Monetary Patterns

// User types "150.50" — convert to integer cents ONCE on submit
export function userInputToCents(input: string): number {
  const parsed = parseFloat(input);
  if (isNaN(parsed) || parsed <= 0) throw new Error('Invalid amount');
  return Math.round(parsed * 100); // Math.round handles float imprecision
}

Display Cents → String (formatting only, never do math on this)

import { dinero, toDecimal } from 'dinero.js';
import { EGP, USD } from '@dinero.js/currencies';

export function centsToDisplay(cents: number, currencyCode: string): string {
  const map: Record<string, typeof EGP> = { EGP, USD };
  const currency = map[currencyCode] ?? EGP;
  const d = dinero({ amount: cents, currency });
  return toDecimal(d, ({ value, currency }) =>
    `${currency.code} ${parseFloat(value).toFixed(2)}`
  );
}
// centsToDisplay(15050, 'EGP') → "EGP 150.50"

Equal Split (distributes remainder fairly)

import { dinero, allocate, toSnapshot } from 'dinero.js';
import { EGP } from '@dinero.js/currencies';

// 100 EGP / 3 people → [3334, 3333, 3333] cents — first person gets the extra cent
export function splitEqually(totalCents: number, count: number): number[] {
  const d = dinero({ amount: totalCents, currency: EGP });
  const ratios = Array(count).fill(1);
  return allocate(d, ratios).map((share) => toSnapshot(share).amount);
}
// splitEqually(10000, 3) → [3334, 3333, 3333] — sum = 10000 ✓

Exact Split Validation

export function validateExactSplit(
  totalCents: number,
  splitAmounts: number[]
): { valid: boolean; difference: number } {
  const splitTotal = splitAmounts.reduce((s, a) => s + a, 0);
  return {
    valid: splitTotal === totalCents,
    difference: totalCents - splitTotal,
  };
}

Forbidden Patterns

// ❌ Float arithmetic
const share = 100.0 / 3; // → 33.333...

// ❌ Storing float in DB
{ amount: 150.50 }

// ❌ JS multiplication without rounding
const fee = 1.10 * 0.05; // → 0.055000000000000007

// ✅ Everything in integer cents
const totalCents = 15050;
const feeCents = Math.round(totalCents * 0.05);
const halves = splitEqually(totalCents, 2);