Day 4

src/day04.scala

@@ -0,0 +1,49 @@
+package aoc
+package day04
+
+val dayNumber = "04"
+
+@main def part1: Unit =
+  println(part1(loadInput(dayNumber)))
+
+@main def part2: Unit =
+  println(part2(loadInput(dayNumber)))
+
+def parseRollsSet(input: String) =
+  val grid = input.split("\n")
+  (for
+    y <- 0 until grid.length
+    x <- 0 until grid.head.length
+    if grid(y)(x) == '@'
+  yield (x, y)).toSet
+
+val offsets =
+  for
+    ox <- -1 to 1
+    oy <- -1 to 1
+    if !(ox == 0 && oy == 0)
+  yield (ox, oy)
+
+def findRemovable(rollsSet: Set[(Int, Int)]): Set[(Int, Int)] =
+  rollsSet
+    .filter((x, y) =>
+      offsets
+        .count((ox, oy) => rollsSet.contains((x + ox, y + oy))) < 4
+    )
+
+def part1(input: String): String =
+  findRemovable(parseRollsSet(input)).size.toString
+
+def part2(input: String): String =
+  val rolls = parseRollsSet(input)
+
+  Iterator
+    .iterate((rolls = rolls, removedCount = 0))((rolls, _) =>
+      val removed = findRemovable(rolls)
+      rolls.diff(removed) -> removed.size
+    )
+    .drop(1)
+    .takeWhile(_.removedCount != 0)
+    .map(_.removedCount)
+    .sum
+    .toString