From 43c0e21657c63445f247ca9f3f71d039d5a8c546 Mon Sep 17 00:00:00 2001
From: Simmo Saan <simmo.saan@gmail.com>
Date: Thu, 21 Dec 2023 08:09:39 +0200
Subject: [PATCH] Add naive solution to 2023 day 21 part 2

---
 .../eu/sim642/adventofcode2023/Day21.scala    | 27 +++++++++++++++++++
 .../sim642/adventofcode2023/Day21Test.scala   | 14 ++++++++++
 2 files changed, 41 insertions(+)

diff --git a/src/main/scala/eu/sim642/adventofcode2023/Day21.scala b/src/main/scala/eu/sim642/adventofcode2023/Day21.scala
index 450c70ce..637156e3 100644
--- a/src/main/scala/eu/sim642/adventofcode2023/Day21.scala
+++ b/src/main/scala/eu/sim642/adventofcode2023/Day21.scala
@@ -4,6 +4,7 @@ import eu.sim642.adventofcodelib.Grid
 import eu.sim642.adventofcodelib.GridImplicits.*
 import eu.sim642.adventofcodelib.graph.{GraphSearch, SimultaneousBFS, UnitNeighbors}
 import eu.sim642.adventofcodelib.pos.Pos
+import eu.sim642.adventofcodelib.IntegralImplicits._
 
 object Day21 {
 
@@ -27,11 +28,37 @@ object Day21 {
     SimultaneousBFS.search(graphSearch).distances.count(_._2 % 2 == steps % 2)
   }
 
+  // copied from 2021 day 25
+  extension (pos: Pos) {
+    def %+(other: Pos): Pos = Pos(pos.x %+ other.x, pos.y %+ other.y)
+  }
+
+  def countReachableExactlyInfinite(grid: Grid[Char], steps: Int = 26501365): Int = {
+    val gridSize = Pos(grid(0).size, grid.size)
+
+    val graphSearch = new GraphSearch[Pos] with UnitNeighbors[Pos] {
+      override val startNode: Pos = grid.posOf('S')
+
+      override def unitNeighbors(pos: Pos): IterableOnce[Pos] = {
+        for {
+          offset <- Pos.axisOffsets
+          newPos = pos + offset
+          if grid(newPos %+ gridSize) != '#'
+        } yield newPos
+      }
+
+      override def isTargetNode(pos: Pos, dist: Int): Boolean = dist == steps
+    }
+
+    SimultaneousBFS.search(graphSearch).distances.count(_._2 % 2 == steps % 2)
+  }
+
   def parseGrid(input: String): Grid[Char] = input.linesIterator.map(_.toVector).toVector
 
   lazy val input: String = scala.io.Source.fromInputStream(getClass.getResourceAsStream("day21.txt")).mkString.trim
 
   def main(args: Array[String]): Unit = {
     println(countReachableExactly(parseGrid(input)))
+    println(countReachableExactlyInfinite(parseGrid(input)))
   }
 }
diff --git a/src/test/scala/eu/sim642/adventofcode2023/Day21Test.scala b/src/test/scala/eu/sim642/adventofcode2023/Day21Test.scala
index 410a5046..9aba7e07 100644
--- a/src/test/scala/eu/sim642/adventofcode2023/Day21Test.scala
+++ b/src/test/scala/eu/sim642/adventofcode2023/Day21Test.scala
@@ -25,4 +25,18 @@ class Day21Test extends AnyFunSuite {
   test("Part 1 input answer") {
     assert(countReachableExactly(parseGrid(input)) == 3687)
   }
+
+  test("Part 2 examples") {
+    assert(countReachableExactlyInfinite(parseGrid(exampleInput), 6) == 16)
+    assert(countReachableExactlyInfinite(parseGrid(exampleInput), 10) == 50)
+    assert(countReachableExactlyInfinite(parseGrid(exampleInput), 50) == 1594)
+    assert(countReachableExactlyInfinite(parseGrid(exampleInput), 100) == 6536)
+    assert(countReachableExactlyInfinite(parseGrid(exampleInput), 500) == 167004)
+    //assert(countReachableExactlyInfinite(parseGrid(exampleInput), 1000) == 668697)
+    //assert(countReachableExactlyInfinite(parseGrid(exampleInput), 5000) == 16733044)
+  }
+
+  test("Part 2 input answer") {
+    //assert(countReachableExactlyInfinite(parseGrid(input)) == 3687)
+  }
 }