Can you crack the Black Box?

Eduard Lebedyuk 19 June 2017

Hi, Community!

And so I continue with publishing of the tasks for the Final round of InterSystems Contest on InterSystems Caché and DeepSee as a part of IT Planet Student Championship in Sochi. This year we had about 2 000 participants in InterSystems Contest.

One of the tasks for the finals was to crack the black box!

Problem description

First of all, download and import the black box:

write ##class(%Studio.Project).InstallFromFile("BlackBox.xml")

Black Box has only one class - ITPlanet.BlackBox and one method - Main. If you call it from a terminal:

write #class(ITPlanet.BlackBox).Main()

it would throw an exception. However, if you call it just right, it would return something slightly more meaningful.

But what? And how do you need to call it?

Answers

Answer by Timothy Leavitt 19 June 2017

Not the most creative output:

USER>write ##class(ITPlanet.BlackBox).Main(<left as a mystery>,<also left as a mystery>)
Hello World!

@Eduard, I'm assuming you don't want spoilers here beyond that?

One non-spoiler: there are actually two valid options for the first argument that will produce this output.

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Comment by Eduard Lebedyuk 19 June 2017

That was fast.

I'm assuming you don't want spoilers here beyond that?

Yes, for now.

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Answer by Dmitry Maslennikov 20 June 2017

Btw, it was a good idea, to deploy code. But I think bad idea to hide original string in $ListBuild

set list=$lb(72,101,108,108,111,32,87,111,114,108,100,33)

Because, such static variables, stored as is in OBJ code, and could be easily recognized.

USER>zzdump list

0000: 03 04 48 03 04 65 03 04 6C 03 04 6C 03 04 6F 03         ..H..e..l..l..o.
0010: 04 20 03 04 57 03 04 6F 03 04 72 03 04 6C 03 04         . ..W..o..r..l..
0020: 64 03 04 21                                             d..!

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Comment by Dmitry Maslennikov 20 June 2017

I don't know how close my version of this method to the original, but it should be something like this. But I think, your version a bit bigger.

ClassMethod Main(cfg As %SystemBase, mode As %String) As %String
{
    set trantabla=cfg.trantable
    set str=""
    set list=$listbuild(72,101,108,108,111,32,87,111,114,108,100,33)
    for i=1:1:$listlength(list) {
        set n=$listget(list, i)
        set hex = $zhex(n)
        set str = str _ "\u" _ $extract("000" _ hex, *-3, *)
    }
    set str=$zconvert(str, mode, trantable)
    set:$length(str)'=12 str="????? ??????"
    quit str
}

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Comment by Eduard Lebedyuk 20 June 2017

How do you propose to store static strings?

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Comment by Dmitry Maslennikov 20 June 2017

maybe something like this

USER>set list="ªÅÊÈÉxÃÄ¼²m"

USER>for i=1:1:$l(list) s c=$a(list,i)-100+(i*2) write $c(c)
Hello World!

or another way a bit easier to decode, I think

USER>set list="Iemlp Xosle!"

USER>for i=1:1:$l(list) s c=$a(list,i)+(i#-2) write $c(c)
Hello World!

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Answer by Vitaliy Serdtsev 20 June 2017

The solution:

USER>w ##class(ITPlanet.BlackBox).Main({"trantable":"JSON"},"I")
Hello World!
USER>w ##class(ITPlanet.BlackBox).Main({"trantable":"JS"},"I")
Hello World!

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