luhn range-v3

luhn.cc

bool luhn_check(const std::string& pan) {
  auto to_int = [](char c) { return c - 48; };

  auto sum_digits = [](int n) { return n / 10 + n % 10; };
  auto do_digits = [&sum_digits](auto&& values) {
    auto const& [index, digit] = values;
    return (index % 2 == 0) ? digit : sum_digits(digit * 2);
  };

  auto rng = pan 
              | views::transform(to_int)
              | views::reverse
              | views::enumerate
              | views::transform(do_digits);

  auto sum = accumulate(rng, 0);

  return sum % 10 == 0;
}