MaRos's blog

By MaRos, history, 2 months ago, In English,

The first online round of the 2017 Facebook Hacker Cup. Open to all. All participatns who answer at least one problem correctly will advance to Round 1. The contest will be going 72 hours, from January 6, 2017 4pm PST till January 9, 2017 4pm PST.

Good luck and have fun in first great competition on 2017!)

To register, visit https://www.facebook.com/hackercup/register.

More information on https://www.facebook.com/hackercup/.

UPD Registration page was updated.

UPD2 Link to the contest: https://www.facebook.com/hackercup/round/1760504744276109/

UPD3 The contest finished, results is here. Congratulations for qualified to the Round 1.

 
 
 
 
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2 months ago, # |
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Auto comment: topic has been updated by MaRos (previous revision, new revision, compare).

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Auto comment: topic has been updated by MaRos (previous revision, new revision, compare).

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2 months ago, # |
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"Good luck and have fun in first great competition on 2017!)" — let me disagree. First great contest will be Snarknews New Year Prime Contest (I hope it will be held as usual) :p.

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2 months ago, # |
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What the hell... It's been almost a week and the registration page still says it's for the 2016 cup.

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Why isn't the registration link working? Is there any special hack for that?

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This is the first time I hear of facebook hacker cup...

What are the problems like...Is there anywhere that I can see the problem set of an older year ?

:)

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2 months ago, # |
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Just for remind.

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Auto comment: topic has been updated by MaRos (previous revision, new revision, compare).

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Why not it's being started yet ?

Please help us providing a link when you get :)

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Where is the link to the Facebook Hacker Cup 2017 contest page ?

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    2 months ago, # ^ |
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    I think it is only published on the Hacker Cup Facebook page close to the start of the contest.

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Bump. Contest has started.

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Which button for submitting our input and selecting the language that we wrote? can we submit solution only once?

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Auto comment: topic has been updated by MaRos (previous revision, new revision, compare).

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2 months ago, # |
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Are outputs of the form "0.4553223e-08" in the third problem acceptable?

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    2 months ago, # ^ |
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    No. Just print 6 digits after point

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    2 months ago, # ^ |
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    It should be okay, don't worry.

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    7 weeks ago, # ^ |
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    Yeah, we do a float cast on the string (in PHP), and that works fine for scientific notation.

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      7 weeks ago, # ^ |
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      Wow, you should be the only popular online judge that actually runs in PHP (or maybe its optimized version you use in production in Facebook, I've heard something about that).

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        7 weeks ago, # ^ |
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        It's called Hack. I find it much more sane than vanilla PHP.

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          7 weeks ago, # ^ |
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          Yeah, it's indeed Hack, and not vanilla PHP. Strong typing ftw!

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            7 weeks ago, # ^ |
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            wjomlex: In later rounds that are 3 hrs long(Round 2 onwards I believe), will the submissions be judged instantly or will there be any sort of pretests during the round itself since the round is just 3 hours long ?

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              7 weeks ago, # ^ |
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              As far as I know they never tell you if your submission is correct or not, before the end of the contest. You only have one try for each problem.

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              7 weeks ago, # ^ |
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              Submissions and scoring work the same way in every round.

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2 months ago, # |
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The reader and writer don't matter, right? I just uploaded an output file(made with fstream) and after that I switched the reader and writer to iostream, and I uploaded the code. Is it correct? The output file had a random name.

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    2 months ago, # ^ |
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    I guess, that source code is needed only to check for plagiarism, but I am not sure

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    2 months ago, # ^ |
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    It's OK to upload any code that you believe it can solve the problem

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What's the need of source code when we submit the output?

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    2 months ago, # ^ |
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    To make sure that you didn't just copied/stole the results from someone else.

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      2 months ago, # ^ |
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      Can I send a hello world program instead?

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        2 months ago, # ^ |
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        As far as I know they have system test as well.

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          2 months ago, # ^ |
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          There are no system tests. They only check, if the output is uploaded output-file contains the correct solutions.

          But I still would submit the used program. Terms and conditions says, that if there is an alleged discrepancy between the source file and the solution, they will check the code and the competitor might get a time penalty or might loose his points. Can't really imagine them doing so (with over 6000 participants), but still ...

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    2 months ago, # ^ |
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    It is stated in terms&rules on their facebook link.

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      2 months ago, # ^ |
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      I believe that your premise is false. Facebook does not specify any requirements about the source code, languages or libraries used at all, so one is free to use anything available in the world. Say, if your solution contains several files, you can zip them together and send an archive. Automating that kind of testing would be incredibly hard.

      As far as I know, there is no automatic testing of submissions. Submitted output file check is the only thing which matters for OK/WA. Source code is probably used for plagiarism check. Some submissions still could be checked manually (say, if it's someone passing to the onsite round).

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        2 months ago, # ^ |
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        I have a question. Do they give random inputs for different participants and check their outputs according to output produced by their own program using same inputs or it's same for everyone? If it is the latter case then they are ought to have screwed everything up. Person A can solve and give output to person B and they upload random sources and tadaa... nobody can prove you did fishy things.

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          2 months ago, # ^ |
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          IIRC, each participant is given a randomized data set.

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          7 weeks ago, # ^ |
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          Even if the input is different. Person A can just run it's code for person B and pass B the output. What's the difference?

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            7 weeks ago, # ^ |
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            So I emailed them and asked about possible ways of cheating including your point and how could they prevent it. They said; "That's true, cheatin­g can never be fully ­prevented in any onli­ne competition."

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              7 weeks ago, # ^ |
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              Ya, my point is that regardless of given random inputs to participants, "cheatin­g can never be fully ­prevented in any onli­ne competition."

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There is a "Download Input" button, clicking on which you'll get a file of the input .We have to generate output and submit it along with our code in 6 minutes. If you fail to do this in 6 minutes than you won't be able to submit the code for the problem. So click on "Download Input " only when you are ready.

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Spoiler
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On the "Progress Pie" problem, is the given point considered black if it lies on the circumference of the pie?

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    2 months ago, # ^ |
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    Do not discuss problem from ongoing competition. And read constraints/problem carefully btw

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      7 weeks ago, # ^ |
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      Well, I agree with this question. It wasn't clear. It's better when you don't need to guess what understanding is more probable.

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        7 weeks ago, # ^ |
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        Whenever a point (X, Y) is queried, it's guaranteed that all points within a distance of 10-6 of (X, Y) are the same color as (X, Y).
        

        I believe this is clear. A given point in the input will never lie on the circumference of the pie.

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        7 weeks ago, # ^ |
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        Maybe it's not specified but it's not needed because you can't get such query

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After submitting, I can theoretically download my output and source to check that everything went as planned. The output can be downloaded alright. However, when I try downloading the source, it gives me a page with the following message:

Sorry, something went wrong.
We're working on getting this fixed as soon as we can.

Now I wonder whether the system actually stored my source.

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    2 months ago, # ^ |
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    I experienced the same thing and asked clarification.

    Answer: "We apologize for the inconvenience, we're working on fixing the issue with downloading source code files. Your source code has been successfully submitted."

    So, most probably your source is stored as well.

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When and where can we get to know the result of Qualification round and check if our program passed on the system test?

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    7 weeks ago, # ^ |
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    Just wait till the round ends, then the system test will take place.

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      7 weeks ago, # ^ |
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      I don't have to wait for the round to end to know I'm going to have at least 1 failed submission ;_;

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to wjomlex and other Facebookers

There were several improvements that were suggested last year such as

Could you share any news on this features (e.g what is done, what is planned and what is not planned) or any other FBHC updates?

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    7 weeks ago, # ^ |
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    Ad statements for non-competitors: I saw the statements without being registered, if that's what you mean (seeing them without being logged in is probably a feature of FB). Dunno if it will be different in other rounds.

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    7 weeks ago, # ^ |
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    Mobile version: You can view the page on mobile, but it's definitely not optimized for mobile. This is a low priority since you basically need to be using a desktop/laptop to be competitive.

    Test distribution: Every problem has a set of tests that you're guaranteed to get, and this includes ones that test edge cases.

    Samples: The samples are always included in your input file, though not as the first K cases. So we don't really have that sort of pre-testing. However, the logic for presentation errors has been made more lenient, and you get a warning if you try to upload something that has the wrong number of lines, or bad "Case #x:" formatting. I talked about this last year: http://codeforces.com/blog/entry/15981?#comment-274649

    Giving encrypted files: We've been trying to avoid this problem by simply not having problems that have large input or output (our goal is to constrain both to about 3MB, and generally smaller than that). At onsite finals we relax this restriction since we can guarantee that everybody is on the same quality connection.

    Downloading output: You can do this now.

    Past rounds page: You can find it here: https://www.intern.facebook.com/hackercup/past_rounds/1760504744276109/

    Statements for non-competitors: The problems are now viewable by anybody, even if they're not logged into Facebook.

    We made the scoreboard show immediately upon round completion this year, since our previous approach required about a 10-minute delay.

    Also, we'll be rolling out a new clarification system, ideally for the next round, which'll make it easier for us to respond to clarifications, and let you view your clarifications in the Hacker Cup UI rather than having to go to your email.

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      7 weeks ago, # ^ |
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      Looks like the new clarification system will be ready for next week, but we'll use the normal one this week again.

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      7 weeks ago, # ^ |
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      Cool, thanks.

      As for usability from mobile: it is redirecting me to the m.facebook.com, which just shows "Sorry, something went wrong"

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        7 weeks ago, # ^ |
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        Good to know. We should at least make sure it doesn't totally break :)

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From the Progress Pie statement: "Envision your screen as a square on the plane with its bottom-left corner at (0, 0), and its upper-right corner at (100, 100)." Does it mean that the circle diameter is 100 or 101? Surely there are 101 pixels between 0 and 100 :) In the editorial solution the center of the circle is (50;50) with the radius=50. Why not (50,5;50,5) and radius=50,5?

ProgressPieCenter

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    7 weeks ago, # ^ |
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    Don't think about pixels. I was confused by this at first too, but in fact the problem statement does not mention pixels at all. Simply think of the points on the plane (0,0) and (100,100).

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      7 weeks ago, # ^ |
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      Since the problem statement started with the description of progress bars and then mentioned the screen and "Every point on the screen is either white or black." I somehow assumed we are not talking about points on the geometry 2D plane, but about the actual screen with the points being the alias for pixels.

      In the context of "a screen" I would be more inclined to read the expression (100, 100) as a pixel coordinate (ie. integer) than a real numbers coordinate.

      Anyway, I have read the problem statement once more and when I forget about the screen metaphor (ie. completely ignore visualizing the progress bar, "There are no pixels." etc.), then your explanation makes perfect sense. Thanks.

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    7 weeks ago, # ^ |
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    The top right point in your pic is (5, 5), not (4, 4).

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      7 weeks ago, # ^ |
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      It really depends on whether you're talking 2D plane geometry or pixels on the screen, on which the progressbar is drawn.

      "While you wait for the progress pie to fill in, you find yourself thinking about whether certain points would be white or black at different amounts of progress."

      I mean, while you are waiting are you thinking about the abstract 2D plane or about the pixels you are looking at? It probably depends on whether you're a math geek or a computer geek:)

      My mistake was focusing too much on the "screen" part of the story and not enough on the "abstract 2D plane" part.

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        7 weeks ago, # ^ |
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        I fully agree: the statement had poor legend which could misdirect some participants (as it did with me). Strictly speaking, the "abstract 2d plane" interpretation is the only one that makes sense if we think that jury has provided all necessary details, but you never can be sure.

        1. If we talk about "screen" and "pixels screen", then it's natural to assume that all pixels are discrete, as they are in real screens. So, I did exactly that and assumed that we have a square grid.
        2. Next question: are the coordinates given of grid points or grid cells? I believe that's the question raised by tafit3 in their first comment. As there were no mention, I've started suspecting something.
        3. Next question: if everything is discrete, we cannot draw a perfect pie. So, some rasterization should be performed. It also depends on the previous question: are we talking about grid points (this case is pretty much obvious) or grid cells (and we probably should apply Bresenham's algorithm)?
        4. Next question: if the world is discrete, what is the sense of talking about answer being same in 10 - 6-ball around the point asked? At this point I finally decided to mostly ignore the statement and solve the problem on continuous 2d plane. Ranging it as the simplest problem of the round helped me to conclude that the solution should be simple and definitely won't include any fuzz with rasterization.
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7 weeks ago, # |
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In the first problem (Progress Pie), corner cases would be 0 50 51 and 100 50 51. The point 50 51 is on the axis corresponding to 0% or 100% progress. For the first one, the entire board is still white. For the second one, the point and its surroundings are all black.

However, there were no cases with substring 0 50 at all in my input. Were they intentionally avoided by the jury? Or maybe I misinterpreted the problem somehow, and they are not in fact corner cases?

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    7 weeks ago, # ^ |
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    "Whenever a point (X, Y) is queried, it's guaranteed that all points within a distance of 10-6 of (X, Y) are the same color as (X, Y)."

    This is not true for the point (0, 50), thus it does not exist in the input.

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      7 weeks ago, # ^ |
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      All points in an epsilon-ball around the center are the same color when the entire circle is the same color.

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        7 weeks ago, # ^ |
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        Maybe I am idiot and my lack of geometry is showing. But (0, 50) is not the center, (50, 50) is the center, and (0, 50) is on the outer edge of circle, and therefore it is exactly at the discontinuous point of color where one side is black and other is white. Or are you saying something else?

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          7 weeks ago, # ^ |
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          The input is given as the progress percentage, then the coordinate. The coordinate in the proposed input is (50, 51), not (0, 50), which is a valid coordinate to be included in the input.

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            7 weeks ago, # ^ |
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            Oh sorry I misunderstood the whole proposed input, you and Gassa are correct, and indeed it is a good question as to why such cases were not included in input.

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    7 weeks ago, # ^ |
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    (50, 51) is a nice hacking case, however (50, 50) for p=0 and 100 are also nice hacking cases. All of them are absent in my input.

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      7 weeks ago, # ^ |
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      Yeah, the point 50 50 is basically the same deal as 50 51. It could have also appeared only at 0% and 100%, and the answers would be also white and black, respectively.

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      7 weeks ago, # ^ |
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      I was expecting to have those edge cases, but my input also doesn't have them.

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      7 weeks ago, # ^ |
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      As Gassa says, those points can only appear with P = 0 or P = 100, so they're not actually very interesting, with the constraints as given.

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        7 weeks ago, # ^ |
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        By far the most common error I saw was using raw atan2() output (0 degrees to the right, 90 degrees up) and not transforming it to match how the progress pie actually fills in (0 degrees up, 90 degrees to the right).

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        7 weeks ago, # ^ |
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        Why aren't they interesting? They are valid and they should fail some solutions, right?

        Or maybe they aren't valid? I think that all input constraints are satisfied for p = 0, x = 50, y = 60 and my accepted program doesn't solve this input correctly.

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        7 weeks ago, # ^ |
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        They are completely valid and I am sure they will cause some solutions to fail (my solutions works on them, but it is just a lucky coincidence, because they didn't come to my mind when I was coding it). They are so called corner cases and even if it is simple to deal with them one has to be aware of their existence and handle them with appropriate care. And it is organizers duty to do their best in order to accept only those solutions which give correct answers on all valid data. Working on real software should teach one that corner cases and error handling are crucial part of creating good software.

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        7 weeks ago, # ^ |
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        Yeah, those cases are valid, I just didn't consider them to be as particularly interesting as some of the other hardcoded corner cases, since the problem is pretty trivial when P = 0 or 100.

        As I read the clarifications for this round though, it became clear that a lot of people were missing the line where I state explicitly that all the points are white when P = 0, so I could have been more thorough about stress testing people on it.

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7 weeks ago, # |
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What mistake I have with my code (Second Problem)? Problem with file?

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    7 weeks ago, # ^ |
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    Seems to fail when N = 1 or all items have weight >= 50

    PS: I didn't run your code

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    7 weeks ago, # ^ |
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    "ans++;" --------> "if(w >= 50) ans++;"

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      7 weeks ago, # ^ |
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      Yes. Cause of this. I didn't qualify

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7 weeks ago, # |
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In problem C, a simple way in C++ to read the input (with optional Z) was

  int H, S, X, Y, Z;
  char dummy;
  string spell;
  cin >> H >> S;
  while (S--) {
    cin >> spell;
    stringstream ss(spell + "+0");
    ss >> X >> dummy >> Y >> Z;

Adding "+0" or just " 0" makes sure that Z = 0 when it is absent in the input.

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    7 weeks ago, # ^ |
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    Initializing Z to 0 also seems to work fine if we do not add "+0".

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    7 weeks ago, # ^ |
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    That's how I did it:

        scanf("%dd%d%d", &spell.times, &spell.sides, &spell.z);
    

    Don't forget that scanf can be used for formatted input too.

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      7 weeks ago, # ^ |
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      I thought about sscanf but I believe you should have checked the return value to know if Z was read or not. Otherwise, I'm not sure, but I suppose the value of spell.z is undefined.

      In any case, I prefer using stringstream because the code is concise and leaves me no doubts of its correctness (same with the comment from anuraganand).

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        7 weeks ago, # ^ |
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        If z is initialized to zero then there is no need to check the return value.

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          7 weeks ago, # ^ |
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          Could you give me a reference for this?

          I did a quick search but could not find a guarantee that scanf or sscanf don't touch arguments that it couldn't successfully read.

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            7 weeks ago, # ^ |
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            There you go. The link contains relevant quotes from the standard so yes, it's safe to use both scanf and sscanf in this case.

            Please don't assume that I'm against the cin usage, but there could be a lot of reasons (outside of the scope of this discussion) one would like to prefer scanf over cin.

            Know your tools and choose what's best, I'm sure my method would be helpful to someone especially because it's a one liner and does not require any additional dummy variables.

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              7 weeks ago, # ^ |
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              Cool. Thanks for the reference!

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When the next round starts? I checked the official page but I didn't find anything about that.

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    7 weeks ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    It's in the FAQ.

    • Online Round 1 will begin on January 14, 2017 at 10:00 AM PST and will last 24 hours.
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7 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

When can we upsolve?

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7 weeks ago, # |
  Vote: I like it +20 Vote: I do not like it

Out of the 12804 participants, 1750 got the perfect (100) score.

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7 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

Can someone please tell me where I made a mistake in my code for first problem.

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    7 weeks ago, # ^ |
    Rev. 2   Vote: I like it +5 Vote: I do not like it

    Just download one of the correct solutions, then compare the result produced by that solution to your own (for the same input), and compare the 2 outputs.

    You can use diffchecker to compare 2 files.

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      7 weeks ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Oops. I already checked them, got wrong on so many tests that I couldn't figure out exactly which part I did wrong.

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7 weeks ago, # |
Rev. 2   Vote: I like it -20 Vote: I do not like it

I downloaded progresspie.in from Dropbox (link is in the editorial). It contains T=2005 tests. While in the problem it is stated that T <= 1000. And the progress_pie.txt I got during quals contains 1000 tests.

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    7 weeks ago, # ^ |
      Vote: I like it +20 Vote: I do not like it

    Each participant gets 1000 tests, but not the same tests. Maybe each participant is given 1000 tests randomly from those 2005 tests.

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      7 weeks ago, # ^ |
        Vote: I like it +20 Vote: I do not like it

      Yes, that is exactly what happened. I read it in their official editorial comments.

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7 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

How does everyone decide that their solution will work within six minutes. What is the bound on the number of calculations for this 10^6 or 10^7 or 10^8 .. ?

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    7 weeks ago, # ^ |
      Vote: I like it +18 Vote: I do not like it

    Depends on your machine. A safe estimate would be 108 per second. And typically you only need a few seconds to solve the problem. The 6 minutes is just to make sure everyone has time to fiddle with files and uploading etc.

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7 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

I passed Elemination Round. Now i want particpate 1 round. When I want log in to facebook,site asked ID from.Then i sended Photo of my Passport.Then this message comed.
We'll get in touch with you at the email address you provided after we've reviewed your ID. You will now be logged out of Facebook.
What can i do,to participate this round quick as possible?