import { isRepeatedDigits, isSequentialDigits } from '../../../utils/validation';

/** A TPIN is always exactly four digits. */
export const TPIN_LENGTH = 4;

/** The shortest run up or down the backend's TPIN policy refuses. */
const SEQUENCE_RUN_LENGTH = 3;

/**
 * True when any three neighbouring digits run straight up or down — 1235 (123),
 * 5987 (987) and 1234 alike. The TPIN policy refuses "a sequence of three
 * digits" and answers "TPIN cannot contain sequential digits", so a run
 * anywhere counts, not only one spanning all four. No wrap-around: 8901 is not
 * a run.
 */
export const containsSequentialRun = (value: string) => {
  for (let start = 0; start + SEQUENCE_RUN_LENGTH <= value.length; start += 1) {
    if (isSequentialDigits(value.slice(start, start + SEQUENCE_RUN_LENGTH))) {
      return true;
    }
  }

  return false;
};

/**
 * Any four digits — for ENTERING the TPIN on a transfer. Deliberately loose:
 * whether they are the right four is the server's to say, with INVALID_TPIN.
 */
export const isValidTpinEntry = (value: string) =>
  new RegExp(`^[0-9]{${TPIN_LENGTH}}$`).test(value);

/**
 * Inline copy for why a NEW TPIN was refused before it ever left the device —
 * for choosing one, not for entering one on a transfer.
 *
 * `mobileNumber`, when known, adds the "not the last four digits of your
 * registered mobile" rule, so the customer is told before a round trip rather
 * than after one.
 *
 * The backend stays the final authority. It also refuses a TPIN matching the
 * customer's date of birth, which the app never holds, and — on a reset — the
 * current TPIN or any of the previous three, which only the server's history
 * knows. Those can only arrive as a VALIDATION_ERROR on confirm.
 */
export const describeTpinProblem = (value: string, mobileNumber?: string): string | undefined => {
  if (value.length === 0) {
    return undefined;
  }

  if (!/^[0-9]*$/.test(value)) {
    return 'Digits only';
  }

  if (value.length !== TPIN_LENGTH) {
    return `A TPIN is ${TPIN_LENGTH} digits`;
  }

  if (isRepeatedDigits(value)) {
    return 'Pick a TPIN that is not the same digit repeated';
  }

  if (containsSequentialRun(value)) {
    return 'Pick a TPIN without a run of digits like 123 or 987';
  }

  const mobileDigits = mobileNumber?.replace(/\D/g, '') ?? '';

  if (mobileDigits.length >= TPIN_LENGTH && mobileDigits.slice(-TPIN_LENGTH) === value) {
    return 'Pick a TPIN that is not the last four digits of your mobile number';
  }

  return undefined;
};

/** The stricter check for CHOOSING a TPIN. */
export const isAcceptableNewTpin = (value: string, mobileNumber?: string) =>
  isValidTpinEntry(value) && describeTpinProblem(value, mobileNumber) === undefined;