convert ISBN 10 to 13

- (NSString *)convertISBN10To13:(NSString *)isbn10
{
  NSString *prefix = [NSString stringWithFormat:@"978%@", [isbn10 substringToIndex:9]];
  NSInteger sum = 0;
  for (int i=0; i<12; i++){
    char c = [prefix characterAtIndex:i];
    int n = c - '0';
    int w = (i % 2) ? 3 : 1;
    sum += w * n;
  }
  int check = 10 - (sum % 10);
  if (check == 10) check = 0;

  return [NSString stringWithFormat:@"%@%d", prefix, check];
}