Wallets and Balances

// money.ts: integers in minor units, a currency with an exponent, named rounding, formatting last
export type Currency = 'NGN' | 'USD' | 'EUR' | 'JPY' | 'BTC'
const EXP: Record<Currency, number> = { NGN: 2, USD: 2, EUR: 2, JPY: 0, BTC: 8 }
export type Money = { readonly amount: bigint; readonly currency: Currency }      // never a float; never a string with a point

export const add = (a: Money, b: Money): Money => { assertSame(a, b); return { amount: a.amount + b.amount, currency: a.currency } }
export const sub = (a: Money, b: Money): Money => { assertSame(a, b); return { amount: a.amount - b.amount, currency: a.currency } }

export type Rounding = 'HALF_EVEN' | 'HALF_UP' | 'DOWN'
function divRound(n: bigint, d: bigint, mode: Rounding): bigint {           // one rounding, at a named point, with a named mode
  const q = n / d, r = n % d, twice = r * 2n
  if (mode === 'DOWN' || r === 0n) return q
  if (mode === 'HALF_UP') return twice >= d ? q + 1n : q
  return twice > d ? q + 1n : twice < d ? q : (q % 2n === 0n ? q : q + 1n)   // HALF_EVEN
}
// a rate as a scaled integer: 0.21% = { num: 21n, den: 10000n }; an FX quote ₦1,580.4321/$ = { num: 15804321n, den: 10000n }
export const applyRate = (m: Money, rate: { num: bigint; den: bigint }, mode: Rounding, currency = m.currency): Money =>
  ({ amount: divRound(m.amount * rate.num, rate.den, mode), currency })
export const fee = (gross: Money) => applyRate(gross, { num: 21n, den: 10000n }, 'HALF_UP')          // the rule names the mode; the receipt shows the result

// formatting, last; digits from the integer, locale from Intl; never parsed back
export function format(m: Money, locale = 'en-NG'): string {
  const exp = EXP[m.currency]; const neg = m.amount < 0n; const abs = neg ? -m.amount : m.amount
  const s = abs.toString().padStart(exp + 1, '0'); const major = s.slice(0, s.length - exp), minor = s.slice(s.length - exp)
  const str = exp ? `${major}.${minor}` : major                                                    // a decimal string, not a float
  return new Intl.NumberFormat(locale, { style: 'currency', currency: m.currency, minimumFractionDigits: exp, maximumFractionDigits: exp }).format(neg ? `-${str}` as any : str as any)
}
// input: parse(locale, "250.000,00") → strip group separators by locale, split on the locale's decimal, accept ≤ exp decimals → 25000000n