# Architecture

```
Status: Current
Owner: _null
Last reviewed: 2026-08-18
Governs: docs/architecture/**, settings.gradle.kts, build.gradle.kts,
         core/database/**, domain/** — 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

Eight today — 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 | `core/designsystem`, `core/data`, `core/datastore`, `core/notifications`, `domain/*` — never `core/database` |
| `core/designsystem` | Android library | Material 3 theme, colour and type tokens | nothing in this project |
| `core/database` | Android library | Room entities, DAOs, converters, the schema export | `domain/cycle`, `domain/prediction` |
| `core/datastore` | Android library | `UserPreferences` and the settings that are not health history | nothing in this project |
| `core/data` | Android library | `CycleRepository`, entity⇄domain mapping, accuracy — the only module that touches a DAO | `core/database`, `domain/cycle`, `domain/prediction` |
| `core/notifications` | Android library | reminder copy, the privacy modes, WorkManager scheduling | `core/data`, `core/datastore`, `domain/*` |
| `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. **Named without backticks on
purpose** — `doc-claims.sh` reads a backticked path as a claim that the file is
there, and none of these are:

| Module | Plugin | Owns | May depend on | Issue |
| --- | --- | --- | --- | --- |
| 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.

### How the Room boundary is actually held

Three mechanisms, and it matters that none of them is "people remember":

1. `core/data` depends on `core/database` with **`implementation`**, not `api`,
   so Room never reaches the compile classpath of anything above it.
2. `CycleRepository`'s constructor is **`internal`** — it names a
   `PeriodDatabase`, and a public constructor would force every caller to be
   able to name that type too. Callers use `CycleData.repository(context)`.
3. Repository reads return domain types. `Mappers.kt` is the one place an entity
   and a domain object meet.

Checkable, not merely intended: `grep -rn "androidx.room" app/src domain` is
empty, and `./gradlew :app:dependencies --configuration debugCompileClasspath`
lists Room zero times.

**No `fallbackToDestructiveMigration`.** It turns a forgotten migration into
silent data loss on update — here, a user's entire cycle history gone with no
error and no way back. A missing migration must be a crash in testing rather
than a wipe in production.

### Ordinary outcomes are values; only faults are exceptions

`period_records.startDate` is UNIQUE and inserts ABORT rather than REPLACE, so a
duplicate cannot destroy the original row. Both are right. The API around them
was not: `confirmPeriodStart` let `SQLiteConstraintException` out, and
`viewModelScope.launch` has no handler, so **tapping "Started today" twice
killed the app** — found on a device, not in a test.

The mistake was treating *already recorded* as an error. It is a completely
reasonable thing for a person to do twice when they are unsure the first tap
registered. So the period writes return `PeriodWriteResult` — `Added`,
`AlreadyRecorded`, `Updated`, `Conflict`, `NotFound` — and a genuine fault (full
disk, corrupt database) still throws, because that is not something a caller can
carry on from.

`Conflict` is refused rather than resolved: moving a record onto a date another
record already holds is a question only the user can settle, and merging would
delete a period they entered.

The ViewModel also installs a `CoroutineExceptionHandler` as a backstop. In a
health app a crash mid-write is adjacent to losing what the user just entered,
and a message they can read beats a process that vanished. The message carries
the exception *type* and never a record's contents — §45.

### One forecast stands at a time, and backfill is not a prediction

Two rules about `prediction_records` that are easy to get wrong and expensive to
notice, because both failure modes produce *plausible* accuracy figures rather
than obviously broken ones.

**Exactly one unscored snapshot exists at any moment.** Every recalculation
replaces the standing forecast instead of appending. A forecast superseded
before its outcome was known is not a wrong forecast — nobody was looking at it
when the period arrived — and counting it as one lets a single cycle contribute
several errors to §16's figures.

**A confirmed start only scores a forecast made on or before it.** Onboarding
asks for earlier periods and a user can add one at any time; scoring today's
forecast against a date in the past invents an error for a prediction nobody was
ever shown. This was a real defect, caught by a test expecting one scored
prediction and finding three, and it is pinned by
`backfilled history does not fabricate accuracy figures`.

### 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`.

### The guard, and the three proofs it survived

`checkModuleBoundaries` in the root `build.gradle.kts` holds the tables above as
a check. It runs as part of `./gradlew check`, lists **every** violation rather
than the first, and refuses to report a pass when it examined no modules at all.

Per [`GUARDS.md`](GUARDS.md) §1 it is not evidence until it has been watched
failing. These three are repeatable, each restores the file from a `trap`, and
each was run:

```bash
# 1. A domain module reaching upward — the leak that would end JVM-only tests
bash scripts/prove-guard.sh domain/prediction/build.gradle.kts \
  'implementation(project(":domain:cycle"))' \
  'implementation(project(":domain:cycle"))
    implementation(project(":core:database"))' \
  ./gradlew checkModuleBoundaries

# 2. :app reaching past the repository straight to Room
bash scripts/prove-guard.sh app/build.gradle.kts \
  'implementation(project(":core:data"))' \
  'implementation(project(":core:data"))
    implementation(project(":core:database"))' \
  ./gradlew checkModuleBoundaries

# 3. A module with no rule is reported as unmeasured, not assumed fine
bash scripts/prove-guard.sh build.gradle.kts \
  '":core:data" to setOf(":core:database", ":domain:cycle", ":domain:prediction"),' \
  '' \
  ./gradlew checkModuleBoundaries
```

**Proof 1 failed the first time, and that is the point.** The guard reported
*"7 modules checked, no violations"* with a forbidden dependency sitting in the
build file. The root project is configured before its subprojects, so reading
`subprojects.configurations` from the root script saw every configuration empty
— the check had been green over an empty map since the moment it was written.
Collection now happens in `afterEvaluate`, and the task throws rather than
passing if it ends up with no modules to examine.

Thirty seconds of proving caught a guard that would otherwise have been trusted
for months.

## Data shapes

Defined in `domain/cycle` and `domain/prediction` as plain Kotlin, plus
`UserPreferences` in `core/datastore`, and 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 — `toCycles()` recomputes from the period records on every read, so an edit cannot leave a stale interval behind it |
| `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 |
| `FertilityEstimate` | estimated ovulation and the fertile window around it | derived from the forecast, never stored, and **null when the forecast is too vague to locate ovulation** |
| `Interval` | one gap between confirmed starts, with its recency weight and whether it looks questionable | derived per calculation, never stored. A questionable interval is **down-weighted, never dropped** — §12 step 2 |
| `UserPreferences` | notification privacy, reminder time, lock, theme, ads entitlement | lives in DataStore, **never** in the cycle database — see below |

### Why settings are not in the database

`core/datastore` could have been two more Room tables. It is not, and the reason
is a deletion semantic rather than a taste in storage.

**Delete My Data removes the health history and must leave the settings alone.**
A user exercising that control has not asked to have notification privacy
returned to a default they did not choose — handing back a weaker setting at the
exact moment somebody is reaching for a privacy control is the worst possible
time to do it. Separate stores make that the easy implementation rather than the
one you have to remember.

`UserPreferencesRepository` takes a `DataStore` rather than a `Context`, which
is what lets its tests run on the JVM against a temporary file. The Android
instance is supplied by DI at the app layer — the only place that should know
where a file lives.

### Permissions are a declared set, not whatever the build produces

The Play listing shows them, the Data Safety form describes them, and a
privacy-first period tracker is judged on them before anybody opens the app. It
is also the list most likely to grow without anyone deciding to: adding
WorkManager brought four permissions in one line — `WAKE_LOCK`,
`ACCESS_NETWORK_STATE`, `RECEIVE_BOOT_COMPLETED`, `FOREGROUND_SERVICE` — none
typed by anybody.

`checkPermissions` in the root `build.gradle.kts` holds the allowed set and a
forbidden set, checks the **release** manifest as well as debug, and fails on
anything outside either. §31's exact-alarm permissions are in the forbidden list
by name.

It failed its own first proof too: without a `dependsOn` on the manifest task it
read whatever was left from a previous build, so an injected
`SCHEDULE_EXACT_ALARM` went unnoticed. Both directions are proved now.

### Notifications, and the two ways privacy leaks through Android

`NotificationCopy` is a pure function — mode plus kind plus day count in, two
versions of the text out — so every combination is tested without an emulator.
`PeriodNotifier` maps that onto Android, and the mapping is where the leaks are:

- **A private notification with no public version** does not blank the lock
  screen, it shows the private text. `setPublicVersion` is mandatory, which is
  why `NotificationText` has no nullable title.
- **A channel is immutable after creation.** Importance and lock-screen
  visibility cannot be changed, so one shared channel would keep whatever the
  user's first privacy mode set forever — the setting would appear to work and
  change nothing. There is one channel per mode. Found by an instrumented test,
  not by reading the docs.

### The prediction engine

`PersonalPredictionEngine` (`modelVersion` `personal-1`) is what the app ships.
It keeps a **discrete probability distribution over candidate start dates**
rather than a date with a margin bolted on, and everything the product needs
falls out of that one structure: the most likely date is its mode, the window is
the narrowest span holding 80% of its mass, and a "Not yet" is the distribution
being conditioned on what the user just said. A date-plus-margin design cannot
express that last one, which is why §13 is the reason for the shape.

Five decisions, each measured rather than assumed:

| Decision | Why |
| --- | --- |
| Weighted **median**, not mean | §51's outlier history moves a mean to 31.7 and leaves a median at 29 |
| **Laplace**, not normal | cycles have heavy tails; under a bell curve a period four days late is nearly impossible, so the model stays confidently wrong |
| Spread is the **larger** of MAD and mean-AD | a user alternating 25 and 37 has half her deviations at zero, so MAD alone reads a wildly variable cycle as perfectly consistent — a test caught exactly that |
| Recency trusted **in proportion to consistency** | recency weighting assumes the recent past predicts the near future, which is false for a variable user; weighting it equally cost three days on the §51 variable fixture |
| Trend damped, with a floor **relative to the user's own spread** | a fixed one-day floor fired on a 42-day-cycle history whose medians differed by one day and turned an exact forecast into a wrong one |

`BaselinePredictionEngine` stays in the tree as the **control**.
`EngineComparisonTest` scores both over the §51 fixtures on every build, so
"better" is a number. At the swap: mean absolute error **0.67 against 1.00**,
and the window contained the actual start **9 times out of 9 against 7**.

**Coverage is the measure, not width.** The personal engine's windows are wider,
and where they are wider they are right to be — a history with a suspected
missing period is genuinely less certain, and the baseline answers it with a
two-day window and misses. What a window promises is that the period starts
inside it; an engine keeping that promise 7 times in 9 has a broken promise, not
a tight forecast. The comparison test asserts coverage, with a ceiling so the
trivial cheat of answering "some time this month" still fails.

### Fertility declines rather than stretches

The fertile window is seven days wide before any uncertainty is added, so a
forecast carrying ±5 produces a seventeen-day window — over half a cycle. That
is honest arithmetic and useless information, and it was on screen before
anybody noticed: a user one cycle into the app was shown *8 Aug – 24 Aug*.

`FertilityEstimate.from` now returns **null** past
`MAX_USEFUL_UNCERTAINTY_DAYS`, and Today says so — *"Not enough history to
estimate"*, with a reason to keep logging. Three stable cycles later the same
user gets *12 Aug – 20 Aug*, which is worth reading.

The general rule this is an instance of: **the app declines rather than
stretches.** The same shape appears in `PredictionAccuracy` refusing figures
below three scored forecasts, and in `CycleInsights` withholding an average
until there are two intervals.

### Unusual is relative to the user, never to a constant

`IntervalAnalysis` decides whether a gap is odd by comparing it to a robust
centre of **this user's own** intervals. A global rule — "over 40 days is
suspicious" — gets exactly one group wrong, and it is the group whose cycles are
already unusual: the person this product exists for, and the one most tired of
apps assuming she is average. 45 days is unremarkable at a usual of 43 and worth
questioning at a usual of 29.

Two flags, because they earn different responses. A gap near a whole *multiple*
of the usual is a probable missed entry and produces §14's question. A gap merely
far from usual is down-weighted and left alone — asking about it would be the
over-questioning §25 warns against.

**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 | Guard |
| --- | --- | --- | --- |
| 1 | initial schema: `period_records`, `spotting_records`, `prediction_records`, `not_yet_observations` | — (first version) | `SchemaTest` + `scripts/schema-guard.sh` |

Room's exported schemas live in `core/database/schemas/` and are **committed**,
so a migration can be tested against the real previous schema rather than a
remembered one.

### The trap in this table, and the guard that closes it

**Room regenerates the schema export during compilation.** Change an entity
without bumping `PeriodDatabase.VERSION` and Room silently overwrites
`schemas/…/1.json` to match — so every in-process check compares two copies of
the new truth and passes. This was not reasoned about; it was proved, by adding
a column and watching the whole unit suite stay green while the committed schema
quietly changed underneath it.

The failure that produces on a device is `Room cannot verify the data
integrity` — a crash on update, in front of a user, after shipping.

`scripts/schema-guard.sh` is the guard, and it works by asking **git**, which is
the one party Room cannot overwrite: an already-committed schema file that now
differs means an entity changed under a shipped version. It runs in
`.githooks/pre-commit` whenever an entity or the schema directory is staged.

So: **adding a row to this table is part of changing a schema, not tidying up
afterwards.** The version bump, the migration, the new schema file and this row
belong in one commit.

## 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 |
| `scripts/schema-guard.sh` | a Room entity may not change without the version changing with it — asks git, because Room overwrites the export during the build |
| `checkNoHealthLogging` (root `build.gradle.kts`) | no logging call may exist in a module that can see a cycle date — §45. Strips comments and matches a call rather than the class, so the `Log.WARN` constant and the KDoc explaining the rule both stay legal |
| `.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.
