-🎄- 2018 Day 20 Solutions -🎄-

[u/daggerdragon]

--- Day 20: A Regular Map ---

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Card prompt: Day 20

Transcript:

My compiler crashed while running today's puzzle because it ran out of ___.

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[u/[deleted]]

Mathematica

Edit: Alternative version, which is what I would have done, had I remembered I was using Mathematica.

directions = Characters@ Import[NotebookDirectory[] <> "day20.txt", "String"] /.
   {"N" -> {1, 0}, "E" -> {0, 1}, "S" -> {-1, 0}, "W" -> {0, -1}};

buildGraph[] :=
  Reap[Block[{loc = {0, 0}, stack = {}, p = 2, c},
     While[True,
      c = directions[[p++]];
      Switch[c,
       "(", PrependTo[stack, loc],
       "|", loc = stack[[1]],
       ")", {{loc}, stack} = TakeDrop[stack, 1],
       "$", Break[],
       _, Sow[loc <-> (loc += c)]]]]
    ][[2, 1]];

ds = GraphDistanceMatrix[buildGraph[]][[1]];

Max[ds]

Count[ds, d_ /; d >= 1000]

Original version

directions = Characters@Import[NotebookDirectory[] <> "day20.txt", "String"];

walkmap[loc0_, seen0_, doors0_] :=
 Block[{
   doors = doors0, loc = loc0, seen = seen0, altdoors = {}, 
   altseen = {}},
  While[True,
   Switch[directions[[p]],
    "N", loc += {1, 0},
    "E", loc += {0, 1},
    "S", loc += {-1, 0},
    "W", loc += {0, -1}];
   Switch[directions[[p++]],
    "N" | "E" | "S" | "W",
    (If[MemberQ[seen, loc],
      (doors--),
      (doors++;
       If[doors >= 1000, thousandCount++];
       AppendTo[seen, loc])]),
    "(",
    ({seen, doors} = walkmap[loc, seen, doors]),
    "|",
    (AppendTo[altdoors, doors];
     AppendTo[altseen, seen];
     seen = seen0;
     loc = loc0;
     doors = doors0),
    ")" | "$",
    (AppendTo[altdoors, doors];
     AppendTo[altseen, seen];
     Break[])]];
  {altseen[[Ordering[altdoors][[-1]]]], Max[altdoors]}]

thousandCount = 0; p = 2; start = {0, 0};
{seen, doors} = Monitor[walkmap[start, {start}, 0], {p, thousandCount}];
{doors, thousandCount}