se-panfilov/jsvat

import { Country } from '../jsvat';

export const unitedKingdom: Country = {
  name: 'United Kingdom',
  codes: ['GB', 'GBR', '826'],
  calcFn: (vat: string): boolean => {
    // Government departments
    if (vat.substr(0, 2) === 'GD') return isGovernmentDepartment(vat);

    // Health authorities
    if (vat.substr(0, 2) === 'HA') return isHealthAuthorities(vat);

    // Standard and commercial numbers
    return isStandardOrCommercialNumber(vat, unitedKingdom.rules.multipliers.common);
  },
  rules: {
    multipliers: {
      common: [8, 7, 6, 5, 4, 3, 2]
    },
    regex: [/^(GB)?(\d{9})$/, /^(GB)?(\d{12})$/, /^(GB)?(GD\d{3})$/, /^(GB)?(HA\d{3})$/]
  }
};

function isGovernmentDepartment(vat: string): boolean {
  const expect = 500;
  return Number(vat.substr(2, 3)) < expect;
}

function isHealthAuthorities(vat: string): boolean {
  const expect = 499;
  return Number(vat.substr(2, 3)) > expect;
}

function isStandardOrCommercialNumber(vat: string, multipliers: ReadonlyArray<number>): boolean {
  let total = 0;

  // 0 VAT numbers disallowed!
  if (Number(vat.slice(0)) === 0) return false;

  // Check range is OK for modulus 97 calculation
  const no = Number(vat.slice(0, 7));

  // Extract the next digit and multiply by the counter.
  for (let i = 0; i < 7; i++) {
    total += Number(vat.charAt(i)) * multipliers[i];
  }

  // Old numbers use a simple 97 modulus, but new numbers use an adaptation of that (less 55). Our
  // VAT number could use either system, so we check it against both.

  // Establish check digits by subtracting 97 from total until negative.
  let checkDigit = total;
  while (checkDigit > 0) {
    checkDigit = checkDigit - 97;
  }

  // Get the absolute value and compare it with the last two characters of the VAT number. If the
  // same, then it is a valid traditional check digit. However, even then the number must fit within
  // certain specified ranges.
  checkDigit = Math.abs(checkDigit);
  if (
    checkDigit === Number(vat.slice(7, 9)) &&
    no < 9990001 &&
    (no < 100000 || no > 999999) &&
    (no < 9490001 || no > 9700000)
  ) {
    return true;
  }
  // Now try the new method by subtracting 55 from the check digit if we can - else add 42
  if (checkDigit >= 55) {
    checkDigit = checkDigit - 55;
  } else {
    checkDigit = checkDigit + 42;
  }

  const expect = Number(vat.slice(7, 9));
  return Boolean(checkDigit === expect && no > 1000000);
}