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budget2.0/docs/DECISIONS.md
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Lucas WintherandClaude Opus 5 8d2b2ca881 feat(money): one decimals setting for the dataset, not one per currency 
`Settings.digits` replaces `CurrencyDef.digits`. Every amount rounds to it,
whatever its currency, and `digitsFor` is gone: the sixteen call sites read the
setting, and the domain functions that took a currency table purely to look
digits up take a `digits: number` instead. Schema 6.

The owner asked for this with the cost in front of them, and the cost is real:
a dataset mixing minor units cannot be described any more. Yen at 0 beside
kroner at 2 has no correct global value — 2 gives ¥1234.56, an amount yen
cannot represent; 0 rounds 49.95 DKK to 50 and drifts the carry from the bank.
Nothing in the app warns. docs/specs/2026-09-02-global-decimals-design.md
argues it in full, including what re-adding a per-currency override would take,
and AGENTS.md §1 now says not to reintroduce one without revisiting the
decision.

The 5 -> 6 step takes the MAXIMUM of the per-currency digits, not the base
currency's. A dataset holding a three-decimal currency already has
three-decimal amounts in it, and adopting the base's 2 would truncate the next
edit of one; the maximum can only keep more precision, never less. On every
dataset this app has written all four currencies record 2, so nothing rounds
differently than before.

Changing the setting does not rewrite stored amounts. 49.95 stays 49.95 and
shows as 50; totals recompute at the new precision. Rewriting each Money is a
destructive whole-dataset write, which §1 owes a backup export first, and it is
lossy one way.

Display turned out to be the hole in the plan. src/ui/format.ts pinned
minimumFractionDigits: 2 in a module-level Intl.NumberFormat, so it never read
the dataset at all — equally broken under per-currency digits, where a JPY
rendered "3,333.00", so a pre-existing gap rather than a regression. Shipping a
Settings control that display ignores would look broken at 0 and be lossy at 3,
hiding precision the dataset holds. So format.ts takes digits like roundMoney
does, useMoneyFormat() binds them for the 64 call sites, and postRowCells takes
the formatter as an argument because it is a plain helper, not a component.

currencyDigits.test.ts is rewritten rather than deleted. Driving a dataset at
ZERO decimals through rounding, both FX directions, both split modes, plan
division, the fold and the month view still tells a real implementation from
one assuming 2 — it just cannot prove per-currency flow, because there is none.
It also asserts that raising the setting to 2 changes what the fold produces.

Verified in Chrome, not only in tests: a v5 dataset with digits 2/0/3 written
straight into IndexedDB migrates to a setting of 3, the table loses its
Decimals column, and editing the control persists schema 6 with digits stripped
from each currency and income left at 20000.55. The same month reads 600/206 at
zero decimals and 600.000/205.561 at three, so the figures recompute rather
than merely reformat.

Not verified here: the .ods in a real spreadsheet.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
2026-09-03 03:12:14 +02:00

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# Decisions
An index of the choices that shaped this codebase, and what each one was chosen
*over*. One or two lines each — enough to know whether a decision is the one you
are looking for.
This file does not argue them. The spec named beside each entry does, including
the alternatives in full; where an entry has no spec, the commit message is the
record.
## Product and storage
- **IndexedDB is the only store. No server, no account, no sync.** A deliberate
product position, not a missing backend: the data never leaves the browser.
→ [`specs/2026-09-01-budget-app-design.md`](specs/2026-09-01-budget-app-design.md),
[`PRODUCT.md`](PRODUCT.md)
- **Single user, permanently.** One person, one browser, one dataset. No roles,
no household mode. → [`PRODUCT.md`](PRODUCT.md)
- **Schema migrations are mandatory, not optional.** `schemaVersion` plus
ordered migration functions applied on load, because IndexedDB holds the
user's only copy of their financial data. Now at version 6.
→ [`specs/2026-09-01-budget-app-design.md`](specs/2026-09-01-budget-app-design.md)
- **A destructive write downloads a backup first.** Import and reset both export
before replacing, and abort if the export fails. → commit `ac3d18a`
- **Reset restores first-run state, not an empty dataset.** The seed posts, the
seed currency table and the baked rates come back, because "reset" and "a
brand-new browser" should be the same call. → commit `ac3d18a`
## Money and the data model
- **Money is a float, not an integer of minor units.** Made safe by two rules
rather than by the type: round at every boundary, and compute a distributed
last part as `total - sum(others)`.
→ [`specs/2026-09-01-budget-app-design.md`](specs/2026-09-01-budget-app-design.md)
- **`roundMoney` takes a digit count, and the argument is required.** Rejected a
default of 2: it is correct while the decimals setting is 2, so a missed call
site would pass every test and fail the moment it moves.
→ [`AGENTS.md`](../AGENTS.md) §1, commit `cdece63`
- **Decimal places are ONE dataset-wide setting, not per currency.**
`Settings.digits`; `CurrencyDef.digits` is gone. Chosen over a per-currency
default and over display-only rounding, and knowingly over correctness for
mixed minor units: a dataset holding both yen and kroner can no longer be
described, and the app does not warn. The owner asked for the simpler model
with that trade-off in front of them.
→ [`specs/2026-09-02-global-decimals-design.md`](specs/2026-09-02-global-decimals-design.md)
- **`Currency` is an open string code; `Dataset.currencies` is the authority**
on which codes exist, their names and their symbols — but no longer on their
decimals. Rejected a closed union, which would have made "add a currency" a
code change. The compile-time safety is replaced by validation at the
boundaries — `parseDatasetJson` rejects a code no currency defines.
→ `src/domain/types.ts`, commit `cdece63`
- **Every derived number comes from one left fold.** `src/domain/fold.ts` is the
only implementation; year and summary views aggregate over it rather than
recomputing. Two screens disagreeing about someone's money is the worst
failure this app has. → [`ARCHITECTURE.md`](ARCHITECTURE.md)
- **Dates widened rather than restructured.** `Purchase.date` may be
`"YYYY-MM-DD"` or `"YYYY-MM"`. Rejected splitting it into `month` plus
optional `day` — the more honest model, and the right call greenfield, but it
changes a type every layer depends on and needs a migration run in the browser
against the user's only copy, for an identical user-visible outcome.
→ [`specs/2026-09-02-fast-entry-refinements-design.md`](specs/2026-09-02-fast-entry-refinements-design.md)
## Allocation rules
- **A post's allocation is a dated series, and per-month overrides still
exist.** Both survive because they answer different questions: "from now on"
versus "just this month". A month override wins outright.
→ [`specs/2026-09-02-versioned-allocation-rules-design.md`](specs/2026-09-02-versioned-allocation-rules-design.md)
- **No rule yet means an allocation of zero.** Rejected applying the earliest
rule backwards, which invents budget for months the user never budgeted.
→ [`specs/2026-09-02-versioned-allocation-rules-design.md`](specs/2026-09-02-versioned-allocation-rules-design.md)
- **The series lives on the post.** Rejected keeping `standingRule` alongside a
map of later changes — it cannot express the history.
→ [`specs/2026-09-02-versioned-allocation-rules-design.md`](specs/2026-09-02-versioned-allocation-rules-design.md)
- **Posts are archived, never deleted**, because purchases reference them.
Archived posts still fold. → [`ARCHITECTURE.md`](ARCHITECTURE.md)
## Design system
- **Two token layers.** `--budget-*` is the app's own vocabulary; shadcn's
entire variable set points at it, so every primitive inherits both themes.
→ [`specs/2026-09-02-design-foundation-design.md`](specs/2026-09-02-design-foundation-design.md)
- **The brand teal is `--primary`, never `--accent`.** shadcn's `--accent` is a
hover *background* whose paired foreground must move with it; a saturated
colour there turns every hover into a slab with the wrong text on it.
→ [`specs/2026-09-02-design-foundation-design.md`](specs/2026-09-02-design-foundation-design.md)
- **Contrast was computed and read back out of the browser**, not chosen by eye.
Two candidate field borders were rejected at 2.17:1 and 2.55:1.
→ [`specs/2026-09-02-design-foundation-design.md`](specs/2026-09-02-design-foundation-design.md)
- **Fonts are self-hosted and injected at boot from `src/fontCss.ts`.** Rejected
`@font-face` in the bundled stylesheet (Bun inlines woff2 as base64 either
way: 70 KB became 303 KB of render-blocking bytes and defeated the subsetting)
and rejected a separate `<link>`ed stylesheet (works in a build, but the dev
server has no `external` option, so dev would lie about typography). A CDN was
rejected on product grounds: a font request to another origin contradicts
"the data never leaves the browser".
→ [`specs/2026-09-02-design-foundation-design.md`](specs/2026-09-02-design-foundation-design.md)
- **Theme preference lives in `localStorage`, not in the dataset.** The dataset
is exported and imported, and restoring a desktop backup must not flip a phone
to dark. Cycles system → light → dark, because "follow the system" is a real
answer and there must be a way back to it.
→ [`specs/2026-09-02-design-foundation-design.md`](specs/2026-09-02-design-foundation-design.md)
- **Two markups per responsive table, not one reflowing grid.** A real `<table>`
where it is a table, a real list where it is not. Rejected one CSS grid
carrying `role="table"`: it deduplicates the markup but describes rows and
columns to a screen reader that no longer exist on screen.
→ [`specs/2026-09-02-month-view-design.md`](specs/2026-09-02-month-view-design.md)
- **A table states its currency once, not per cell.** Every figure in the post
table is base currency; repeating the code only wrapped figures onto two lines
on a phone. `formatMoney` still applies wherever the currency can vary.
→ [`specs/2026-09-02-month-view-design.md`](specs/2026-09-02-month-view-design.md)
- **Meter geometry is pure and clamped.** All meter arithmetic lives in
`src/ui/meterSegments.ts`; negatives clamp rather than invert, because a
segment of negative width puts a gradient's stops out of order and paints
something arbitrary instead of failing.
→ [`specs/2026-09-02-month-view-design.md`](specs/2026-09-02-month-view-design.md)
- **The allocation meter scales to `max(income, allocated)`.** Over-budget is
always allowed, so over-allocation has to be drawable; capping at a full bar
would make it look identical to allocating income exactly.
→ [`specs/2026-09-02-month-view-design.md`](specs/2026-09-02-month-view-design.md)
## Build, offline and deploy
- **Bun is the runtime, package manager, bundler and test runner.** shadcn/ui
components are vendored into the repo and owned by this project.
→ [`specs/2026-09-01-budget-app-design.md`](specs/2026-09-01-budget-app-design.md)
- **The service worker and manifest are hand-written.** No PWA plugin exists for
Bun, so `build.ts` emits the precache list from `Bun.build`'s outputs, keeping
hashed filenames in sync automatically.
→ [`specs/2026-09-01-budget-app-design.md`](specs/2026-09-01-budget-app-design.md)
- **A new build prompts, and calls `skipWaiting` only on user action.** Without
it a stale service worker pins the user to an old version indefinitely.
→ [`specs/2026-09-01-budget-app-design.md`](specs/2026-09-01-budget-app-design.md)
- **FX rates are baked at build time, and seed a new dataset only.** They are
never a fallback for a missing rate: clearing a rate must surface
`MissingRateError` rather than silently converting at a number the owner never
chose. On a network failure the build falls back to committed constants marked
`source: "manual"`, so Settings never claims a hardcoded number came from an
API. → commit `b4e1933`
- **The rate service must be a host that answers directly, not one that
redirects.** `api.frankfurter.app` began 301-ing to `api.frankfurter.dev/v1`,
and a redirect response has to carry `Access-Control-Allow-Origin` itself for
a browser to follow it cross-origin — that 301 carries none, so "Fetch rates
now" died with only a CORS error to go on. The build-time fetch never noticed,
because a server-side fetch follows redirects freely. → commit history, and
`src/store/fxApi.ts`
- **A migration must never read today's defaults.** The 2 -> 3 step seeded the
live seed-currency table by reference, so adding sterling to that table would
have retroactively changed what an old dataset got from a step it had already
been through — silently, and only for whoever had not migrated yet. Each step
now carries its own frozen copy, and a source-level guard fails the build if a
live default is imported again. → `src/store/migrations.ts`
- **Sterling ships with the app, and is migrated into datasets that already
exist.** Baked currencies and rates seed a NEW dataset and nothing else, so
the seed table alone would have left every existing user typing the code, name
symbol and decimals by hand. The 4 -> 5 step supplies the rate as well, which
is the one thing the "rates never backstop a cleared rate" rule permits here:
it forbids re-supplying a number the owner deleted, not introducing one for a
currency they have never had. A GBP the owner already defined is left exactly
as it is. → `src/store/migrations.ts`
- **The v3 -> v4 migration DROPS the stale URL rather than rewriting it** to the
new endpoint, so a dataset that stored it follows whatever the current default
is and the next move of the service needs no second migration for the same
user. A URL the owner chose themselves is left strictly alone.
→ `src/store/migrations.ts`
- **ODS export is generated client-side with `fflate`.** No server exists to
generate it. → [`specs/2026-09-01-budget-app-design.md`](specs/2026-09-01-budget-app-design.md)
- **Base path is a build-time variable, read through a `try/catch` accessor.**
Rejected a `typeof process` guard: it survives Bun's inlining and discards the
inlined value, silently breaking every subpath deploy.
→ [`AGENTS.md`](../AGENTS.md) §2, `src/ui/basePath.ts`
## Things deliberately left undone
Not oversights. Each was decided against, and reopening one needs a reason.
- Over-budget, percentages past 100%, splits that do not sum, and negative
amounts are all **allowed**. Adding validation is a regression.
→ [`AGENTS.md`](../AGENTS.md) §3
- The income field and the split editor's fixed-amount fields **do not parse a
currency**; only the purchase total does. → [`TODO.md`](TODO.md) §3
- **No currency picker.** Adding a currency means typing code, name, symbol and
decimals by hand. → [`TODO.md`](TODO.md) §3
- **Symbol collisions resolve to the first currency defined with that symbol.**
Tested and documented rather than solved. → [`TODO.md`](TODO.md) §3
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