The hidden tests do not always test for numbers like "7000" or "400". Code, that does not handle those numbers, can pass the hidden tests if the user is lucky. I suggest to hardcode a number like "7000" or "400" into the hidden test.

unsigned long long would be even better since the input is never negative. it also avoids the problem that long is 32-bit in Windows world and 64-bit in Linux world, making code less portable for no reason (long long is 64-bit in both)

• In modern C, the return type should be a bool (from <stdbool.h>)
• the input type should be marked as unsigned
• you need to put the fixed tests in the full tests suite as well
• suggested edge cases that should be in the fixed tests: 0, 10, 1000, 1001, 2001
• the functions defined in the tests must all be static, otherwise the user might call them
• you should extract your reference solution to a function to make the random tests more maintainable

Okay I think I've fixed it, could you check if it works for you now?

Are you sure your solution is still correct after making changes below?

For random input: 735027429

smallest: 202345779

largest: 977543220

actual result: 0

expected result: 1

oh okay, I'll try to fix it

No, don't use floating point numbers (float, single, double, half, quad, extended, ...) to solve integer problems.

No, I changed it to long

Okay ty, I changed it to double

Reference solution fails when num > $2^{23}$.

Never use floats unless strictly necessary.

Is there any reason why the input is int and not long? long would be more appropriate for this kata.

Thanks for the suggestions, I've added that now

I see a viable kata here, but the tests should be tougher. Currently there are only three difficult inputs, all hard-coded. I suggest:

1. Creating random difficult inputs (i.e., those with lots of zeros), e.g., 100 such inputs.
2. Increasing the number of random tests in general, e.g., to 500 random inputs.

No, that does become a decimal number; since 1034456789 (the lowest, can't start with 0) + 9876544310 = 10911001099 which ends in 9 so is a decimal point when devided by 2. I also added that numbers can't start with 0 to the description since it was unclear before
Edit: wait nevermind I was just wrong it's fixed now though cuz it has random numbers

Ty I fixed it