Controlling the Tower

I recently came across this problem and it looked interesting enough, so I solved it. It’s from Topcoder, which is nice because they provide test cases so you can be sure that you have solved it.

Unfortunately, Topcoder has dracanion copyright laws, so I will paraphrase the problem in case the link to the description dies. You want to control some territory, the territory has t (an integer) towers and each tower is a certain number of stories s (an integer). To control the territory you must control the majority of the active towers. A tower is active if someone controls the majority of stories in a tower. If a tower has no majority controller, it is inactive and does not count. What is the minimum number of stories you must win to guarantee control of the territory?

The first thing that helped me was converting the test cases into the following two lists and created a table:

inputs:((47);(9;9);(1;1;1;1;1;1);(2;2;2;2;2;2);(1;2;3;4;5;6;7;8;9);(1;2;3;4;5;6;7;8;9;10);(1;1;100;100;100;100;200;200);(2);(999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999;999);(2;3;9);(1);(3);(4);(5);(6);(1;1);(1;2);(2;2);(1;3);(1;4);(2;3);(2;4);(3;3);(100;100);(1;1;1);(1;2;1);(2;2;1);(2;2;2);(10;100;900);(9;99;999);(2;8;4);(2;2;2;2);(2;2;2;2;4);(784;451;424;422;224;478;509;380;174;797;340;513;842;233;527;342;654;816;637);(342;574;20;606;413;479;51;630;802;257;16;925;938;951;764;393;766;414;764;208;959;222;85;526;104;848;53;638;969;214;806;318;461;950;768;200;972;746;171;463);(877;587;587;325;528;5;500;714;548;688;243;5;508;783;561;659;297;44;70;131;329;621;374;268;3;113;782;295;902);(159;797;323;629;647;941;75;226);(528;259;717;920;633;957;712;966;216;812;362;146;472;462;140;743;672;512;232;16;186;721;171;831;641;446;183;483;198;55;610;489;884;396;344;499;368;618;812;211;771;449;289);(608;405;630;751;288;836))
results:(24;14;4;7;36;45;699;2;37451;12;1;2;3;3;4;2;3;3;4;5;4;5;5;150;2;3;4;4;955;1053;11;5;8;7806;18330;10838;3404;18074;2866)
t:([]inputs;results)

q)select results, inputs from asc t  / just to give a visual sample

results inputs
———————————–
1      1
2      2
2      3
3      4
3      5
4      6
24     47
2      1 1
2      1 1 1
4      1 1 1 1 1 1
699  1 1 100 100 100 100 200 200

That way, I could write a function and start testing and seeing if I was making progress.

If there is only one tower, then I obviously need to take it, which means I need to take the majority.

So that is pretty easy:

f{[x] if[count[x]=1; :sum 1+floor x%2]; :0} /one tower,
/ otherwise return 0 this is wrong but we want to return a number

I can run my function and see which cases I still haven’t solved:

q) select myResult, results, inputs from (update myResult:f each inputs from t ) where myResult<>results

This gave me a way to check if I was making progress.

Next, we know that we need to win the majority of the towers and we need to win them in the most expensive way possible in order to guarantee a victory because we don’t get to chose how our stories our distributed. So we sort the towers desc by height and fill up half of them. Then comes the tricky part, the opposition can have a simple majority in all the other towers. In that case, there will be one final tower that decides everything.

For example, suppose there are 5 towers, 6 6 5 5 4. We need to win three towers. So we win the first 2 by getting 12 stories. Then we only need one more tower but the opposition can strategically win the shortest towers. It’s important to note that winning a 4 story tower and a 5 story tower takes 3 either way. So they can do better by winning the two 5 story towers and simply stop me from winning the 4 story tower by taking 2 stories.

This explains the following slightly convoluted algorithm:

f:{
if[count[x]=1;:sum 1+floor x%2]; /one tower
t:floor count[x]%2; / majority of towers, one less if there are an odd number
v:sum t#x:desc x; /most expensive way to win t towers
r:reverse (t-count[x])#x; /remaining towers
o:sum 1+floor %[;2] t#r; /opposition needs to take at least t towers
o+:sum floor 0.5+%[;2] d _ r; /oppositions needs to prevent the rest
o-:(any(d _ r)mod 2)*(any not(d#r)mod 2) ;
/subtract if opposition wins during preventing
/ and you can strategically exchange an odd tower with an even one
v+1+sum[r]-o
};

If anyone has a simpler, more straightforward approach, or wants to tell me how my approach is wrong, I would love to hear about it.

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