# 15.1.205. tablet_qt/lib/nhs.cpp¶

```/*
Copyright (C) 2012, University of Cambridge, Department of Psychiatry.
Created by Rudolf Cardinal (rnc1001@cam.ac.uk).

This file is part of CamCOPS.

CamCOPS is free software: you can redistribute it and/or modify
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

CamCOPS is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with CamCOPS. If not, see <https://www.gnu.org/licenses/>.
*/

#include "nhs.h"

const QVector<int> NHS_DIGIT_WEIGHTINGS{10, 9, 8, 7, 6, 5, 4, 3, 2};

namespace nhs {

int nhsCheckDigit(const QVector<int>& ninedigits, int failure_code)
{
// Calculates an NHS number check digit.
//
//    1. Multiply each of the first nine digits by the corresponding
//       digit weighting (see NHS_DIGIT_WEIGHTINGS).
//    2. Sum the results.
//    3. Take remainder after division by 11.
//    4. Subtract the remainder from 11
//    5. If this is 11, use 0 instead
//    If it's 10, the number is invalid
//    If it doesn't match the actual check digit, the number is invalid

const int len = ninedigits.length();
if (len != 9) {
return failure_code;
}
int total = 0;
for (int i = 0; i < len; ++i) {
const int digit = ninedigits.at(i);
if (digit < 0 || digit > 9) {
return failure_code;
}
total += digit * NHS_DIGIT_WEIGHTINGS.at(i);
}
const int mod_total = total % 11;
// ... % 11 yields something in the range 0-10
int check_digit = 11 - mod_total;
// ... (11 - that) yields something in the range 1-11
if (check_digit == 11) {
check_digit = 0;
}
return check_digit;
}

}  // namespace nhs
```