validation.ts

function isValidItalianTaxCode(data: string | null | undefined): boolean {
    if (!data) return false;

    if (data.length !== 16 || !isTaxCodeValidForIT(data)) {
        return false;
    }

    return true;
}

function isTaxCodeValidForIT(taxCode: string): boolean {
    const EVEN: { [key: string]: number } = {
        '0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7,
        '8': 8, '9': 9, 'A': 0, 'B': 1, 'C': 2, 'D': 3, 'E': 4, 'F': 5,
        'G': 6, 'H': 7, 'I': 8, 'J': 9, 'K': 10, 'L': 11, 'M': 12, 'N': 13,
        'O': 14, 'P': 15, 'Q': 16, 'R': 17, 'S': 18, 'T': 19, 'U': 20, 'V': 21,
        'W': 22, 'X': 23, 'Y': 24, 'Z': 25
    };

    const ODD: { [key: string]: number } = {
        '0': 1, '1': 0, '2': 5, '3': 7, '4': 9, '5': 13, '6': 15, '7': 17,
        '8': 19, '9': 21, 'A': 1, 'B': 0, 'C': 5, 'D': 7, 'E': 9, 'F': 13,
        'G': 15, 'H': 17, 'I': 19, 'J': 21, 'K': 2, 'L': 4, 'M': 18, 'N': 20,
        'O': 11, 'P': 3, 'Q': 6, 'R': 8, 'S': 12, 'T': 14, 'U': 16, 'V': 10,
        'W': 22, 'X': 25, 'Y': 24, 'Z': 23
    };

    const taxCodeUpper = taxCode.toUpperCase();

    const asciiValue =
        [...Array(13).keys()].filter(i => i % 2 === 1).reduce((acc, curr) => acc + EVEN[taxCodeUpper[curr]]!, 0) +
        [...Array(15).keys()].filter(i => i % 2 === 0).reduce((acc, curr) => acc + ODD[taxCodeUpper[curr]]!, 0);

    return (asciiValue % 26 + 'A'.charCodeAt(0)) === taxCodeUpper.charCodeAt(15);
}