Thanks! You can’t view replies to tweets without an account so didn’t see it for OpenAI.

My vague memory of 10 years ago recalled seeing judgements like WA42, but maybe my memory confused it with some other competition or things changed.

Out of curiosity, is there submission limit at ICPC? At IOI you're limited to 50 submissions no matter what. At ICPC if there is no submission limit, you could make 1000000 submissions, completely send your time to the shadow realm but still win on solved problems.

Also, I don't believe ICPC scrambles tests, and you are told which test number has the first failure. So one could start extracting characteristics of a particular test (e.g. hash all of the input, get it out through WA/TLE/AC statuses and then hardcode in the code) and make sure that for that test the logic isn't modified further from AC and keep trying different things for next test.

So I'd be really curious in full results (including time) and also how affected would AI be if you add a couple of new tests.

Second part of the problem is simple DP after you replace every ? with 1 as it always yields a string with maximal number of splits.

First part of the problem was to split the string in groups of length 1 or 2, with each group having independent choice of options. My cases were:

• Every 0 in string forms a group of length 2 with previous character and itself. If previous character is ?, options are 20 or 10; otherwise there is 1 option.
• After cutting out 0 groups, we get everything else split into sub-groups.
• Note that it's never valid to replace ? with 0.
• If the sub-group is of length 1: if the character is ?, we have 9 options; otherwise there is 1 option.
• Otherwise process the sub-group from right to left.
• If the character is not ?, it forms a group of its own with 1 option.
• If we have ? at the end of the sub-group, if the previous character is ? a group of size 2 and 15 options (26, 25, ..., 21, 19, ..., 11); if the previous character is 2 a group of size 1 and 6 options (6, ..., 1); otherwise it's a group of size 1 and 9 options (9, ..., 1).
• If we have ? not at the end of sub-group, if the next character is 7, 8 or 9, we have 1 option (1); otherwise we have 2 options (2, 1).

The key intuitions here was to split out 0 groups, and after that 3 — 9 can only be put at the end of subgroups, 2 can be put only if the next character allows it, 1 can always be put and 0 can never be put. So I think that ??0 was trickiest case as it has 18 options, because first character will always be on its own.

And one German contestant. According to Regulations, everyone who competed remotely (9 contestants) would not count towards calculating medal boundaries but would be inserted into the table and given appropriate awards. The asterisk near rank denotes that as it does result in duplicate ranks, since on IOI Stats I keep ranks identical for medal boundary computations.

Some updates will happen at tomorrow’s General Assembly

I was not part of the investigation, but from what I’ve been told, the main focus of the investigation was the concrete violation reported to relevant committees. During this process, it was deemed necessary to speak with entirety of delegation. At that time, the competition has ended so there was no possibility to search every bag on the contest floor.

So while it is possible that there were other violations of the contest rules, I personally don’t believe that it should prevent IC sanctioning known violations. On a personal note, as a past contestant, I am really surprised from this thread that there are a lot of contestants that are not personally familiar with contest rules that dictate what is and what is not allowed. That is not a risk I personally afforded when I was competing.

I’m speaking my personal thoughts now.

Ultimately, it is delegation responsibility to fully understand contest rules:

Delegation Leaders have the responsibility to ensure that all members of their delegation fully understand these rules and abide by them.

Bags are not allowed, although not explicitly. It was a surprise to learn that they were allowed on the contest floor. IC will work closely with the future hosts to ensure that this does not happen again. As the result, no action was taken because someone brought their bag. Mobile phones are explicitly prohibited however. But, given that it is possible to just accidentally forget the phone in the big bag, this resulted in different sanction for two of the contestants.

That is incorrect.

I will first point out that according to IOI Contest Rules, mobile phones are explicitly prohibited and having one in the competition room is considered cheating.

IC has considered the case anonymously (only one voting member who was part of investigation was aware of the identity of contestants or country in question and the rest were not told) and after long deliberations IC has decided on disqualification of one contestant and 50% score reduction in Day 2 for the other two contestants.

This was then presented to the General Assembly who need to ratify IOI results according to Regulations. Although I cannot say currently with 100% certainty, but from my memory the ratification was unanimous in GA.

At no point IC has presented to GA any other options.

Due to circumstances, Australia is unable to send its delegation this year and IC has allowed the four nationally selected students from Australia to compete under IOI flag. For more details, feel free to ask Ben [IOI President].

IOI 2024 data is published at IOI Stats. Photos are coming at a later time. Please let me know if you find any errors.

It's going live this weekend, I had to delay the publishing unfortunately.

I'm aiming to process the registration data this or the next weekend.

All committees travel to the IOI venue for a week half a year before IOI. If half of us will be unable to make it to the meetings, I hope we'll notice.

Unexaggerated version of this is a valid concern. And although there isn't a place where everyone will be in the same conditions (time zones being the most basic example), I'm sure that potential mitigations of this will be discussed in-depth with the hosts closer to IOI 2025.

Yeah, that is the intent. IOI Regulations specifically state in S5.14: "contestants must not bring objects onto the stage that may obstruct other people" and flags are just an example given in N5.14.

IOI 2023 had a unique closing ceremony format where there were no need for group photos. If this format sticks, perhaps this might not even be needed in the future anymore.

So, by that logic, why is having a flag of the country university is affiliated with in the official scoreboard of the competition OK, but having it in the closing ceremony is suddenly not OK?

I learned treap in preparation for IOI 2011. It was exciting. On both competition days I ended up implementing treap for some task before realizing it doesn't actually work and throwing implementation into the bin.

It's real. You learn some new shiny and powerful thing and want to use it everywhere regardless of whether it makes sense or doesn't.

IOI International Committee is aware of ICPC decision.

I just saw this, sorry for the delay. kostka's general understanding is correct.

Point N2.6.2 of IOI Regulations states:

N2.6.2. If a Country wishes to attend IOI as an Invited Observer, they should contact the Secretary of the IOI for information on how to apply. A typical application would include information about programmes that are already established within the Country. The IC is responsible for deciding whether to approve such applications.


Programmes that are already established within the Country are crucial because to also answer your first question, it is required to have a National Olympiad to select students that will attend IOI, as is indicated in point A2.5 of IOI Regulations:

A2.5. Students who are to be considered for representing their delegations at an IOI must, in addition to meeting the eligibility requirements set forth in S2.5, have participated in a selection procedure that:

• Is based on ability and includes a test of the students’ programming and problem solving ability;

• Is open to all eligible students in their delegation’s Country, although restrictions may be placed on where, when and how students can enter the procedure, and a student’s nationality can be limited to that of the relevant Country.

If, after you graduate, you are serious about being the long-term main driving force behind national olympiads with support of your Country's official institutions, I would highly encourage you to contact the Secretary of the IOI, who would be able to explain the process and expectations in more details. The contact information is available at IOI website.

Unfortunately though, given the timeline of these applications, and the general framework of being Invited Observer at first, you shouldn't expect your Country to be able to send contestants to the IOI earlier than IOI 2025 at best.

Not sure I'll have the time to process them in mass unfortunately, but if people submit their handles through the system, I'll try to approve them in a timely manner.

IOI 2022 data has been released on IOI Stats.

The limit on the source code is 100KB. You only get 1 byte per possible answer for precomputation.

For A I think I came up with a reasonably easy to implement solution. If N <= 5, just hardcode all solutions. For larger N+2, all lengths can be achieved by going to the top-left corner of N – either by not using shortcuts at all or going immediately right and down; so we check if we can still finish by avoiding shortcuts on this level and reduce the problem to the smaller one.

Still didn't enjoy the task though.

Realize that positive and negative positions are independent problems, so assume positive position. Sort a list of zeroes and a list of ones. Any operation you do is beneficial to do on closest remaining numbers. So DP is a[i][j] = the minimal cost to deliver i closest zeroes and j closest ones. From every position there are at most 5 transitions.

Was enough to qualify though!

Thank you.

I apologize if you felt that the message was disrespectful and appreciate the feedback. It was however strongly worded because I strongly disagreed with the original decision.

To me your announcement basically stated that you believe that not letting unproctored online mirror participants figure out the difficulty of tasks if they really wanted to is more important than celebrating the achievements of BOI 2022 participants which I took offence at.

Organizational issues happen. I have absolutely no problems if the publishing of results is delayed due to that. But that was not what was communicated I believe.

Given that the results still aren't published in full (although I do not have any issues with that at this point), it seems to me that maybe I'm completely missing some other benefits of intentionally delaying results. Please let me know if that's the case.

I think the Edit functionality on IOI Stats is starting to outlive its usefulness to be honest. It made sense while we were trying to recover a bunch of data from past IOIs, nowadays it's mostly requests amending data from IOI Registration system, and it's not possible to verify authenticity of requests (aka is it the person in question or someone trolling). I think apart from social links I'd like to move to a world where all such requests are handled by e-mail soon.

We will publish the official results of this year's BOI after the online mirror.

As someone who follows the competition outside of delegation, I want to know how my country did. I want to know who is going to IOI from my country. I don't think withholding this information because someone might estimate task difficulty in an unofficial mirror is acceptable to be honest, especially when that mirror is almost a week later. If someone really wants to cheat in an unofficial mirror they can always get the tasks from an actual competitor. Or are you going to ban people from the actual competition if they do that?

I'll blame my particular compiler (g++.EXE (tdm64-1) 5.1.0) which is an old version of tdm-gcc, which does compile successfully while specifying --std=c++17.

If I test g++ in Windows Subsystem for Linux (g++ (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0) it indeed errors out.

I guess I expected the compiler to either compile error or complain about --std=c++17, but maybe in my old compiler version C++17 was experimental and not yet feature complete.

Anyway, thanks a lot for looking into this!

I have a tech question for someone way more familiar than me with C++.

I was using variable name data for a long time. However today I got compilation error which was fixed after I renamed variable data to d.

#include <bits/stdc++.h>
using namespace std;
static long long data[1][1];

int main () {
    data[0][0] = 0;
    return 0;
}

Solution.cpp: In function 'int main()':
Solution.cpp:6:5: error: reference to 'data' is ambiguous
     data[0][0] = 0;
     ^~~~
In file included from /usr/include/c++/8/string:51,
                 from /usr/include/c++/8/bits/locale_classes.h:40,
                 from /usr/include/c++/8/bits/ios_base.h:41,
                 from /usr/include/c++/8/ios:42,
                 from /usr/include/c++/8/istream:38,
                 from /usr/include/c++/8/sstream:38,
                 from /usr/include/c++/8/complex:45,
                 from /usr/include/c++/8/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/8/bits/stdc++.h:52,
                 from Solution.cpp:1:
/usr/include/c++/8/bits/range_access.h:318:5: note: candidates are: 'template<class _Tp> constexpr const _Tp* std::data(std::initializer_list<_Tp>)'
     data(initializer_list<_Tp> __il) noexcept
     ^~~~
/usr/include/c++/8/bits/range_access.h:309:5: note:                 'template<class _Tp, long unsigned int _Nm> constexpr _Tp* std::data(_Tp (&)[_Nm])'
     data(_Tp (&__array)[_Nm]) noexcept
     ^~~~
/usr/include/c++/8/bits/range_access.h:299:5: note:                 'template<class _Container> constexpr decltype (__cont.data()) std::data(const _Container&)'
     data(const _Container& __cont) noexcept(noexcept(__cont.data()))
     ^~~~
/usr/include/c++/8/bits/range_access.h:289:5: note:                 'template<class _Container> constexpr decltype (__cont.data()) std::data(_Container&)'
     data(_Container& __cont) noexcept(noexcept(__cont.data()))
     ^~~~
Solution.cpp:3:18: note:                 'long long int data [1][1]'
 static long long data[1][1];
                  ^~~~

I cannot reproduce this locally using C++17. Removing either first or second line makes it compile. Can anyone explain what exactly is failing and why I might not be able to reproduce this locally? Thanks!

I thought this post would be about selection of anime that Team Moldova will watch at IOI 2022 from the title. Slightly disappointed.

My solution was slightly different, but using same ideas.

• Rather than walking one room and then teleporting away, I kept walking until I visited 2 rooms I already was in in a row.
• I actually didn't differentiate between how I reached the node when I learned its degree. So to accomodate large-degree nodes, I took median instead of average.

On the proof, I had flashbacks to my Master's degree's thesis in sublinear algortihms. For example if you want to tell with a certain confidence whether the graph is bipartite, it can be proven that if you choose a specific sublinear size sample of nodes and check whether the sample graph is bipartite or not, you can actually prove that that answer will hold for the whole graph with certain probability.

So I had a hunch that you just need to sample, and the fact that they gave us walks was a big hint.

Although I imagine there's certain element of luck to it and some sensible ideas probably wouldn't work in reality.

Statement on the IOI website was updated with similar clarification.

The update on IOI 2022 format was shared to GA two weeks ago. Unfortunately, that wasn't shared to the wider audience, so I am going to do so now.

The International Committee met online in February, and will continue to meet regularly as the COVID-19 situation evolves. The plan for IOI 2022 remains the same: we are preparing for an on-site IOI, but we are committed to having an online option for those delegations that cannot attend in person. This is, of course, an enormous ask for the host, and we are very grateful to Indonesia for their work in making such a “hybrid IOI” happen.

Unlike the last two IOIs, the on-site IOI will be the main event, and the online option will (by necessity) be a lesser experience. For example: culturally, on-site participants will be able to experience Indonesia fully, and will have more opportunities to interact with contestants from across the world. Technically, on-site participants will be in a competition hall with hardware managed and supported by the host country, and with the technical committees working in the next room. Logistically, the IOI timetable will be planned around Indonesian time.

We do understand if delegations cannot attend on-site, and we are committed to offering the online option as a fallback. However, we strongly encourage delegations to attend in person if it is safe and feasible to do so.

We will continue to send updates in the coming months. In particular, the COVID-19 safety arrangements and IOI schedule are still under development, and we will announce these when we can. Regarding quarantine: we are hopeful that it will be possible for delegations to attend without the need to quarantine, and the hosts are currently in negotiation with the Indonesian authorities. Vaccination is likely to remain important, however, and we urge delegations to ensure that their team leaders and potential contestants will be fully vaccinated against COVID-19 before August if this is at all possible, preferably with boosters also. Again, the situation is still evolving, and we will announce more precise requirements once we know them.

As IOI 2022 was announced to be a "hybrid IOI", I fully expect that from the competition point of view, all online contestants will be considered official contestants and as such be ranked and eligible for awards.

I was able to download them from https://www.imo-official.org/problems.aspx.

As announced at IOI General Assembly, from the IOI perspective, IOI 2021 regulations do not prohibit simultaneous secondary and tertiary enrolment.

After seeking additional information from the delegation, IOI International Committee are satisfied that the secondary enrolment of the contestant is genuine and significant; and confirm that the student is eligible for IOI 2021.

IC would like to congratulate the contestant on their achievements at IOI 2021.

Generally, any regulation changes approved by GA on IOI’n are taking effect from IOI’(n+1).

It is possible to fast-track regulation changes, but it needs to be presented to GA a month before IOI and the vote happens at the first GA for it to take effect immediately. This happened with HM proposal at IOI 2020, and if it would have been passed, it would have taken effect immediately. But as stated above, that proposal was rejected.

Given that this year we specifically allocated time at GA to discuss and decide on HM criteria, it was not possible to fast-track this, and as such it will take effect from IOI 2022.

There were many discussions during this IOI within the GA, so I am not able to present all the opinions that were discussed either within the GA (that should be minuted) or on the e-mail chains.

This IOI there first was a ranked vote where GA chose the criteria they like the most; and at the final GA the vote was taken for introducing HMs with the previously chosen criteria. It received a necessary 2/3 majority support within the GA to be passed.

P.S. Note that since this proposal changed the Statutes of the Regulation, 2/3 majority was needed rather than a simple majority.

Day 1 preliminary results are now also available at https://stats.ioinformatics.org/results/2021/preliminary if you prefer.

IOI 2021 data has been released on IOI Stats.

I'll try to add Codeforces handles from kostka's list at some point.

They have mentioned this approach in "Flip Flop" section of the Analysis and show that it's finite and the asymptotics are enough. They didn't however mention greedy which makes this task even easier (since flows are only helpful if you solve the task the intended way I feel).

This is the reason why I personally disliked this task since it clearly has two ways two go about it with one significantly easier than the other one. From reading the analysis I got the impression that they realized that there exists this alternative solution quite late in the problemsetting, and I'm not sure why the task was kept after that or not swapped with the first one.

Wouldn't greedy (where for every row you mark the columns with the most marks still needed) work? Can't come up with a counter-example.

All I could think of during the round: who hurt the problemsetter?

An update from the host was posted to ioi-announce@. The text is provided in the spoiler below.

In last month's message from the hosts it was stated:

Singapore is also NOT considering to postpone the planned onsite IOI2021 any further because:

...

• last year when we agreed to postpone the onsite event, most of our host organizing committee members have agreed to make time for 20-27 June 2021 new time slot but most of the members may not be available afterwards

I'm afraid that this argument is still in-play for the online competition as it still requires a large team from the host committee to host a successful online IOI. So the fact that the competition has moved online does not necessarily mean that it's any easier to change the dates unfortunately.

I will keep your point in mind and will try to inquire the hosts about this given the opportunity. But, personally, I would not expect the possibility of changing the date announced last year to gain any serious traction unless there are significant reasons behind it (for example, if a lot of teams would be unable to hold their selections for IOI in June).

This is not a conclusion that was surfaced to IC from the surveying of team and deputy leaders shortly before IC meeting.

While I can't speak for the entire IC, here is my reasoning on the matter, which would hopefully provide some context behind the choice.

• The survey was sent shortly before IC meeting to all last year's team and deputy leaders. While you could see some support for onsite competition it definitely scaled in the direction of online competition due to uncertainties in many countries. A non-single-digit-amount of countries told us in February that they would be unable to go should anything happen onsite in June.
• Business travel is still a thing. However, with heavy restrictions. At this moment in time if I have a valid reason to go to Singapore from Latvia, for example, I am required to sit 14-day quarantine in Singapore and 10-day quarantine back in Latvia. With the 7-day event, this is a month-long commitment. While it's easy to have that commitment as a student, it gets very tricky for everyone else, and this will also reduce the amount of delegations who would be able to travel, should such restrictions still be in place by June.
• A portion of contestants are minors. Possibilities of parents and/or ministries refusing to send minors to Singapore is a real thing.
• For various reasons, decision needed to be made now. At some point in time we would have to go back to onsite IOIs and quite possibly take a hit in a number of participating countries. However, at this point in time, it's impossible to predict what will happen in June with any certainty. If we had an onsite IOI, I estimate that we may have anywhere from 25% to 80% countries participating. For proposed onsite event, small number of countries is not an issue, but for the contest itself, it certainly is. While the range is so wide and uncertainty is still so large, it felt utterly irresponsible to me to make any other decision.

Glad to hear that Croatia is interested in travelling to Singapore should an onsite event happen in the end!

The IC is exploring the possibility of an optional on-site contest.

I wouldn't be summarising this as an "onsite contest". While more details would be released by the host when they become available, the intention is that should anything develop further from this proposal, there would still be only one contest and it will be online, as outlined by "such teams would still sit the contest online from within Singapore, using their own computers".

ez

No, the intention is to release the scoreboard as soon as possible.

Just keep in mind that 10 years ago things were much worse for contestants. :)

I suspect because the tasks just got released to the public and as for the actual contestants, I'd usually advise to forget about Day 1 as soon as it's over and start mentally preparing for Day 2.

UPD: Updated the main post with Day 1 tasks.

Once I get the snapshot of the results from Singapore, they will appear on IOI Stats. Hopefully soon.

Well, we have at least a whole year to decide, GA has just voted down the proposal for honourable mentions at IOI 2020 (with the definition of HMs as next 20% after the bronze).

A note on “No medal”, it will change to “Participation” or something else and it’s only present in one place on the website (whereas HMs would be present a lot more), so I have no issues changing its colour to anything else.

IOI 2020 is live on IOI Stats.

I will update (at least) the Codeforces handles from this list soon.

P.S. kostka: your list contained at least one link to IOI Stats profile without any participations (usually happens with late changes to reg). You can't find it unless you know the ID, so I'm impressed. I usually delete those, but forgot for 2019; so I've done it before I matched anyone for 2020. As such, in your list if you have a contestant with no past participation but with IOI Stats link, it's now likely broken, so just FYI.

In Russian, you would usually call a 4th place medal the wooden medal. I wonder how many different names there are in different cultures...

I like it, and the argumentation is amazing.

This is actually a very good point, thanks. Now that I think about it, I definitely seen a colour close to this used as a silver in some programming competition.

There is actually a stronger reason for using pink, which is that this is what IMO website is using for HMs. But I would have to make it light enough by which point the similarities are a bit gone.

Light blue is actually one of my favourites, but I just can't rationalize my preference to it compared to light green or some other light colours.

The issue here is that what essentially would be light brown would still be associated with bronze medal on quick sight.

Write some simple 2-player game (e.g. pong) with AI, have AI you have written control both players, order as many bananas and water through CMS as you can and enjoy the show!

Correction: Rokas Urbonas from Lithuania participated in IOI 2018 not IOI 2019.

Latvia has managed to hold their national olympiad before lockdowns began and used Baltic Olympiad in Informatics to select its IOI team, where all Latvian contestants were assembled in a single location.

From what I know, Lithuania held their national olympiad online with multiple cameras for proctoring purposes; and then used BOI to select its IOI team.

I would hope that not many countries did that. First of all, not many countries can (since students eligible for IOI 2019 can be non-eligible for IOI 2020); and second of all this denies a final year student who wasn't good enough to qualify for IOI 2019 a chance to compete for IOI 2020.

I'm not trying to dismiss this opinion, I think these are good points, I'm just trying to understand for my own curiosity. I don't remember that many complaints about code execution on contestants' PCs when Google Code Jam did it not too long ago. I genuinely wonder what's the reason of this?

1. My memory is just wrong.
2. People were assuming it as a given for Code Jam and not voicing their concerns, but for Hacker Cup it's a new rule change.
3. People started to use less powerful machines more to participate in programming competitions (e.g. Chromebooks).
4. Competitive programming has become more popular over the last years and as such the number of issues we are hearing about this has increased.
5. Codeforces has become more popular over the last years and as such the number of issues we are hearing about this has increased.
6. Something else.

Well, I decided not to bother with coordinates compression and decided to have a 500M array of ints for A1. Which exceeded Windows' 32-bit 2GB RAM limit. Thankfully, after panicking for 5 minutes (it was the last hour of the contest) I just launched Ubuntu using Windows Subsystem for Linux; and everything worked like a charm there.

It's available now.

Keep in mind that there may be some differences between onsite and Codeforces tasks (e.g. colors became multi-test-case-per-input problem).

I liked old Code Jam format because it allowed to have some fun.

Like submitting everything in Excel.

Or racing for the top of languages used leaderboard (hard-core players would deliberately fail Rounds 1A & 1B to get more languages in).

Or just looking at what crazy languages did people use and their submissions.

Or just solving the problem in two parts in two different languages if you really wanted it (say in the first part you wanted to use C++ STL, but for the second part you wanted to use Python long arithmetics).

Or if you really need it, you can even use multi-threading to get just a bit more performance.

The current format doesn't limit you in anything, it's basically just use your computer to solve this problem, in whatever way you want without limiting your tools in any way. When Code Jam used this format, it had this unique feeling for it, which it lost when it became exactly like the other contests. Which in my eyes, was a shame.

The biggest downside to this format I see is Internet stability/speed. But that can easily be addressed by distributing tests in advance in a password-protected archive. When the you start the timer, you could just reveal the password. IPSC has been doing this for years.

Well, now that you've started it, I'll share a video I made yesterday as a joke. Hopefully, I won't offend anyone.

On a more serious note, I think that perhaps it wasn't an ideal task for subtask-structure. What I mean by that is I think it was way harder than usual to score points even on the first subtask (it's reasonable if you notice simplification to 2D, but if you keep working in 3D, good luck). Additionally, long testing queue during the first onsite day, made it a bit more difficult as well. Finally, pre-written code may have helped a lot in this task as well, so comparing onsite results to online mirror isn't exactly fair in my opinion either. So, while this is definitely an amusing situation, it was the hardest task in the set in my opinion, and I'm not overly surprised, especially since I feel that a lot of people kept working on other tasks as well instead. Which maybe, given the difficulty to score even some points here, was in most cases absolutely the right decision.

We have now published the analysis for Day 2.

BOI will not be a good representative for how IOI will be proctored I’m afraid.

Regarding IOI, Singapore has started to distribute drafts of proposed proctoring requirements to the delegations. I believe these weren’t released to public because it’s still work in progress and large parts of it would only affect delegations themselves rather than students.

Having said that, let me ask the hosts if they are willing to provide some sort of short summary for how IOI is planned to run to the students & general public.

UPD: I have checked and more information is expected to be released to public in early August.

We are at the moment still planning to hold an online mirror for the onsite competition next week but dates/times aren’t confirmed yet. We will send an announcement as soon as this is confirmed.

These are the two relevant articles:

• http://e-maxx.ru/algo/binary_pow
• https://cp-algorithms.com/algebra/binary-exp.html

English users had a disadvantage, since code on the Russian website had int. Implying that Vasya copy-pasted the code without realizing internally it needs to use long long.

So, this binpow "works":

int binpow (int a, int n, int m) {
	if (n == 0)
		return 1;
	if (n % 2 == 1)
		return (binpow (a, n-1, m) * a) % m;
	else {
		int b = binpow (a, n/2, m) % m;
		return (b * b) % m;
	}
}

In another announcement today (see below), it was announced that IOI 2020 will happen online from September 13 — 19 with an additional day on September 23.

I certainly don't and I'd like to know especially if they actually considered it or just picked a date that seemed good for them and later found out there's a clash. You imply that it's the former case, so you know something more?

As a member of IOI International Committee I indeed know more, but am very careful with my statements as to not intentionally or accidentally release non-public information. Having said that, I can say the following:

The clash was discovered earlier than any side announced the dates publicly. To the best of my knowledge both sides tried to resolve the clash but were unfortunately unable to do so. For the IOI side, the dates chosen turned out to be the only ones possible due to people and venue constraints. A year of notice with already finalized budget (for IOI 2020 onsite) to organize event of such scale isn't "all the time in the world" and severely restricts available options unfortunately. Please also understand the additional challenges of scheduling under these circumstances – a lot of events get rescheduled at a short notice and at the same time, which is definitely not what usually happens in regular circumstances.

The limiting factor is the amount of attention a human can give. You lose out on one or both.

I agree with you. Which is why effort is being made so that affected people do not have to follow two competitions in one day.

As jonathanirvings's response implies, some of the constraints may involve people resourcing or financial aspect as well, which you can't just magically avoid. I suspect that you do not know all the factors Singaporean side had to consider before deciding that the clash would be unfortunately necessary.

If you find that avoiding clash between IOI/ICPC (which is very unfortunate for some delegation members indeed but at least doesn't affect contestants at all) is the absolute top priority in all circumstances, I have no issues stating that I personally can consider many factors to be much more important than having no clash between IOI/ICPC.

Taking care that contest days don't overlap in this instance is helpful for members involved in both competitions to be able to follow both of them separately. If I'm the coach of both teams and my priority is tending to both teams, even if one of them has to be online; this is absolutely crucial. I believe that in most of the cases it should be possible to find replacements for onsite delegation representatives. In my personal opinion, this causes the biggest problems for Singapore itself, where almost everyone involved in competitive programming would be involved in IOI 2021.

Had a roller-coaster with C after the contest.

• Wrote a simple DP over bitmasks for sizes of pen with 3 moves (take 2 random, remove 1 learning its size, try all pens removing 0 if it was present).
• This gave around 69% so expected to get all three.
• Coding this was a pain because of merged test cases.
• Had bugs, couldn't debug in time.
• Saw that A + B1 + C is enough for WF.
• FFFUUUUUU.
• Shortly after the contest re-read the statement and found that the only bug was that I thought pens' initial capacity were from 1 to N rather than from 0 to N-1.
• FFFFFFFFFUUUUUUUUUUUUUU.
• Submit during practice.
• WA on C3.
• A + B1 + C1 + C2 wasn't enough anyway. MASSIVE RELIEF.
• Rerun locally, with reducing the amount of ink in pens, it's now 60.44%.
• (facepalm) (facetable)

Conclusion: after all those years I still haven't learned to read problem statements.

The only released information so far is that, as communicated previously, it will happen no earlier than September.

Also, as this statement said, many details need to be discussed and decided upon. We understand that a lot of people are waiting for the specifics and are aiming to update the community as soon as possible after decisions are made.

There is some information on EJOI's Facebook page.

Formally, if you look at IOI regulations, you have a couple of interesting points there.

S3.1 The General Assembly is a temporary, short-term committee during IOI’n, which is composed of the Delegation Leaders and the Deputy Leaders of all Participating Countries. The GA is the owner of the IOI.

Stating that "IOI doesn't trust team/deputy leaders" essentially translates to "IOI doesn't trust IOI". Of course, it doesn't mean that there should be no oversight altogether from the hosts, but I don't think that not trusting team/deputy leaders at all is a good look for IOI either. There is a level of trust in team/deputy leaders at regular IOIs as well, so this isn't suddenly introduced by online IOI. Depending on how exactly would online IOI be organized, there level of that trust does indeed vary, and everyone involved is aware and mindful of that.

E3.1 The GA acts in the general spirit of the Regulations. The GA has the following tasks:

• Supervise and participate in the selection of Competition Tasks and confirm the awarding of medals

So even if we talk specifically about the act of releasing tasks to team/deputy leaders early, it's done not only for translation. GA actively participates in task selection and while it usually goes without any major changes, there have been incidents when task was removed or modified (for example, because it was too similar to already existing task when ISC wasn't aware about this). So this concern of yours isn't actually exclusive to task translations.

As mentioned, many working details will need to be discussed by various committees, so I have nothing on this at this point. I'm always happy to hear opinions though which will hopefully lead committees to more informed decisions.

What exactly is your concern with task translations? How is that concern exclusive to task translations?

I actually miss the days when a lot more competitive programming contests were evaluating the skill of testing your implementation. In the long run I feel that it certainly helped me to write working code on the first try more often.

So the last 15 minutes of this round, for example, instead of focusing on D.small, I specifically was testing my solutions (1-line max test for A, multiple manual tests for B.large since B.small is useless for testing that etc.). Every contest has different rules and you have to be able to adapt your strategy to each one of them.

I do not expect host or IOI Committees to be able to answer questions on country-specific selection plans. If you have questions or concerns regarding country-specific plans to host such selections, please contact your country’s contact person with the details available on IOI website.

It’s highly likely that countries might not yet have that plan ready since situation is changing rapidly and given the current information, there are still almost four months to select teams. So understand that you are not likely to get any definitive answer at this stage, which may mean that you might need to make decisions around your workload based on very incomplete information unfortunately. But such are times we live in now.

Cancellation of team selection tests doesn’t imply that team cannot be selected at a later time or in a different format, especially when there hasn’t been a decision reached about IOI 2020 yet.

It is absolutely pointless to speculate on who will or will not participate in an event for which no dates have been set yet. So I kindly suggest you stop creating anonymous accounts to ask speculative questions to which you will not get any answers.

As I said previously, it is known that many countries cannot hold their selections in a regular timeframe, hence an announcement was made that IOI 2020 will not happen until at least September, giving delegations more time. Hosts and IC is aware that delegations need to finish their selections and are factoring this into discussions of potential options for IOI 2020.

There are no updates since the last announcement.

I do not believe IOI Committees or hosts have the ability to know whether any country has or has not completed their team selection. However, I believe that there is a significant amount of delegations who were unable to complete their selection (Latvia is one of them, for example), hence an announcement was made that “In whatever form it runs, IOI 2020 will not take place any earlier than September 2020” to give delegations more time to complete their team selection.

I haven't solve E, but here are my thoughts process if that helps.

• Run some brute force on small numbers. Get a hypothesis that for $$$N \geq 4$$$ only $$$N+1$$$ and $$$N^2-1$$$ traces aren't possible.
• Read from Wikipedia that "The problem of determining if a partially filled square can be completed to form a Latin square is NP-complete".
• From there you can guess that it's likely not some algorithmic problem (at the top level), but a construction one.
• Then I decided I have better things to do. :D

Now, after reading the analysis, turns out it was indeed construction problem at the top level.

Quoting regulations:

A Contestant is a student who

1. was enrolled in a school at a level not higher than secondary education, in the Country they are representing, for the majority of the period 1 September to 31 December in the year before IOI’n. Students who are studying abroad may represent the Country of their nationality. Exceptions may be requested through the IC.
2. is not older than twenty years on the 1st of July of the year of IOI’n

Remember that GA does not only translate tasks. They select tasks and can file minor or major objections. Which would be hard to do with unknown solutions, since everyone at GA would be forced to solve the task to see if the approach used is extremely similar to what they've used at their national competition/training, for example. See point #13 from IOI 2013 GA minutes for an example of what may happen during task selection.

So as long as GA selects the tasks and does not delegate it entirely to ISC/HSC, I'm not seeing how this aspect can be avoided.

As far as I know leaders don't get the solutions as well

That is incorrect. Every IOI I've been at GA I can remember, short notes describing a solution were presented to GA either with the problems or shortly afterwards.

As e-mail stated, "as a backup plan, we are looking seriously for an online IOI". That means that the plan is still being developed. However, maintaining the integrity of IOI is the top priority when designing such a plan, so I personally don't believe that will result in online access for the IOI competitors. At the moment, my understanding of the word "online" in this context is that we are looking seriously for an online IOI, where everyone would not be physically in the same location.

My personal opinion, is that there are ways to organize an IOI online while ensuring the integrity of the competition, so my position is strongly at "online IOI is better than no IOI". Some of your points are very important and I believe both hosts and IC are aware of them while developing that plan. However, I personally, don't think that some arguments such as timezones (which in my eyes is always a factor in IOI – I often couldn't sleep before competitions) are enough of an argumentation to deny the whole year of students an opportunity to showcase their skills and results of years of preparation, but those arguments are definitely something still worth considering, so thanks for bringing them up.

P.S. Your accidental cheating point is an interesting one in general; and I personally believe that it happens often in onsite programming competitions. I wouldn't be surprised if at some competition I mumbled something a bit too loud and someone could have used that information. But it's something that I'll definitely keep in mind, as whereas usually it benefits a random participant (however, not always, some ICPC quarterfinals with multiple sites may be another example); if that pool is a random participant from your country or university; that becomes something that needs addressing in my opinion.

Yes, please see the official e-mail from IOI President I've just posted here as a comment.

The latest update regardless IOI and COVID-19 has just been circulated on ioi-announce@.

It doesn't offend me, I'm just very tired of hearing about this game in every single YouTube video... :D

Glad the explanation helped!

I'll downvote anything containing words "raid", "shadow" and "legends" in that combination. :D But the fact that I had to find the editorial myself, probably didn't help.

On a serious note, you're correct, time complexity would still be quadratic. But in this problem, you're not interested in time complexity, you're interested in the amount of comparison calls you need to make, and in that aspect, switching to binary and ternary insertions reduce that complexity to O(N log N).

As for approaches, in all you have an array of $$$x$$$ indices sorted and you want to add $$$x+1$$$ in the right position.

• Linear insertion just check all consecutive two indices and your new index, until it is reported as a median – that would be the right position for it.
• Binary insertion – check two consecutive elements in the middle: if the median isn't your new index, then based on which one of the two old ones is reported as median you can eliminate half of possible positions.
• Ternary insertion – take two elements at around 1/3 and 2/3 of interval. Based on median reported, leave only the appropriate third of the interval.

There was another public announcement regarding IOI 2020 from hosts recently, so I'll quote it here (in case someone else like me, missed the update on official IOI 2020 website):

As you can see, the hosts have already done a short survey on the situation and have shared the results with IC. I am also always monitoring Codeforces for IOI-related topics and in the event of some great feedback/opinions/options/discussions/questions appearing here, I will make sure to summarize it and let the rest of the IC know of it when we'll have our emergency meetings.

From my understanding (since I wasn't attending that discussion in person) at the IOI 2019 group discussions various members of GA discussed this and expressed its desire for IC to introduce Honourable Mentions. The chosen amount of 70% was to be consistent with other major Science Olympiads as well, where you have:

• IMO: HM is using a different criteria, but effectively over the last 5 years average of 74.25% received an award (medal or HM)
• IPhO: 67% receive an award
• IChO: 70-71% receive an award
• IBO: 70% receive an award

In my personal opinion, Honourable Mentions are well-established in other Science Olympiads and do not dilute the value of the medals themselves. Similar to IMO (can't vouch for other Science Olympiads), the recipients of Honourable Mentions would not be a part of the Closing Ceremony.

So I believe that your concern is a valid one and to answer that IC believed that there was a clear desire to look into this from GA and this is similar to other major Science Olympiads. However, as I mentioned, all of this is due to approval vote form GA at IOI 2020, and unless I'm mistaken, this would also require a 2/3 majority (rather than a simple majority) vote to pass; so this will not actually take effect without explicit GA approval.

C is not the boundary of the length of a number. It is a boundary of a number. 10 is coming from the fact that the maximum allowed value of C has 10 digits.