each group iterates users calling `user_signal_count()` (new method on `UserSignalIndex` that sums AllTime counts across all entities for a user+signal_type pair) and `user_activity_split()` for return rate.

## Key Design Decisions

1. **No new storage** — all metrics derived from existing `UserSignalIndex` DashMap scans
2. **New UserSignalIndex methods** — `user_signal_count(user_id, type_id)` and `user_activity_split(user_id, boundary_ns)` added to support per-user aggregation across all entities
3. **chronological profile** — uses existing `Sort::New`, no new sort variant needed
4. **Thread safety** — all types are `Send + Sync`, `assign_group` is pure, report takes `&self`

## Error Handling

- `InvalidInput`: treatment_fraction out of range, empty signal lists
- `Internal`: no schema/ledger available
- No new error variants needed