It's lambda-function, which finds out is number even (return true) or not (return false).

Random tests are wrong, there are several rows with the same product_id and date. But description says "Only one transaction for a product_id per date is possible."

Hehe newbie problems... Made "size_t" version https://www.codewars.com/kumite/62f45878bd09cd0030ee4513?sel=63021bd8acfb8d000e5b4cfa

Oh, I haven't seen mistake "Kumite was published with failed tests!" while republishing.
When I test it in Kumite, everything is ok...

Changed return type to size_t, thank you for advice :)

Oh, was return value type meant?

I translated kata from Python, there were really big numbers (bigger even than ull), and I wanted to make tests with big numbers. But then I was adviced to limit numbers at 1.000.000.000 and now Ull maybe is too big for it.

Thank you, I have fixed this.

C-translation :) Please, review and approve!
https://www.codewars.com/kumite/62f45878bd09cd0030ee4513?sel=62f45878bd09cd0030ee4513

Check the C Translation, please

Hm, right... I'll try to fix this :)

C++ translation

I wrote my own func for messages, anyway :)

Increased chance to get r*r to 80% :)

The number of tests was taken from original kata, but I've increased it.

I didn't forget about sol's, just thought the error message will be nicer with functions. So renamed var's a little.