@0xdivyanshh/tab-money

@0xdivyanshh/tab-money

Tier 0 · zero runtime dependencies · zero I/O · no floating point

Every monetary amount in Tab, and every rate applied to one.

Why this is its own package

verify-tab claims the ledger reconciles to the cent. Tab also multiplies amounts by fractions repeatedly — a 0.6 unattested-revenue weight, tier multiples, a ramp stepping ±15%/30%, interest accrual per window. Many small numbers, repeated fractional multiplication, exact reconciliation required: that is the exact shape of problem where doubles produce a ledger a few micro-USDC out that nobody can explain at 2am.

Zero runtime dependencies means this package is auditable in one sitting, and everything else rests on it. See ADR-0006.

Contents

The rules

• No number ever holds an amount. MicroUsdc is bigint branded, so a raw bigint — a timestamp, a sequence number, a hop count — cannot be passed where an amount belongs. This has already caught two real slips in apps/web.

• Rates are integer basis points. A 0.6 weight is bp(6000). A 40% concentration cap is bp(4000).

  This is not cosmetic. share > 0.4 in floats can flip at exactly 40% depending on how the ratio was derived, because 0.4 is not representable. shareBp > 4000 cannot. The concentration cap is a spend decision, so that difference is a refusal that should have fired and didn't.

• mulBp requires a rounding direction. mulBp(amount, rate, 'down'), never a default. Rounding is a decision; a default is an accident, and interest and the ramp both compound.

• Display truncates, never rounds up. The docs promise "truncated, not rounded", so the code means it: 0.999999 shows as 0.9999, never 1.0000.

• Display uses U+2212 MINUS, not a hyphen — same width as a digit in tabular figures, so signed columns stay aligned.

• Wire form keeps all six decimals (toWire). JSON.stringify cannot handle bigint, so amounts cross every boundary as decimal strings. This is also what makes the HCS ceiling input hash byte-stable.

Consumers

apps/web imports it through @/lib/money, which re-exports this package and adds one presentation-only helper. There is exactly one money implementation in the repo; a second would drift, and a display layer that rounds differently from the ledger is how "reconciles to the cent" stops being true on screen.

Still to do

• formatBpMultiple is written but unused; the tier display in the landing playground uses it.
• No property-based tests yet. The invariants worth generating over: mulBp never leaves [0, amount] for bp ≤ 10000, parse(format(x)) === x, and summing is associative. The first and third have example-based coverage; the second needs toWire round-tripping.
• Interest accrual and ramp rounding directions must be pinned in @0xdivyanshh/tab-params when that package is written, not chosen at each call site.

node --experimental-strip-types --test src/*.test.ts