Solve 2020 day 24 part 2

sim642 · sim642 · commit 4284bcd8e5ba · 2020-12-24T10:22:58.000+02:00
diff --git a/src/main/scala/eu/sim642/adventofcode2020/Day24.scala b/src/main/scala/eu/sim642/adventofcode2020/Day24.scala
@@ -1,6 +1,7 @@
 package eu.sim642.adventofcode2020
 
 import eu.sim642.adventofcodelib.pos.HexPos
+import eu.sim642.adventofcodelib.IteratorImplicits._
 
 object Day24 {
 
@@ -15,16 +16,39 @@ object Day24 {
 
   def applyMoves(pos: HexPos, moves: Seq[String]): HexPos = moves.foldLeft(pos)(_ + neighbors(_))
 
-  def countBlackTiles(directions: Seq[Seq[String]]): Int = {
+  def getBlackTiles(directions: Seq[Seq[String]]): Set[HexPos] = {
     directions
       .view
       .map(applyMoves(HexPos.zero, _))
       .groupMapReduce(identity)(_ => 1)(_ + _)
       .filter(_._2 % 2 == 1)
-      .keys
-      .size
+      .keySet
   }
 
+  def countBlackTiles(directions: Seq[Seq[String]]): Int = getBlackTiles(directions).size
+
+  def step(blackTiles: Set[HexPos]): Set[HexPos] = {
+    // based on 2020 Day 17
+    blackTiles.iterator
+      .flatMap(pos =>
+        neighbors.values.iterator
+          .map(pos + _)
+      )
+      .groupMapReduce(identity)(_ => 1)(_ + _)
+      .collect({
+        case (pos, 2) => pos
+        case (pos, 1) if blackTiles(pos) => pos
+      })
+      .toSet
+  }
+
+  def countBlackTilesAfter(directions: Seq[Seq[String]], days: Int = 100): Int = {
+    val initialBlackTiles = getBlackTiles(directions)
+    val finalBlackTiles = Iterator.iterate(initialBlackTiles)(step)(days)
+    finalBlackTiles.size
+  }
+
+
   private val moveRegex = """e|se|sw|w|nw|ne""".r
 
   def parseMoves(s: String): Seq[String] = moveRegex.findAllIn(s).toSeq
@@ -35,5 +59,6 @@ object Day24 {
 
   def main(args: Array[String]): Unit = {
     println(countBlackTiles(parseDirections(input)))
+    println(countBlackTilesAfter(parseDirections(input)))
   }
 }
diff --git a/src/test/scala/eu/sim642/adventofcode2020/Day24Test.scala b/src/test/scala/eu/sim642/adventofcode2020/Day24Test.scala
@@ -34,4 +34,12 @@ class Day24Test extends AnyFunSuite {
   test("Part 1 input answer") {
     assert(countBlackTiles(parseDirections(input)) == 277)
   }
+
+  test("Part 2 examples") {
+    assert(countBlackTilesAfter(parseDirections(exampleInput)) == 2208)
+  }
+
+  test("Part 2 input answer") {
+    assert(countBlackTilesAfter(parseDirections(input)) == 3531)
+  }
 }

Original file line number	Diff line number	Diff line change
`@@ -34,4 +34,12 @@ class Day24Test extends AnyFunSuite {`
`34`	`34`	`test("Part 1 input answer") {`
`35`	`35`	`assert(countBlackTiles(parseDirections(input)) == 277)`
`36`	`36`	`}`
	`37`	`+`
	`38`	`+ test("Part 2 examples") {`
	`39`	`+ assert(countBlackTilesAfter(parseDirections(exampleInput)) == 2208)`
	`40`	`+ }`
	`41`	`+`
	`42`	`+ test("Part 2 input answer") {`
	`43`	`+ assert(countBlackTilesAfter(parseDirections(input)) == 3531)`
	`44`	`+ }`
`37`	`45`	`}`