Solved day 21

Author: FlorianCassayre

README.md

@@ -36,7 +36,7 @@ _My solutions to the 2024 edition of [Advent of Code](https://adventofcode.com/2
 | **[18](https://adventofcode.com/2024/day/18)** | [solution](src/main/scala/adventofcode/solutions/Day18.scala) |
 | **[19](https://adventofcode.com/2024/day/19)** | [solution](src/main/scala/adventofcode/solutions/Day19.scala) |
 | **[20](https://adventofcode.com/2024/day/20)** | [solution](src/main/scala/adventofcode/solutions/Day20.scala) |
-| **[21](https://adventofcode.com/2024/day/21)** | [](src/main/scala/adventofcode/solutions/Day21.scala) |
+| **[21](https://adventofcode.com/2024/day/21)** | [solution](src/main/scala/adventofcode/solutions/Day21.scala) |
 | **[22](https://adventofcode.com/2024/day/22)** | [](src/main/scala/adventofcode/solutions/Day22.scala) |
 | **[23](https://adventofcode.com/2024/day/23)** | [](src/main/scala/adventofcode/solutions/Day23.scala) |
 | **[24](https://adventofcode.com/2024/day/24)** | [](src/main/scala/adventofcode/solutions/Day24.scala) |

input/21.txt

@@ -0,0 +1,5 @@
+149A
+582A
+540A
+246A
+805A

output/21-1.txt

@@ -0,0 +1 @@
+164960

output/21-2.txt

@@ -0,0 +1 @@
+205620604017764

src/main/scala/adventofcode/solutions/Day21.scala

@@ -0,0 +1,96 @@
+package adventofcode.solutions
+
+import adventofcode.Definitions.*
+
+@main def Day21 = Day(21) { (input, part) =>
+
+  case class Vec(i: Int, j: Int):
+    infix def +(that: Vec): Vec = Vec(i + that.i, j + that.j)
+    infix def -(that: Vec): Vec = Vec(i - that.i, j - that.j)
+    def manhattan: Int = i.abs + j.abs
+
+  def parseLayout(s: String): Map[Char, Vec] =
+    s.split(lineSeparator).zipWithIndex.flatMap((row, i) => row.zipWithIndex.collect { case (c, j) if c != ' ' => c -> Vec(i, j) }).toMap
+
+  val layout1 = parseLayout(
+    """789
+      |456
+      |123
+      | 0A
+      |""".stripMargin
+  )
+
+  val layout2 = parseLayout(
+    """ ^A
+      |<v>
+      |""".stripMargin
+  )
+
+  val directions = Map(
+    '^' -> Vec(-1, 0),
+    '<' -> Vec(0, -1),
+    'v' -> Vec(1, 0),
+    '>' -> Vec(0, 1)
+  )
+  val directionsInverse = directions.toSeq.map(_.swap).toMap
+
+  val A = 'A'
+
+  def isValidPath(position: Vec, path: Seq[Vec], validPositions: Set[Vec]): Boolean =
+    if validPositions.contains(position) then
+      path match
+        case head +: tail => isValidPath(position + head, tail, validPositions)
+        case _ => true
+    else
+      false
+
+  def optimalPaths(from: Vec, to: Vec): Set[Seq[Vec]] =
+    val d = to - from
+    val path = Seq(Vec(d.i, 0), Vec(0, d.j)).map(v => Vec(v.i.sign, v.j.sign) -> (v.i.abs + v.j.abs)).filter((_, d) => d != 0)
+    Seq(path, path.reverse).toSet.map(_.flatMap((vec, count) => Seq.fill(count)(vec)))
+
+  def buildLevelMap(layout: Map[Char, Vec]): Map[(Char, Char), Set[Seq[Vec]]] =
+    val validPositions = layout.values.toSet
+    (for
+      a <- layout.keys
+      b <- layout.keys
+      (pa, pb) = (layout(a), layout(b))
+      paths = optimalPaths(pa, pb).filter(isValidPath(pa, _, validPositions))
+    yield (a, b) -> paths).toMap
+
+  val (digitsLevelMap, arrowsLevelMap) = (buildLevelMap(layout1), buildLevelMap(layout2))
+
+  type Cache = Map[(Char, Char, Int), Long]
+
+  def count(level: Map[(Char, Char), Set[Seq[Vec]]], start: Char, end: Char, depth: Int, cache: Cache): (Long, Cache) =
+    val cacheKey = (start, end, depth)
+    cache.get(cacheKey) match
+      case Some(value) => (value, cache)
+      case _ =>
+        if depth >= 0 then
+          val paths = level((start, end)).map(_.map(directionsInverse) :+ A)
+          val (values, cache3) = paths.foldLeft((Seq.empty[Long], cache)) { case ((costs, cache1), path) =>
+            val (c, cache2) = countAll(arrowsLevelMap, path, depth - 1, cache1)
+            (costs :+ c, cache2)
+          }
+          val value = values.min
+          (value, cache3 + (cacheKey -> value))
+        else
+          (1, cache)
+
+  def countAll(level: Map[(Char, Char), Set[Seq[Vec]]], keys: Seq[Char], depth: Int, cache: Cache): (Long, Cache) =
+    (A +: keys)
+      .sliding(2).collect { case Seq(start, end) => (start, end) }
+      .foldLeft((0L, cache)) { case ((sum, cache1), (start, end)) =>
+        val (result, cache2) = count(level, start, end, depth, cache1)
+        (sum + result, cache2)
+      }
+
+  def solve(robots: Int): Long =
+    input.toLines.map(code => countAll(digitsLevelMap, code, robots, Map.empty)._1 * code.init.mkString.toInt).sum
+
+  part(1) = solve(2)
+
+  part(2) = solve(25)
+
+}