Day 21

aoc/src/dev/ctsk/aoc/Main.scala

@@ -22,7 +22,8 @@ val solvers = Map[Int, Solver](
   17 -> Day17,
   18 -> Day18,
   19 -> Day19,
-  20 -> Day20
+  20 -> Day20,
+  21 -> Day21
 )
 
 def runSolver(solver: Solver, input: os.Path): Unit =

aoc/src/dev/ctsk/aoc/days/Day20.scala

@@ -20,18 +20,18 @@ object Day20 extends Solver(20):
 
     def cheats(radius: Int): Int =
       trace.indices.par
-        .map(a =>
-          (a + 1 until trace.length)
-            .count(b =>
-              val (from, fromDist) = (trace(a), trace.length - a)
-              val (to, toDist) = (trace(b), trace.length - b)
+        .map(toDist =>
+          (toDist + 100 until trace.length)
+            .count(fromDist =>
+              val from = trace(toDist)
+              val to = trace(fromDist)
               val dist = (from - to).manhattan
 
               dist <= radius && (fromDist - toDist >= 100 + dist)
             )
         )
         .sum
-      
+
     val p1 = timed { cheats(2) }
     val p2 = timed { cheats(20) }
 

aoc/src/dev/ctsk/aoc/days/Day21.scala

@@ -0,0 +1,48 @@
+package dev.ctsk.aoc.days
+
+import dev.ctsk.aoc.*
+
+object Day21 extends Solver(21):
+  private val keyPad = Grid(Array("789", "456", "123", " 0A").map(_.toArray))
+  private val buttonPad = Grid(Array(" ^A", "<v>").map(_.toArray))
+
+  private val paths = Memo(paths_)
+  private def paths_(args: (Char, Char, Grid[Char])): Array[Array[Char]] =
+    val (from, to, grid) = args
+    val Point(fx, fy) = grid.findFirst(_ == from).get
+    val Point(tx, ty) = grid.findFirst(_ == to).get
+    val ops = Seq((fx, tx, 'v'), (tx, fx, '^'), (fy, ty, '>'), (ty, fy, '<'))
+    val seq = ops.flatMap((a, b, c) => (a until b).map(_ => c))
+
+    def gap(path: Array[Char]): Boolean =
+      path
+        .scanLeft(Point(fx, fy))((acc, c) => Direction.from(c)(acc))
+        .exists(grid(_).contains(' '))
+
+    seq.permutations.map(_.toArray).filterNot(gap).toArray
+
+  private def countPresses(codes: Array[String], numRobots: Int): Long =
+    type State = (Char, Char, Int)
+    def minCost_(state: State, f: State => Long): Long =
+      val (from, to, robots) = state
+      if robots == 0 then return 1
+      paths(from, to, if robots == numRobots then keyPad else buttonPad)
+        .map('A' +: _ :+ 'A')
+        .map(path => path.sliding(2).map(w => f(w(0), w(1), robots - 1)).sum)
+        .min
+    val minCost = Memo.Y(minCost_)
+
+    def evalCode(line: String): Long =
+      ('A' +: line).sliding(2).map(w => minCost(w(0), w(1), numRobots)).sum
+
+    codes.map(code => code.init.toInt * evalCode(code)).sum
+
+  override def run(input: os.ReadablePath): (Timings, Solution) =
+    val codes = os.read.lines(input).toArray
+    val (p1_time, p1_solution) = timed { countPresses(codes, 3) }
+    val (p2_time, p2_solution) = timed { countPresses(codes, 26) }
+
+    (
+      Timings(0, p1_time, p2_time),
+      Solution(Long.box(p1_solution), Long.box(p2_solution))
+    )

data/21.ans

@@ -0,0 +1,2 @@
+202274
+245881705840972

data/21.in

@@ -0,0 +1,5 @@
+319A
+670A
+349A
+964A
+586A