On the most recent Bleddest Div 2, I used gcd instead of mod, thinking I wouldn't get punished.

Becuase O(n^2 log 10^9) = 30 * 5000 * 5000 = 7.5 * 10^8 which should pass.

But it turns out that the C++ built in gcd uses the Euclidian Algorithm:

int gcd (int a, int b) {
    return b ? gcd (b, a % b) : a;
}

Hacked Submission: https://codeforces.me/contest/2283/submission/397357386

So the next time you want to compute GCD, use Stein’s algorithm:

int gcd(int a, int b) {
    if (!a || !b)
        return a | b;
    unsigned shift = __builtin_ctz(a | b);
    a >>= __builtin_ctz(a);
    do {
        b >>= __builtin_ctz(b);
        if (a > b)
            swap(a, b);
        b -= a;
    } while (b);
    return a << shift;
}

I get the easy AC after getting hacked :( https://codeforces.me/contest/2283/submission/397490186

So what do you guys think about the built in gcd?

Edit: newer versions of C++ (later than 17) actually don't use the Euclidian Algorithm. (Read: https://codeforces.me/blog/entry/137056)

Sniping Cheaters Episode 2: A Philosophical Rant

Background/Context:

After posting a bunch of these CF blogs, many people in my university discord asked me to leave and quit CP (since I allegedly used LLM's during contests -- if I did why am I still green). Now that I have qualified for the ICPC ECNA (regional contest) twice in a row, I want people to understand that I have never cheated on this account since the start of my journey in college. Finally, I really don't care if a lot of people on this platform used AI on the contests, if I lose rating because I couldn't solve a problem, then I view that as a personal SKILL ISSUE.

Why should one bother doing competitive programming?

1. To get cracked at coding interviews
2. Understand basic programming ball knowledge
3. Improve problem solving skills through pattern recognition

The hard truth

Codeforces rating is just a number, and it unfortunately doesn't objectively represent your skill level (because of how easy it is to cheat). Just because you are a master on CF, that doesn't mean that you're going to get a high paying job because of that.

I'm done ranting, now here's the cheater: Prashant_ksy

He participated in the DeepInfra Round 1 and placed #141 with 2457 performance: https://codeforces.me/contest/2271/standings/participant/248292029#p248292029

Yes, he could have improved from Expert -> GrandMaster over the course of a year. (Last time, I didn't include a lot of evidence supporting my arguments with AK_YZY, other than the fact his submissions looked too clean for a pupil/speciailist.) So here is the evidence:

1. His normal code looks like this, you can check his old contests too. There are no spaces between (x=y, a+=b, etc...): 393988643
2. But his submissions for C and D have plenty of breathing room like an AI wrote the code with a good code standard (a = b, a += b, etc.): 393991411
3. On problems E and F, he probably told the AI to change its coding style to matching his own to be less suspicious.

If you don't buy my argument, lets keep an eye on this guy in future contests and accumulate evidence against him.

Edit — Original Blog: https://codeforces.me/blog/entry/157137

Well, Well, Well -- was I wrong to accuse AK_YZY of cheating. I first posted a blog about this guy when he was a pupil, and now look at how high rated he is. But I was right, finally, his submissions to DeepInfra round 1 were all skipped: https://codeforces.me/submissions/AK_YZY.

But unfortunately, his rating was not rolled back nor was his account banned. I can't wait for this cheater to get deleted off of this platform.

So, our cheater (AK_YZY) posted a comment after AK'ing todays div 2:

Submissions: https://codeforces.me/submissions/AK_YZY Ranking: https://codeforces.me/contest/2267/standings/participant/247291877#p247291877

He boasted down in the comments section about how good he was at coding and how easy the contest was...

Link to Comment: https://codeforces.me/blog/entry/157059?#comment-1394707

His original comment was "Pretty easy for a Div. 2, but still a really great round." (he changed it -- check revisions). I decided to respond with, that's "easy for a cheater to say".

What's intresting is that after a few hours, his comment gets +7 contribution and I get -11 contribution. Why? because he's abusing his cheated GM account to manipulate the voting system!

Take a look: https://codeforces.me/profile/zltz4 He literally smoked Benq, Tourist, (and many other legit LGM's) in a Div 1: https://codeforces.me/contest/2262/standings/participant/246384853#p246384853

Please listen to me Codeforces Mods, ban AK_YZY and his cheated GM account zltz4. His code is simply too clean to be written by a human with how small of a brain he has. I swear, I could beat this guy in a fair in-person 1v1 as a pupil.

Also, can I get a few upvotes from any viewers who agree with me, as compensation for helping catch this blatant cheater? I appreciate anyone who read this post.

These cheaters are stealing good handles, why can't cheater accounts be fully deleted so that their haaandles get freed up again.

Examples:

https://codeforces.me/profile/PumpyPig

https://codeforces.me/profile/XVIII