Day 10

input/day10

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src/day10.scala

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+package aoc
+package day10
+
+val dayNumber = "10"
+
+@main def part1: Unit =
+  println(part1(loadInput(dayNumber)))
+
+@main def part2: Unit =
+  println(part2(loadInput(dayNumber)))
+
+def part1(input: String): String =
+  val (topoMap, trailheads) = parseInput(input)
+
+  def findTrails(from: Point): Vector[List[Point]] =
+    val visited = scala.collection.mutable.HashSet.empty[Point]
+    def aux(cur: Point, alt: Int, path: List[Point]): Vector[List[Point]] =
+      visited += cur
+      if alt == 9 then Vector(cur :: path)
+      else
+        cur.neighbors
+          .flatMap(n => topoMap.get(n).map(n -> _))
+          .filter((n, nAlt) => nAlt == alt + 1 && !visited.contains(n))
+          .flatMap((n, nAlt) => aux(n, nAlt, cur :: path))
+    aux(from, topoMap(from), Nil)
+
+  trailheads.map(findTrails(_).length).sum.toString
+
+def part2(input: String): String =
+  val (topoMap, trailheads) = parseInput(input)
+
+  def findTrails(from: Point): Vector[List[Point]] =
+    def aux(cur: Point, alt: Int, path: List[Point]): Vector[List[Point]] =
+      if alt == 9 then Vector(cur :: path)
+      else
+        cur.neighbors
+          .flatMap(n => topoMap.get(n).map(n -> _))
+          .filter((_, nAlt) => nAlt == alt + 1)
+          .flatMap((n, nAlt) => aux(n, nAlt, cur :: path))
+    aux(from, topoMap(from), Nil)
+
+  trailheads.map(findTrails(_).length).sum.toString
+
+type Point = (Int, Int)
+extension (point: Point)
+  def plus(offset: Point) = (point._1 + offset._1, point._2 + offset._2)
+  def neighbors = Vector(
+    point.plus(-1 -> 0),
+    point.plus(1 -> 0),
+    point.plus(0 -> -1),
+    point.plus(0 -> 1)
+  )
+
+def parseInput(input: String): (Map[Point, Int], Seq[Point]) =
+  val grid = input.split("\n")
+  val coords =
+    for
+      y <- 0 until grid.length
+      x <- 0 until grid.head.length
+    yield (x, y) -> grid(y)(x).asDigit
+  val topoMap = coords.toMap
+  val trailheads = coords.collect { case (x, y) -> 0 => (x, y) }
+  (topoMap, trailheads)