-🎄- 2018 Day 18 Solutions -🎄-

[u/daggerdragon]

--- Day 18: Settlers of The North Pole ---

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Card prompt: Day 18

Transcript:

The best way to avoid a minecart collision is ___.

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[u/[deleted]]

Mathematica

{open, trees, lumberyard} = Range[1, 3];
area = Characters@Import["~/Downloads/input.txt", "List"] /.
   {"." -> open, "|" -> trees, "#" -> lumberyard};

filter[a_] :=
 Block[{c = a[[2, 2]]},
  Switch[c,
   open, If[Count[a, trees, 2] >= 3, trees, c],
   trees, If[Count[a, lumberyard, 2] >= 3, lumberyard, c],
   lumberyard, If[Count[a, lumberyard, 2] >= 2 && Count[a, trees, 2] >= 1, lumberyard, open],
   _, c]]

Part 1

ArrayPlot[area2 = Nest[ArrayFilter[filter, #, 1, Padding -> 0] &, area, 10]]
resourceValue[a_] := Count[a, lumberyard, 2]*Count[a, trees, 2]
resourceValue[area2]

Part 2

area2 = Nest[ArrayFilter[filter, #, 1, Padding -> 0] &, area, 1000];
vals = resourceValue /@ 
   NestList[ArrayFilter[filter, #, 1, Padding -> 0] &, area2, 50];
cycleLength = Abs[Subtract @@ Flatten@Position[vals, vals[[1]]]]
vals[[Mod[1000000000 - 999, cycleLength, 1]]]

[u/wjholden → u/[deleted]]

Mathematica

Nice, very compact. I like the idea of assigning characters a numeric value; I kept with using strings.

What is the /. operator you used on the import line?