const HEX_REGEX = /^#([0-9a-fA-F]{3}|[0-9a-fA-F]{6})$/;

/**
 * Validates a hex color string. Accepts 3-digit and 6-digit hex values
 * with a leading `#`.
 */
export function isValidHex(value: string): boolean {
  return HEX_REGEX.test(value.trim());
}

/**
 * Returns an error message if the hex value is invalid, or undefined if valid.
 */
export function validateHex(value: string): string | undefined {
  if (!value.trim()) {
    return undefined;
  }
  if (!value.startsWith('#')) {
    return 'Color must start with #';
  }
  if (!HEX_REGEX.test(value.trim())) {
    return 'Invalid hex format (e.g. #1A2B3C)';
  }
  return undefined;
}