14. Validating ISBNs

The International Standard Book Number (ISBN) is a unique numeric identifier for books. Currently, a 13-digit format is used. However, for this problem, you are to validate the former format that used 10 digits. The last of the 10 digits is a checksum. This digit is chosen so that the sum of all the ten digits, each multiplied by its (integer) weight, descending from 10 to 1, is a multiple of 11.

The validate_isbn_10 function, shown as follows, takes an ISBN as a string, and returns true if the length of the string is 10, all ten elements are digits, and the sum of all digits multiplied by their weight (or position) is a multiple of 11:

bool validate_isbn_10(std::string_view isbn)
{
   auto valid = false;
   if (isbn.size() == 10 &&
       std::count_if(std::begin(isbn), std::end(isbn), isdigit) == 10)
   {
      auto w = 10;
      auto sum = std::accumulate(
         std::begin(isbn), std::end(isbn), 0,
         [&w](int const total, char const c) {
            return total + w-- * (c - '0'); });

     valid = !(sum % 11);
   }
   return valid;
}

You can take it as a further exercise to improve this function to also correctly validate ISBN-10 numbers that include hyphens, such as 3-16-148410-0. Also, you can write a function that validates ISBN-13 numbers.