Day 23: Unstable Diffusion

README.md

@@ -28,4 +28,4 @@ Development occurs in language-specific directories:
 |[Day20.hs](hs/src/Day20.hs)|[Day20.kt](kt/src/commonMain/kotlin/com/github/ephemient/aoc2022/Day20.kt)|[day20.py](py/aoc2022/day20.py)|[day20.rs](rs/src/day20.rs)|
 |[Day21.hs](hs/src/Day21.hs)|[Day21.kt](kt/src/commonMain/kotlin/com/github/ephemient/aoc2022/Day21.kt)|[day21.py](py/aoc2022/day21.py)|[day21.rs](rs/src/day21.rs)|
 |[Day22.hs](hs/src/Day22.hs)|[Day22.kt](kt/src/commonMain/kotlin/com/github/ephemient/aoc2022/Day22.kt)|[day22.py](py/aoc2022/day22.py)|[day22.rs](rs/src/day22.rs)|
-|[Day23.hs](hs/src/Day23.hs)|
+|[Day23.hs](hs/src/Day23.hs)|[Day23.kt](kt/src/commonMain/kotlin/com/github/ephemient/aoc2022/Day23.kt)|

kt/src/commonMain/kotlin/com/github/ephemient/aoc2022/Day23.kt

@@ -0,0 +1,76 @@
+package com.github.ephemient.aoc2022
+
+@Day
+class Day23(lines: List<String>) {
+    private val states = directions.scan(
+        buildSet { lines.forEachIndexed { y, line -> line.forEachIndexed { x, c -> if (c == '#') add(x to y) } } }
+    ) { state, directions ->
+        val proposals = state.groupingBy { position ->
+            if (position.neighbors().none { it in state }) {
+                position
+            } else {
+                directions.firstNotNullOfOrNull { direction ->
+                    if (position.neighbors(direction).none { it in state }) position.move(direction) else null
+                } ?: position
+            }
+        }.aggregateTo(mutableMapOf()) { _, _: IntPair?, value, first -> if (first) value else null }
+        proposals.entries.removeAll { (key, value) -> value == null || key == value }
+        buildSet(state.size) {
+            addAll(state)
+            removeAll(proposals.values.toSet())
+            addAll(proposals.keys)
+        }
+    }
+
+    @Day.Part
+    fun part1(): Int {
+        val state = states.elementAt(10)
+        val minX = state.minOf { it.first }
+        val maxX = state.maxOf { it.first }
+        val minY = state.minOf { it.second }
+        val maxY = state.maxOf { it.second }
+        return (maxX - minX + 1) * (maxY - minY + 1) - state.size
+    }
+
+    @Day.Part
+    fun part2(): Int = states.zipWithNext().indexOfFirst { (prev, cur) -> prev == cur } + 1
+
+    private enum class Direction {
+        N, S, W, E
+    }
+
+    companion object {
+        private val directions = sequence {
+            val directions = ArrayDeque(Direction.values().asList())
+            while (true) {
+                yield(directions.toList())
+                directions.add(directions.removeFirst())
+            }
+        }
+
+        private fun IntPair.neighbors(): Array<IntPair> = arrayOf(
+            first + 1 to second,
+            first + 1 to second + 1,
+            first to second + 1,
+            first - 1 to second + 1,
+            first - 1 to second,
+            first - 1 to second - 1,
+            first to second - 1,
+            first + 1 to second - 1,
+        )
+
+        private fun IntPair.neighbors(direction: Direction): Array<IntPair> = when (direction) {
+            Direction.N -> arrayOf(first - 1 to second - 1, first to second - 1, first + 1 to second - 1)
+            Direction.S -> arrayOf(first - 1 to second + 1, first to second + 1, first + 1 to second + 1)
+            Direction.W -> arrayOf(first - 1 to second - 1, first - 1 to second, first - 1 to second + 1)
+            Direction.E -> arrayOf(first + 1 to second - 1, first + 1 to second, first + 1 to second + 1)
+        }
+
+        private fun IntPair.move(direction: Direction) = when (direction) {
+            Direction.N -> first to second - 1
+            Direction.S -> first to second + 1
+            Direction.W -> first - 1 to second
+            Direction.E -> first + 1 to second
+        }
+    }
+}

kt/src/commonTest/kotlin/com/github/ephemient/aoc2022/Day23Test.kt

@@ -0,0 +1,16 @@
+package com.github.ephemient.aoc2022
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class Day23Test {
+    @Test
+    fun part1() {
+        assertEquals(110, Day23(getTestInput(23)).part1())
+    }
+
+    @Test
+    fun part2() {
+        assertEquals(20, Day23(getTestInput(23)).part2())
+    }
+}

kt/src/jvmTest/resources/day23.txt

@@ -0,0 +1,7 @@
+....#..
+..###.#
+#...#.#
+.#...##
+#.###..
+##.#.##
+.#..#..