Horrifying? It’s mathematically correct.
https://stackoverflow.com/questions/13683563/whats-the-diffe...
Think of a clock which is a ring of size 12. In a clock, going backwards 15 hours (-15) is the same as going backwards 3 hours (-3) which is the same as going forwards 9 hours.
-15 = -3 = 9 modulo 12
Math usually considers (-7 mod 5) === (3 mod 5). But in C, (-7 % 5 != 3 % 5).
The issue is that -7 and 3 are congruent, but the % operator keeps the sign. So -7 % 5 yields -2, not +3. Those are congruent, but not equal. I've never had a use for this behaviour, but I've definitely had to work around it. The lazy way is ((x % n) + n) % n which is safe (assuming n > 0).