const DECIMAL_RE = /^\d+(\.\d+)?$/;

/** Parse a decimal string ("100.25") into integer base units. Rejects floats, negatives, exponents. */
export function parseDecimalToUnits(value: string, decimals: number): bigint {
  const trimmed = value.trim();
  if (!DECIMAL_RE.test(trimmed)) {
    throw new Error(`invalid decimal amount: "${value}"`);
  }
  const [whole = '0', frac = ''] = trimmed.split('.');
  if (frac.length > decimals) {
    throw new Error(`too many decimal places (max ${decimals}): "${value}"`);
  }
  return BigInt(whole) * 10n ** BigInt(decimals) + BigInt(frac.padEnd(decimals, '0') || '0');
}

/** Format integer base units back to a decimal string, trimming trailing zeros. */
export function formatUnits(units: bigint, decimals: number): string {
  const negative = units < 0n;
  const abs = negative ? -units : units;
  const digits = abs.toString().padStart(decimals + 1, '0');
  const whole = digits.slice(0, digits.length - decimals);
  const frac = decimals > 0 ? digits.slice(digits.length - decimals).replace(/0+$/, '') : '';
  return (negative ? '-' : '') + whole + (frac ? '.' + frac : '');
}

/** Ceiling division for non-negative bigints. */
export function ceilDiv(numerator: bigint, denominator: bigint): bigint {
  if (denominator <= 0n) throw new Error('denominator must be positive');
  if (numerator < 0n) throw new Error('numerator must be non-negative');
  return (numerator + denominator - 1n) / denominator;
}

/**
 * The minimum confirmed amount that satisfies an option:
 * required × (1 − tolerance_bp/10000), rounded UP so a payment one base unit
 * below the true tolerance floor never qualifies.
 */
export function toleranceFloor(requiredAmount: bigint, toleranceBp: number): bigint {
  if (!Number.isInteger(toleranceBp) || toleranceBp < 0 || toleranceBp > 10000) {
    throw new Error(`tolerance_bp out of range: ${toleranceBp}`);
  }
  return ceilDiv(requiredAmount * BigInt(10000 - toleranceBp), 10000n);
}