[DAY 20]

src/main/kotlin/lv/n3o/aoc2021/Task.kt

@@ -28,6 +28,8 @@ abstract class Input {
         }
     }.toMap()
 
+    fun asBlocks(): List<Input> = input.replace("\r", "").split("\n\n").map { StringInput(it) }
+
     fun asLinesPerBlock(trim: Boolean = true): List<List<String>> = input.replace("\r", "").split("\n\n").map {
         it.split("\n").let { block ->
             if (trim) block.map(String::trim) else block
@@ -43,6 +45,10 @@ class ClasspathInput(val name: String) : Input() {
     }
 }
 
+class StringInput(val data: String) : Input() {
+    override fun readInput() = data
+}
+
 abstract class Task(val input: Input) {
     var debugListener: DebugListener? = null
 

src/main/kotlin/lv/n3o/aoc2021/main.kt

@@ -47,6 +47,7 @@ val testCases: List<TestCase> = listOf(
     ci(17, "5253", "1770"),
     ci(18, "4480", "4676"),
     ci(19, "308", "12124"),
+    ci(20, "5259", "15287"),
 )
 
 fun main() = runBlocking {

src/main/kotlin/lv/n3o/aoc2021/tasks/T20.kt

@@ -0,0 +1,41 @@
+package lv.n3o.aoc2021.tasks
+
+import lv.n3o.aoc2021.Input
+import lv.n3o.aoc2021.Task
+import lv.n3o.aoc2021.coords.C2
+import lv.n3o.aoc2021.get
+
+class T20(input: Input) : Task(input) {
+    private val data = input.asBlocks()
+    private val decoder = data.first().raw().toList().map { it == '#' }
+    private val initialState = data.last().asCoordGrid().filterValues { it == '#' }.keys.let { pixels ->
+        val size = pixels.maxOf { it.x.coerceAtLeast(it.y) } + 1
+        Image(BooleanArray(size * size) { i -> pixels.contains(C2(i % size, i / size)) }, size)
+    }
+
+    inner class Image(val pixels: BooleanArray, val size: Int, val borderPixel: Boolean = false) {
+        val pixelsOn get() = if (borderPixel) Int.MAX_VALUE else pixels.count { it }
+        fun enhance() = Image(
+            BooleanArray((size + 2) * (size + 2)) { c ->
+                val x = (c % (size + 2)) - 1
+                val y = (c / (size + 2)) - 1
+                var index = 0
+                for (j in y + -1..y + 1) {
+                    for (i in x - 1..x + 1) {
+                        index = index.shl(1)
+                        if (if (j < 0 || j >= size || i < 0 || i >= size) borderPixel else pixels[i + j * size]) {
+                            index++
+                        }
+                    }
+                }
+                decoder[index]
+            }, size + 2, if (borderPixel) decoder[0b111111111] else decoder[0]
+        )
+    }
+
+    private val enhancements = generateSequence(initialState) { it.enhance() }
+
+    override fun a() = enhancements[2].pixelsOn
+
+    override fun b() = enhancements[50].pixelsOn
+}

src/main/kotlin/lv/n3o/aoc2021/utils.kt

@@ -210,4 +210,6 @@ fun <T> prioritizingComparator(priority: List<T>, fallbackComparator: Comparator
         bIndex < 0 -> -1
         else -> aIndex - bIndex
     }
-}
+}
+
+operator fun <T> Sequence<T>.get(index: Int) = drop(index).first()

src/main/resources/i20.txt

@@ -0,0 +1,102 @@
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+
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