Finish day 20

1 files changed, 60 insertions, 0 deletions

diff --git a/aoc-2021-kotlin/src/Day20.kt b/aoc-2021-kotlin/src/Day20.kt
new file mode 100644
index 0000000..9bc3b01
--- /dev/null
+++ b/aoc-2021-kotlin/src/Day20.kt
@@ -0,0 +1,60 @@
+object Day20 {
+    private class Enhancement private constructor(
+        private val algorithm: BooleanArray,
+        private val initialImage: Set<Pos2D>
+    ) {
+        companion object {
+            fun fromString(string: String) = string.split("\n\n").let { (algorithm, image) ->
+                Enhancement(
+                    algorithm.map { it == '#' }.toBooleanArray(),
+                    image
+                        .split("\n")
+                        .withIndex()
+                        .flatMap { (y, row) ->
+                            row.withIndex().mapNotNull { (x, char) -> if (char == '#') Pos2D(x, y) else null }
+                        }
+                        .toSet()
+                )
+            }
+        }
+
+        private fun nextStateFor(currentImage: Set<Pos2D>, pos: Pos2D) = combinations(1 downTo -1)
+            .map { (dy, dx) -> if (currentImage.contains(pos + Pos2D(dx, dy))) 1 else 0 }
+            .withIndex()
+            .sumOf { (index, value) -> value shl index }
+            .let { algorithm[it] }
+
+        fun enhance(times: Int): Int {
+            return generateSequence(StepState.initial(initialImage)) { (image, topLeft, bottomRight) ->
+                StepState(
+                    combinations(topLeft.y..bottomRight.y, topLeft.x..bottomRight.x)
+                        .mapNotNull { (y, x) ->
+                            Pos2D(x, y).let { if (nextStateFor(image, it)) it else null }
+                        }.toSet(),
+                    topLeft + Pos2D(1, 1),
+                    bottomRight - Pos2D(1, 1)
+                )
+            }.drop(times).first().image.size
+        }
+    }
+
+    private data class StepState(val image: Set<Pos2D>, val topLeft: Pos2D, val bottomRight: Pos2D) {
+        companion object {
+            fun initial(image: Set<Pos2D>) = StepState(image, Pos2D(-100, -100), Pos2D(200, 200))
+        }
+    }
+
+    fun part1(input: String) = Enhancement.fromString(input).enhance(2)
+
+    fun part2(input: String) = Enhancement.fromString(input).enhance(50)
+}
+
+fun main() {
+    val testInput = readInputAsString("Day20_test")
+    check(Day20.part1(testInput) == 35)
+    check(Day20.part2(testInput) == 3351)
+
+    val input = readInputAsString("Day20")
+    println(Day20.part1(input))
+    println(Day20.part2(input))
+}