Year 2015 Day 18

Author: maneatingape

README.md

@@ -227,3 +227,4 @@ The minimal SBT project provides:
 | 15 | [Science for Hungry People](https://adventofcode.com/2015/day/15) | [Source](src/main/scala/AdventOfCode2015/Day15.scala) |
 | 16 | [Aunt Sue](https://adventofcode.com/2015/day/16) | [Source](src/main/scala/AdventOfCode2015/Day16.scala) |
 | 17 | [No Such Thing as Too Much](https://adventofcode.com/2015/day/17) | [Source](src/main/scala/AdventOfCode2015/Day17.scala) |
+| 18 | [Like a GIF For Your Yard](https://adventofcode.com/2015/day/18) | [Source](src/main/scala/AdventOfCode2015/Day18.scala) |

src/main/resources/AdventOfCode2015/Day18.txt

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src/main/scala/AdventOfCode2015/Day18.scala

@@ -0,0 +1,30 @@
+package AdventOfCode2015
+
+object Day18:
+  val adjacent = for y <- -1 to 1; x <- -1 to 1 if !(x == 0 && y == 0) yield (x, y)
+
+  case class Point(x: Int, y: Int):
+    def delta(dx: Int, dy: Int): Point = Point(x + dx, y + dy)
+    def neighbours: Seq[Point] = adjacent.map(delta)
+
+  def parse(input: Seq[String]): Set[Point] =
+    Set.tabulate(100, 100)(Point).flatten.filter(p => input(p.y)(p.x) == '#')
+
+  def step(grid: Set[Point]): Set[Point] =
+    def rule(point: Point) =
+      val active = grid.contains(point)
+      val count = point.neighbours.count(grid.contains)
+      count == 3 || (active && count == 2)
+    Set.tabulate(100, 100)(Point).flatten.filter(rule)
+
+  def corners(grid: Set[Point]): Set[Point] =
+    step(grid) ++ Set(Point(0, 0), Point(0, 99), Point(99, 0), Point(99, 99))
+
+  def part1(input: Seq[String]): Int = Iterator.iterate(parse(input))(step).drop(100).next().size
+
+  def part2(input: Seq[String]): Int = Iterator.iterate(parse(input))(corners).drop(100).next().size
+
+  def main(args: Array[String]): Unit =
+    val data = io.Source.fromResource("AdventOfCode2015/Day18.txt").getLines().toSeq
+    println(part1(data))
+    println(part2(data))

src/test/scala/AdventOfCode2015/Day18Suite.scala

@@ -0,0 +1,6 @@
+package AdventOfCode2015
+
+import org.scalatest.funsuite.AnyFunSuite
+
+class Day18Suite extends AnyFunSuite
+// No unit tests possible