turash/bugulma/frontend/lib/fin/index.ts

// Generic finance utilities for formatting and arithmetic
export type Currency = string; // ISO 4217, e.g. 'EUR', 'USD'

export interface FormatOptions {
  locale?: string; // defaults to 'en-US'
  currency?: Currency; // defaults to 'EUR'
  minimumFractionDigits?: number;
  maximumFractionDigits?: number;
}

const DEFAULT_LOCALE = 'en-US';
const DEFAULT_CURRENCY: Currency = 'EUR';

export const formatNumber = (value: number, locale = DEFAULT_LOCALE) => {
  return new Intl.NumberFormat(locale).format(value);
};

export const formatCurrency = (value: number | bigint, opts?: Partial<FormatOptions>) => {
  const locale = opts?.locale ?? DEFAULT_LOCALE;
  const currency = opts?.currency ?? DEFAULT_CURRENCY;
  const minimumFractionDigits = opts?.minimumFractionDigits ?? 0;
  const maximumFractionDigits = opts?.maximumFractionDigits ?? 0;

  // If passed a bigint, treat it as cents and convert to major units if maximumFractionDigits>0
  let numericValue: number;
  if (typeof value === 'bigint') {
    numericValue = Number(value) / Math.pow(10, maximumFractionDigits);
  } else {
    numericValue = value;
  }

  return new Intl.NumberFormat(locale, {
    style: 'currency',
    currency,
    minimumFractionDigits,
    maximumFractionDigits,
  }).format(numericValue);
};

// A small helper to avoid floating point rounding issues by using integer cents
export class Money {
  // internal representation: cents for a 2-decimal currency by default
  private cents: bigint;
  private readonly scale: number; // number of fraction digits (e.g. 2 for cents)

  constructor(amount: number | bigint, scale = 2) {
    this.scale = scale;
    if (typeof amount === 'bigint') {
      this.cents = amount;
    } else {
      // convert float to scaled integer
      const multiplier = Math.pow(10, this.scale);
      this.cents = BigInt(Math.round(amount * multiplier));
    }
  }

  static fromCents(cents: bigint, scale = 2) {
    const m = new Money(0, scale);
    m.cents = cents;
    return m;
  }

  add(other: Money) {
    this._assertSameScale(other);
    return Money.fromCents(this.cents + other.cents, this.scale);
  }

  sub(other: Money) {
    this._assertSameScale(other);
    return Money.fromCents(this.cents - other.cents, this.scale);
  }

  mul(factor: number) {
    const result = Number(this.cents) * factor;
    return Money.fromCents(BigInt(Math.round(result)), this.scale);
  }

  div(divisor: number) {
    const result = Number(this.cents) / divisor;
    return Money.fromCents(BigInt(Math.round(result)), this.scale);
  }

  toNumber() {
    return Number(this.cents) / Math.pow(10, this.scale);
  }

  toCents(): bigint {
    return this.cents;
  }

  format(opts?: Partial<FormatOptions>) {
    const maximumFractionDigits = opts?.maximumFractionDigits ?? this.scale;
    return formatCurrency(this.cents, {
      locale: opts?.locale,
      currency: opts?.currency,
      minimumFractionDigits: opts?.minimumFractionDigits ?? this.scale,
      maximumFractionDigits,
    });
  }

  private _assertSameScale(other: Money) {
    if (other.scale !== this.scale) {
      throw new Error('Money scale mismatch');
    }
  }
}

export default {
  formatCurrency,
  formatNumber,
  Money,
};