import { RuleError } from './errors';

const DIGITS_ONLY = /^\d{1,8}$/;

/**
 * RULES.md section 2, modulo 11 basis 2:
 *   right to left, multiply each digit by 2,3,4,5,6,7,8,9 then cycle back to 2,
 *   sum, r = sum % 11, dv = r > 1 ? 11 - r : 0.
 *
 * TODO-TAX-VERIFY: confirm against at least five real published RUCs. The seed and the
 * fixtures generate their RUCs with this same function, so the product stays internally
 * consistent whatever the verification concludes. Pinned by ruc.test.ts.
 */
export function computeRucDv(base: string): number {
  assertRucBase(base);

  let factor = 2;
  let sum = 0;
  for (let i = base.length - 1; i >= 0; i--) {
    sum += Number(base[i]) * factor;
    factor = factor === 9 ? 2 : factor + 1;
  }

  const remainder = sum % 11;
  return remainder > 1 ? 11 - remainder : 0;
}

/** True when `dv` is the check digit for `base`. Never throws: invalid input is invalid. */
export function validateRuc(base: string, dv: number): boolean {
  if (!DIGITS_ONLY.test(base)) return false;
  if (!Number.isInteger(dv) || dv < 0 || dv > 9) return false;
  return computeRucDv(base) === dv;
}

/**
 * The digit that decides the calendario perpetuo day: the last digit of the base, NOT the
 * check digit (RULES.md section 2).
 */
export function deadlineDigit(base: string): number {
  assertRucBase(base);
  return Number(base[base.length - 1]);
}

/** `4123456` and `4` become `4123456-4`. */
export function formatRuc(base: string, dv: number): string {
  return `${base}-${dv}`;
}

/** Splits `4123456-4` into its parts. Returns null for anything malformed. */
export function parseRuc(value: string): { base: string; dv: number } | null {
  const match = /^(\d{1,8})-(\d)$/.exec(value.trim());
  if (!match) return null;
  return { base: match[1] as string, dv: Number(match[2]) };
}

function assertRucBase(base: string): void {
  if (!DIGITS_ONLY.test(base)) {
    throw new RuleError('invalid_ruc', `RUC base must be 1 to 8 digits, got: ${base}`, { base });
  }
}