Privacy-Period-Tracker/docs/architecture/README.md

# Architecture

```
Status: Current
Owner: _null
Last reviewed: 2026-08-18
Governs: docs/architecture/**, the Gradle module graph, and the data shapes that
         outlive a function
Review trigger: Any new Gradle module, any change to a module boundary, any change
                to a Room entity or a DAO, any new Room migration, any dependency
                added to a domain/* module
```

## The shape

```text
Compose UI  (app, feature/*)
    ↓
ViewModel — immutable StateFlow of screen state
    ↓
Use case / prediction engine  (domain/*)
    ↓
Repository  (core/data)
    ↓
Room + DataStore  (core/database, core/datastore)
```

Unidirectional: state flows down as an immutable `UiState`, events flow up as
function calls. Nothing below the ViewModel knows Compose exists.

## Modules

Four today. `core/database` and `core/datastore` are Batch 01 issues #3 and #4
and **do not exist yet** — a module created before it has contents is a place
for things to be put by accident. The wider layout sketched in
[`../planning/PRODUCT_PLAN.md` §9](../planning/PRODUCT_PLAN.md) arrives the same
way, with the batch that needs it.

| Module | Plugin | Owns | May depend on |
| --- | --- | --- | --- |
| `app` | Android application | `MainActivity`, the four-tab navigation shell, DI wiring | everything below |
| `core/designsystem` | Android library | Material 3 theme, colour and type tokens | nothing in this project |
| `domain/cycle` | **Kotlin JVM** | `PeriodRecord`, `SpottingRecord`, `CycleRecord` and the rules over them | nothing |
| `domain/prediction` | **Kotlin JVM** | the forecast, the window, confidence, `NotYetObservation` | `domain/cycle` |

Planned, with the issue that brings each one:

| Module | Plugin | Owns | May depend on | Issue |
| --- | --- | --- | --- | --- |
| `core/database` | Android library | Room entities, DAOs, migrations | `domain/cycle` | #3 |
| `core/datastore` | Android library | `UserPreferences` | `domain/cycle` | #4 |
| `core/data` | Android library | the repositories — the only things that touch a DAO | `core/database`, `core/datastore`, `domain/cycle` | #5 |
| `core/ads` | Android library | the `AdProvider` implementation | **neither `core/database` nor `domain/*`** | Batch 07 |

### Why `domain/*` is `kotlin("jvm")` and not an Android library

[`PRODUCT_PLAN.md` §57.10](../planning/PRODUCT_PLAN.md) asks for the prediction
engine to be unit-testable without Android. A convention saying "do not import
`android.*` here" is a convention somebody breaks at 11pm; a module that
**cannot see the Android SDK at all** is a compile error instead.

It buys the thing §50 depends on: the acceptance tests in §51 — stable 35-day
user, variable user, 45-day outlier, "not yet" — run on the JVM in under a
second, so they run on every commit rather than on an emulator when someone
remembers.

### The boundary that is not negotiable

> The advertising subsystem must never receive menstrual dates, cycle length,
> period duration, fertility status, ovulation estimates, prediction confidence,
> prediction history, spotting records, or any other health-derived attribute.
> — [`PRODUCT_PLAN.md` §34](../planning/PRODUCT_PLAN.md)

Expressed structurally rather than as a rule people remember: when `core/ads`
exists it will declare no dependency on `core/database` or `domain/*`, and a
Gradle check enforces the whole table above by enumerating each module's allowed
dependencies. Ads reach the UI through an `AdProvider` interface owned by `app`.

Per [`GUARDS.md`](GUARDS.md) §1, that check is proved to fail — a deliberate
forbidden dependency added, the guard watched going red, the file restored —
before it is treated as evidence. `scripts/prove-guard.sh` performs it.

## Data shapes

Defined in `domain/cycle` as plain Kotlin, mirrored by Room entities in
`core/database`. The full field lists are
[`PRODUCT_PLAN.md` §10](../planning/PRODUCT_PLAN.md); what matters here is why
each exists and what must not happen to it.

| Type | Why it exists | The rule that goes with it |
| --- | --- | --- |
| `PeriodRecord` | a confirmed period, with its source and whether it is confirmed | a record's `source` is kept; edits are recorded, never silent |
| `SpottingRecord` | spotting, tracked separately | **must not** start a cycle or reset one |
| `CycleRecord` | derived interval between two confirmed starts | derived, never stored as truth — recomputed from period records |
| `PredictionRecord` | a snapshot taken *before* the outcome is known | this is what makes accuracy measurable at all; never overwritten in place |
| `NotYetObservation` | the user said the period had not started by a date | a censoring observation — the forecast is re-conditioned on it, not shifted by +1 day |
| `UserPreferences` | notification privacy, reminder time, lock, theme, ads entitlement | lives in DataStore, never in the cycle database |

**Never secretly modify health history.** A gap that looks like a missing entry
([§14](../planning/PRODUCT_PLAN.md)) produces a question, not a correction. That
is an architectural constraint as much as a UX one: nothing in the data layer
may write a `PeriodRecord` the user did not confirm.

## Migrations

Room migrations are numbered, tested, and **listed in this document** — one row
per migration, added in the same commit as the migration itself. The template
this repository came from records why: a manual's migration table sat six
behind, and every reader in between trusted it.

| Version | What changed | Migration test |
| --- | --- | --- |
| 1 | initial schema | — |

Room's exported schemas are committed, so a migration can be tested against the
real previous schema rather than a remembered one.

## Documents here

- **[`GUARDS.md`](GUARDS.md)** — how to write a check that actually checks. Read
  it before adding a structural test or a probe.

## What ships in this folder

Nothing. This project took six scripts from the template into `scripts/`, and
[`../TOOLS.md`](../TOOLS.md) explains why the rest are absent and where the
menu is.

| Path | What it is |
| --- | --- |
| `scripts/secrets.sh` | credential shapes in a staged diff — the one that stops a keystore reaching a commit |
| `scripts/doc-claims.sh` | every file a document names must exist; `--covers` asks the inverse |
| `scripts/doc-triggers.py` | which documents a pending change fires, read from the `Governs:` headers |
| `scripts/commit-mine.sh` | commits only the paths you name, by pathspec, after the secret scan |
| `scripts/forgejo-issue.py` | files and closes issues in the tracker convention, with every rule of it as a check |
| `scripts/prove-guard.sh` | breaks what a guard protects and requires the guard to go red |
| `.githooks/` | pre-commit, commit-msg, post-commit — see [githooks/README.md](githooks/README.md) |

## What does not belong here

- Product intent — that is [`../planning/PROJECT_PLAN.md`](../planning/PROJECT_PLAN.md)
- What it should feel like — that is [`../design/README.md`](../design/README.md)
- What happened while building it — that is [`../history/DEVELOPMENT_LOG.md`](../history/DEVELOPMENT_LOG.md)

## A note on drift

Architecture docs go stale faster than any other kind, because code changes
under them silently. That is what the **Review trigger** above is for, and why
it names a new Gradle module and a new Room migration specifically: those are
the two changes here that make this document wrong without touching it.