§17 and §18. Ovulation is estimated a luteal phase before the PREDICTED next
period rather than counted forwards from the last one — the luteal phase is the
stable half of the cycle, which is why §17 asks for it that way — and the
fertile window opens five days before ovulation and closes one day after,
because sperm survive and the egg does not.
The uncertainty is inherited, not invented. Ovulation is derived from a
predicted date, so it can never be more certain than that prediction.
THE PART THE DEVICE TAUGHT
The first version showed a user one cycle in a fertile window of 8 Aug – 24 Aug.
Seventeen days. Arithmetically honest, and completely useless — over half a
cycle, dressed up as a feature.
So the estimate now returns null past a usable uncertainty, and Today says "Not
enough history to estimate. Log a few more cycles and the app will be able to
estimate ovulation." Three stable cycles later the same user gets 12 Aug – 20
Aug, which is worth reading. Verified in both states on a device.
That is the same shape as PredictionAccuracy refusing figures below three scored
forecasts and CycleInsights withholding an average below two intervals, and it
is now written down in the architecture doc as a rule rather than three
coincidences: the app declines rather than stretches.
§18'S PROHIBITION IS A TYPE, NOT A CONVENTION
FertilityLikelihood has LOWER, HIGHER and UNKNOWN and no fourth value. Somebody
reading "safe" would take a decision on it; the estimate comes from a predicted
date carrying days of uncertainty; and §18 has already promised this is not
contraception. A test asserts no label contains a permission word, so adding one
is a deliberate act with a failing test.
The disclaimer travels with the feature — same screen, same time. A disclaimer
one tab away is a disclaimer nobody read.
ANOTHER GREYSCALE COLLISION
The ovulation star was centred, which put it directly behind the numeral: in
greyscale "18" and the mark merged into one smudge. Ovulation is now the fertile
ring plus a small star low in the cell, which is also semantically right — that
day IS inside the window, and the pair reads as "that window, and this day".
Nine Compose ModifierParameter warnings fixed properly rather than suppressed.
164 tests, all passing. ./gradlew check green, 0 lint errors.
closes#21closes#22closes#23
§27's purpose is one sentence — show the user what the app has learned — which
means the screen has to be honest about how little that sometimes is. Every
figure is absent rather than approximated below the history that supports it,
because the easiest way to overstate accuracy is to average two numbers and
print a decimal place.
- one interval is an anecdote, not an average: no "average cycle" until two
- accuracy figures wait for three scored forecasts, and say why they are
waiting rather than showing a mean of one
- §27's learning copy is chosen by what the data supports, never by mood.
"Personalized to your cycle" is a claim, and it appears only when there are
enough confirmed cycles for the forecast to genuinely be hers
A BUG THE OUTLIER TEST CAUGHT
The typical-range quartiles were indexed off `size` instead of `size - 1`, which
on an even-length list puts the upper index on the largest value. A history of
29, 28, 30, 29, 61, 29 reported a "typical range" of 29–61 — describing a
regular cycle as wildly erratic, on the one screen whose whole job is to say
what has been learned about her. Now 29–30.
Nothing on this screen leaves the device. §46 names prediction_error= among the
values that must never become an analytics event, and this is exactly the screen
that would tempt somebody to send one. There is no network call in these files
and there must never be.
153 tests, all passing. ./gradlew check green.
closes#20
§26's four states plus spotting, distinguished by SHAPE: a solid disc for a
period you logged, a dotted ring for one predicted, a small low dot for
spotting, a continuous ring for the fertile window, a four-pointed mark for
ovulation. Verified the way the requirement is written — the screenshot was
converted to greyscale and every mark is still tellable apart.
Predicted is never a lighter confirmed. §26 forbids them looking identical, and
opacity alone fails exactly when the screen is dim or the user is not looking
closely.
TWO PROBLEMS THE GREYSCALE CHECK FOUND
Today was drawn as an underline under the numeral. Spotting is a dot low in the
same cell. On the one day that was both — which was, of course, the day I looked
at — they merged into an unreadable smudge. Today is now an outline around the
whole cell, which cannot overlap anything drawn inside it.
The legend's spotting row was misaligned for the same reason: a mark anchored to
the bottom of its box next to vertically centred text. Fixed by moving the dot
up off the floor of the cell.
Neither was visible in the colour screenshot. Both were obvious in greyscale,
which is the argument for doing the check the way §43 describes rather than
assuming shapes differ because they were designed to.
Marks are derived per call, never stored: a calendar_days table would be a
second copy of facts the period records and the forecast already hold, stale the
moment either changed. An unclosed period marks only its start day, because
filling forward to today would draw days the user never said were period days.
Every day carries a content description — "18, spotting, today" — because §43 is
right that a grid of bare numerals carries none of a calendar's information.
Weekday initials repeat, so those are labelled too.
Tapping a day logs or removes an entry there; future days do nothing rather than
opening a sheet whose every action is disabled.
145 tests, all passing. ./gradlew check green.
closes#19
§21 and §22, each state its own screen rather than a variant of one. Which one
applies is decided by CycleStatusRules in a pure module with twelve tests on its
boundaries — and the boundaries are the point, because they are the days this
screen is most read: the day a period is due, the day after one ends, the day a
forecast slips.
The number dominates (§38): displayLarge at 72sp, in the primary colour, with
the unit as a separate quiet line so "4" reads instantly and "DAYS" is there if
you look. Its content description carries the whole sentence, so TalkBack says
"Period likely in: 4 days" rather than reading a bare numeral.
The state this screen exists to get right is the last one. Past the forecast the
app NEVER says late — late implies a schedule the user failed to keep, and the
truth is that an estimate was imprecise. It shows what it originally said, what
it says now, and asks.
A UX DEFECT FOUND BY DRIVING IT
Tapping "Period ended" changed nothing on screen. The logic was right — a period
that ends today still includes today, so the state does not change — but the
button looked broken, which is worse than being broken somewhere visible.
DuringPeriod now carries the end date, so the screen shows "Ended 18 August" and
offers only the useful action (undo) rather than a button that visibly does
nothing. §24's "Updated ✓" acknowledgement is there too. No test would have
caught this; it needed somebody to tap the button and look.
The banner slot is reserved and empty. §48 wants no layout jump when an ad loads
and a graceful gap when one fails, and both are properties of the space existing
whether or not it is filled — reserving it in Batch 07 instead means shipping
the jump first. Deliberately not a "your ad here" box, which would be a
placeholder for the thing a user pays to remove.
Fertility lines are absent rather than faked: §22 shows them and Batch 04
estimates them, and a placeholder number there would be inventing a fertility
estimate, which is the one thing this screen must not do.
Preview pairs for every state, light and dark. 129 tests, all passing.
closes#17
PersonalPredictionEngine keeps a discrete probability distribution over
candidate start dates rather than a date with a margin bolted on. 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 the mass, and a "Not yet" is the
distribution conditioned on what the user just said — which is what §13 asks for
and what a date-plus-margin design cannot express at all.
It is better, and that is a number rather than an opinion. EngineComparisonTest
scores both engines over the §51 fixtures on every build:
engine MAE mean window within +/-2 window covered
baseline 1.00 2.67 7/9 7/9
personal 0.67 4.56 9/9 9/9
COVERAGE IS THE MEASURE, NOT WIDTH
The first version of that test asserted the new windows must not be wider, and
it failed. Measuring showed why the assertion was wrong: the fixtures where the
personal engine is wider are the ones that are genuinely less certain — a
history with a suspected missing period, and one with a 45-day outlier — and the
baseline answers both 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 test now asserts coverage, with
a ceiling so "some time this month" still fails.
THREE MODELLING BUGS THE TESTS FOUND
Each was found by a test failing, not by reading the code:
- Median absolute deviation alone reads a user alternating 25 and 37 as
perfectly consistent, because half her deviations are zero. Twenty
disagreeing cycles came back High, breaking §15's rule that volume alone
must never buy High confidence. Spread is now the larger of MAD and mean
absolute deviation; robustness comes from IntervalAnalysis down-weighting
what is questionable, which is a better place for it.
- Recency weighting assumes the recent past predicts the near future. For a
variable user that is false — her latest cycle is a draw from a wide
distribution, not a signal — and weighting it equally cost three days on the
§51 variable fixture. Recency is now trusted in proportion to how much her
cycles actually agree.
- A fixed one-day floor on trend detection fired on a 42-day-cycle history
whose medians differed by a single day, turning an exact forecast into a
wrong one. One day is a real trend at 28 and rounding error at 42, so the
floor is relative to the user's own spread.
WIRED THROUGH, NOT JUST TESTED
PredictionInput carries recentAbsoluteErrors, and CycleRepository feeds the
scored errors back in. Without that the app stores every error it makes and
never reads one back — measuring accuracy rather than learning from it, with the
widening happening only in a unit test. A repository test asserts the errors
actually reach the engine.
BaselinePredictionEngine stays as the control, and both engines run the same
§51 acceptance suite, so the next engine's improvement is measurable too.
108 tests, all passing. ./gradlew check green. Verified on a device.
closes#10closes#11closes#12closes#14
IntervalAnalysis turns confirmed starts into weighted intervals and decides what
looks questionable — relative to this user's own history, never to a constant.
That distinction is the whole point. A global "over 40 days is suspicious" rule
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. Both are tests.
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 would be the
over-questioning §25 warns against, and §51's 45-against-29 outlier is exactly
that case.
Nothing is ever dropped. §12 step 2: unusual data is marked for review or given
less influence, never silently deleted. A questionable interval keeps a quarter
of its recency weight, and a test asserts it is still present and still counts.
Recency decay is here too, ready for the centre in #10: newest cycle weight 1.0,
each older one 0.85 of the last.
12 new tests. The §51 acceptance cases still pass unchanged.
closes#13
Period was a bare directory holding one 2,527-line specification, with no git
repository, no tracker and no documentation convention. This is the adoption
from Projects/Template/START-HERE-New-Project.md, plus a project that compiles
so the hooks and future guards have something real to run against.
Documents. scaffold.sh created 19 paths, 0 skipped. The specification moved to
docs/planning/PRODUCT_PLAN.md unchanged in substance, with a status header; the
capitalised Docs/ is gone. Every scaffolded document was filled in for Period.
docs/OPERATIONS.md deleted — an offline app is not a deployed service.
DOC_TRUST_MAP.md written last, describing what is actually here, including what
this project deliberately does not have.
Code. Four Gradle modules. domain/cycle and domain/prediction are kotlin("jvm")
and cannot see the Android SDK, 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 explicitly not the
product; it exists so Batch 02's replacement can be shown to be better rather
than merely different.
Versions verified against their official sources today rather than inherited
from the specification's own numbers, which that document asks for: 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. AGP 9 ships Kotlin built in, so org.jetbrains.kotlin.android is no
longer applied. compileSdk is 37 because current AndroidX requires it; targetSdk
stays 36, Play's floor from 2026-08-31, and the difference is deliberate.
Six scripts taken into scripts/; the rest declined and named in docs/TOOLS.md.
Three hooks in .githooks/, with pre-commit adapted to Gradle.
closes#1closes#2
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