Privacy-Period-Tracker/docs/history/DEVELOPMENT_LOG.md

# Development log — Period

```
Status: Current
Owner: _null
Last reviewed: 2026-08-18
Governs: the dated record of what happened
Review trigger: Nothing. This file is appended to, never revised.
```

## How to use this

Newest first. **One entry per work session**, written before you stop — that is
step 6 of `docs/WORK_CYCLE.md`, and the two lines it insists on are `Next
action` and `Blockers`.

Those two are not decoration. The next session starts by reading the top of this
file, and a session that ended without saying what came next hands the one after
it a re-derivation instead of a starting point — which is where drift enters.
Neither line competes with anything: the live next action is the field on the
project at privacyllc.dev and the live blockers are issues in the tracker, while
these say what both were **at this date**. A record of then never disagrees with
a record of now.

**Append-only by convention.** Correcting an old entry rewrites the record of
what was known at the time, which is the one thing this file is for. If an entry
turns out to be wrong, add a later entry saying so; do not edit the first.

Note the Review trigger above says "nothing", deliberately. A dated log cannot
rot the way a description of current state can — the entries were true when
written and stay true. It is exempt from review for the same reason a receipt is.

## Entries

### 2026-08-18 — Batch 02 shipped: the prediction engine, and three bugs the tests found

All five Batch 02 issues closed and the milestone closed with them. The app now
ships the engine PRODUCT_PLAN.md §12 specifies rather than the prototype it was
explicitly labelled as.

**The shape is the design.** `PersonalPredictionEngine` keeps a discrete
probability distribution over candidate start dates, not a date with a margin.
The mode is the forecast, the window is the narrowest span holding 80% of the
mass, and a "Not yet" is the distribution conditioned on what the user said.
§13's requirement — that a "Not yet" updates the date, the window *and* the
confidence rather than shifting a fixed prediction by a day — is not extra work
in that design; it is the only thing that structure can do.

**It is better, as a number.** `EngineComparisonTest` scores both engines over
the §51 fixtures on every build: 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 — and measuring taught that.** The first
version of the comparison asserted the new windows must not be wider. It failed,
and it was the assertion that was wrong: where the personal engine is wider, it
is right to be, and the baseline answers a history with a suspected missing
period using a two-day window and misses. A window that misses is a broken
promise rather than a tight forecast.

**Three modelling bugs, each found by a test failing rather than by reading:**

1. Median absolute deviation alone reads a user alternating 25 and 37 days as
   perfectly consistent — half her deviations are zero. Twenty disagreeing
   cycles came back High, which is exactly the §15 rule about volume not buying
   confidence. Spread is now the larger of MAD and mean absolute deviation.
2. Recency weighting assumes the recent past predicts the near future. For a
   variable user that is false, and weighting it equally cost three days on the
   §51 variable fixture. Recency is now trusted in proportion to how much her
   cycles agree — which is a better statement of what recency weighting is
   actually for.
3. A fixed one-day floor on trend detection fired on a 42-day-cycle history
   whose medians differed by one day. One day is a real trend at 28 and rounding
   error at 42, so the floor is relative to the user's own spread.

That is now three consecutive pieces of work — the schema guard, the boundary
guard, and the engine — where the thing that found the defect was **running it,
not reading it**. Worth stating as a habit rather than a coincidence.

**Wired through, not just tested.** `PredictionInput` carries recent absolute
errors and the repository feeds scored errors back in. Without that the app
would store every error it makes and never read one back — measuring accuracy
rather than learning from it, with §12 step 5's widening happening only in a
unit test.

- **Closed:** #10, #11, #12, #13, #14 — and the `Batch 02 — Prediction Engine`
  milestone, which closing the last issue does not do.
- **Next action:** Batch 03 — Core UX. The onboarding flow in §19, the designed
  Today screen and its six dynamic states in §21–§22, period start and end
  logging in §23–§24, the calendar in §26 and Insights in §27. The working
  surface built in Batch 01 is deliberately ugly and says so on screen; it is
  the thing Batch 03 replaces. Before starting, file the Batch 03 issues — the
  milestone exists and is empty.
- **Blockers:** Unchanged and both need a person. #8, the three branding marks,
  which an agent must not fake. #9, the webhook, whose URL and secret are not
  readable from this machine — until it is registered an opened `P0` raises no
  alert at all. Neither blocks Batch 03.

### 2026-08-18 — Batch 01 foundation: seven of nine issues, and three guards that were wrong

Room, DataStore, the repository layer, period CRUD end to end on a device, and
the module boundary guard. #3 to #7 closed; #8 (branding) and #9 (webhook) are
the two that need a person rather than an agent.

**What was built.** `core/database` with the four entities from §10, DAOs
returning `Flow`, and the schema exported and committed. `core/datastore` for
settings, deliberately separate from the cycle database so Delete My Data cannot
reset a privacy choice the user made. `core/data` as the seam: domain types out,
cycles derived rather than stored, the forecast a function of the data instead
of a field somebody has to refresh. Hilt wiring and a working Today surface that
says "Batch 01 · working surface" so nobody mistakes it for the designed screen.

**Three guards were written, and all three were wrong at first.** This is the
day's real lesson and it is worth carrying forward:

1. `SchemaTest` looked like a Room schema-drift guard. Room regenerates the
   schema export during compilation, so both sides of every comparison agreed by
   construction — adding a column without bumping the version left it green.
   `scripts/schema-guard.sh` asks git instead, which Room cannot overwrite.
2. `checkModuleBoundaries` reported "7 modules checked, no violations" while
   checking nothing: the root project is configured before its subprojects, so
   every configuration read as empty. Caught by `prove-guard.sh` on its first
   run. Collection moved to `afterEvaluate`, and the task now throws rather than
   passing when it examined nothing.
3. The repository let `SQLiteConstraintException` escape into
   `viewModelScope.launch`, so **tapping the primary button twice killed the
   app** — found by hand on an emulator, with 70 unit tests green.

Each was caught by actually trying to break it. None would have been caught by
reading the code, and two of them would have been trusted for months.

**Two more bugs came from wiring the guard into `./gradlew check`**, which ran
Android lint for the first time: `LocalDate.ofInstant` and `LocalDate.EPOCH` are
API 34 and `minSdk` is 26. Both sit on the recalculation path — a crash on every
device below Android 14, invisible to the unit tests and to an API 36 emulator.

**QA.** Round 1 recorded as partial in `docs/qa/`. Passes A and B green, C and D
and H partial, the rest not run and each saying why.

- **Closed:** #3, #4, #5, #6, #7
- **Next action:** Batch 02 — replace `BaselinePredictionEngine` with the engine
  §12 specifies: recency weighting, a robust centre, variability-driven windows,
  trend detection, and "not yet" as a real conditioning step rather than a floor
  on the window. The §51 acceptance tests already exist and must keep passing
  against the new engine, which is what makes the replacement demonstrably
  better rather than merely different. Before that, one cheap thing worth doing:
  run the app once on a device at `minSdk` 26, because nothing here ever has.
- **Blockers:** None for code. #8 needs the three branding marks drawn — an
  agent must not fake them, so the project card shows an initials tile until
  somebody does. #9 needs the Command Center's webhook URL and secret, which are
  not readable from this machine; until it is registered an opened `P0` raises
  no alert at all.

### 2026-08-18 — Template adopted; Kotlin/Compose skeleton builds

Period went from a bare directory holding one specification file to a git
repository with the standard documentation tree, a tracker, and a project that
compiles. Adoption followed `Projects/Template/START-HERE-New-Project.md`.

**Documents.** `scaffold.sh` created 19 paths, 0 skipped. The specification moved
from `Docs/period_tracker_product_plan.md` to `docs/planning/PRODUCT_PLAN.md`
unchanged in substance, with a status header added; the capitalised `Docs/` is
gone, since every script and the Command Center expect the lowercase tree. Every
scaffolded document was filled in for Period rather than left with placeholders.
docs/OPERATIONS.md was deleted — an offline app is not a deployed service.
`docs/DOC_TRUST_MAP.md` was written last and describes what is actually here,
including a section naming what this project deliberately does **not** have.

**Code.** Four Gradle modules: `app`, `core/designsystem`, and `domain/cycle`
and `domain/prediction` as `kotlin("jvm")` so the engine is testable without an
emulator. 17 tests pass, 12 of them the acceptance cases from `PRODUCT_PLAN.md`
§51. `BaselinePredictionEngine` is a robust-median prototype and is explicitly
not the product — it exists so Batch 02's replacement can be shown to be better
rather than merely different.

**Three things that cost time and are worth knowing next session:**

- **AGP 9 ships Kotlin built in.** Applying `org.jetbrains.kotlin.android` is now
  a hard error, not a redundancy. The Compose compiler plugin is still separate.
- **Current AndroidX requires `compileSdk 37`.** Only up to 36 was installed;
  `platforms;android-37.0` and `build-tools;37.0.0` were installed into
  `~/Android/Sdk`. `targetSdk` stays at 36 — Play's floor from 2026-08-31 — and
  the two being different is deliberate, not an oversight to tidy up.
- **Versions were verified, not inherited.** Kotlin 2.4.10, AGP 9.3.1, Gradle
  9.7.0, Compose BOM 2026.08.00, Room 2.8.4, Hilt 2.60.1 — each checked against
  its official source today, which `PRODUCT_PLAN.md` asks for rather than
  trusting its own numbers.

**Tracker.** Eight milestones opened, `Batch 01 — Foundation` through
`Batch 08 — Polish`, and nine issues filed under Batch 01 only. Seven milestones
are deliberately empty: the roadmap is genuinely known and worth being visible,
but the work items under it are not, and inventing them would make every tracker
percentage permanently wrong. `forgejo-issue.py check` warns about this, and the
warning is correct about the mechanism and expected here.

- **Closed:** #1, #2
- **Next action:** Start issue #3 — core/database with Room entities for
  `PeriodRecord`, `SpottingRecord`, `PredictionRecord` and `NotYetObservation`,
  DAOs returning `Flow`, schema export committed, and a version-1 migration test
  that proves the harness works before there is a migration that matters. Its row
  goes in `docs/architecture/README.md`'s migration table in the same commit.
- **Blockers:** None for the code. Two things need a person rather than an agent:
  the three branding marks (#8), which cannot be drawn here and must not be
  faked, and the Command Center webhook (#9), whose URL and secret are not in any
  credential file readable from this machine. Without the webhook an opened `P0`
  raises no alert at all — it waits for the next reconcile.