export type Decimal = { value: bigint; scale: number };

export function decimalFromString(value: string): Decimal {
  if (!/^-?(?:0|[1-9]\d*)(?:\.\d+)?$/.test(value))
    throw new Error('Invalid decimal string');
  const negative = value.startsWith('-');
  const [whole, fraction = ''] = (negative ? value.slice(1) : value).split('.');
  return {
    value: (negative ? -1n : 1n) * BigInt(`${whole}${fraction}`),
    scale: fraction.length,
  };
}

export function add(left: Decimal, right: Decimal): Decimal {
  const scale = Math.max(left.scale, right.scale);
  return {
    value:
      left.value * 10n ** BigInt(scale - left.scale) +
      right.value * 10n ** BigInt(scale - right.scale),
    scale,
  };
}

export function subtract(left: Decimal, right: Decimal): Decimal {
  return add(left, { value: -right.value, scale: right.scale });
}

export function multiply(left: Decimal, right: Decimal): Decimal {
  return { value: left.value * right.value, scale: left.scale + right.scale };
}

export function isZero(value: Decimal): boolean {
  return value.value === 0n;
}

export function compare(left: Decimal, right: Decimal): number {
  const difference = subtract(left, right).value;
  return difference === 0n ? 0 : difference > 0n ? 1 : -1;
}

export function formatDecimal({ value, scale }: Decimal): string {
  if (value === 0n) return '0';
  const sign = value < 0n ? '-' : '';
  if (scale === 0) return `${sign}${value < 0n ? -value : value}`;
  const digits = (value < 0n ? -value : value)
    .toString()
    .padStart(scale + 1, '0');
  const whole = digits.slice(0, -scale) || '0';
  const fraction = digits.slice(-scale).replace(/0+$/, '');
  return `${sign}${whole}${fraction ? `.${fraction}` : ''}`;
}

// Round half away from zero; callers choose presentation precision explicitly.
export function divide(left: Decimal, right: Decimal, scale: number): Decimal {
  if (right.value === 0n) throw new Error('Division by zero');
  const numerator = left.value * 10n ** BigInt(right.scale + scale);
  const denominator = right.value * 10n ** BigInt(left.scale);
  const sign = numerator < 0n !== denominator < 0n ? -1n : 1n;
  const n = numerator < 0n ? -numerator : numerator;
  const d = denominator < 0n ? -denominator : denominator;
  return { value: sign * ((n + d / 2n) / d), scale };
}