// - NPI format validation, client side.
//
// THIS IS THE CONVENIENCE COPY. The gate is the server (`Practitioner/put.ts`); this exists so a
// provider entering their own NPI sees the problem in the field instead of receiving a 400 after pressing
// Save. - a client-side check is never the gate, and the
// two implementations are deliberately identical so they cannot disagree about what "valid" means.
//
// FORMAT, NOT IDENTITY. A well-formed NPI is not a verified one - registry lookup against NPPES is its
// own card and is unbuilt. We deliberately do NOT call NPPES from the browser
// ( ); when real verification lands it goes behind our own
// endpoint with a deterministic fallback.
//
// Format still matters more than it sounds: a typo'd NPI is WORSE for identity matching than an absent
// one, because it either fails to link a human who should be linked or links two who should not.
// measured three false-positive NPI pairs in the seed data alone.

const TEN_DIGITS = /^\d{10}$/;

/**
 * The official NPI check digit: Luhn over the number prefixed with the NPPES issuer id `80840`.
 * A single mistyped digit fails, which is the whole reason this beats a length check.
 */
export function isValidNpi(value: string): boolean {
    const npi = value.trim();
    if (!TEN_DIGITS.test(npi)) return false;

    const digits = ('80840' + npi.slice(0, 9)).split('').map(Number);
    let sum = 0;

    // Luhn from the right: double every second digit, subtracting 9 when the result exceeds 9.
    for (let i = digits.length - 1, position = 0; i >= 0; i--, position++) {
        let digit = digits[i];
        if (position % 2 === 0) {
            digit *= 2;
            if (digit > 9) digit -= 9;
        }
        sum += digit;
    }

    const checkDigit = (10 - (sum % 10)) % 10;
    return checkDigit === Number(npi[9]);
}

export const NPI_FORMAT_MESSAGE =
    'An NPI is 10 digits and includes a check digit, so a single mistyped digit is rejected.';