Support Modulus 10 check digit on CHI

[Moohan]

There's a long email but I think here's the bit about what's happening.

At the CHI NUG meeting on 21st May 2025 a solution which has been identified and approved by the CHI Management Board was presented:
· Implement Modulus 10 check digit for CHI numbers that have no available Modulus 11 check digit numbers available (i.e. once Modulus 11 numbers are complete for a given date, then commence issuing CHI numbers with Modulus 10 check digit for that date).
· Implementation on 1st November 2025.

And here's the relevant bits for implementation:

Where systems do implement current Modulus 11 check digit validation, and need to support Modulus 10 CHI numbers from 1st November 2025, they have choices to;

1. Implement Modulus 10 check digit validation in addition to the existing Modulus 11 validation;
2. Remove the existing Modulus 11 validation, so the system does not carry out any check digit validation.

I'm assuming we want option 1...

[Moohan]

LUHN method:

Double every second digit:
Starting from the rightmost digit (the check digit), move left and double every second digit.
Sum the digits of doubled numbers:
If doubling a digit results in a two-digit number, add its two digits together. For example, if you double a '7' to get '14', you add 1 + 4 = 5.
Sum all digits:
Add all the resulting digits together.
Check the total:
If the final sum is perfectly divisible by 10 (meaning the sum modulo 10 is 0), the number is considered valid.

[alan-y]

@Moohan I've tried adding in the modulus 10 check according to what you've written and it fails for CHIs that pass the mod11 check. Are all CHIs meant to pass both mod11 and mod10? Otherwise I might be doing something wrong.

[Moohan]

@Tina815 and I had this chat on Monday ... Essentially, we're not sure how the mod10 algorithm should be applied, and Tina is going to get more details from the CHILLI team.

Are all CHIs meant to pass both mod11 and mod10? Otherwise, I might be doing something wrong.
Not 100% sure, but I think not... I think most CHIS will/should pass Mod11 and only certain CHIs will only pass Mod10, but to be honest I'm not super clear.

I think options for implementation are:

1. Allow a CHI to 'pass' if it passes the Mod 10 OR Mod 11 check. This will weaken the whole checking methodology, but it will work with the new 'Mod 10 CHIs'
2. Check Mod11 for all CHIs and if it fails AND if some other condition applies, check Mod10 - We'll need more detail to decide what that condition(s) might be...

[alan-y]

sounds good, I'll wait a bit before going further then.

I found a trick that subtracting 9 from any 2 digit number (when this happens after doubling) gives the same result so that makes the coding a bit easier haha.

[Moohan]

I think the Mod 10 algorithm can be written and tested, so no time wasted there, it's just getting clarity on how we should actually use it!

[alan-y]

It would be ideal if CHILI team can provide an example of a CHI number we can use to test this additional check on. I think it would really clear up requirements if we have an example.