Finish day 19

4 files changed, 241 insertions, 7 deletions

diff --git a/README.md b/README.md
index 98f0d2e..07f2639 100644
--- a/README.md
+++ b/README.md
@@ -8,8 +8,8 @@ Repository storing my solutions to Advent of Code. See other people's solutions
 ## Years 📅
 ### [2021](aoc-2021-kotlin)
 ![Kotlin](https://img.shields.io/badge/Kotlin-grey?logo=Kotlin)
-![39/50 stars](https://img.shields.io/badge/🌟%20stars-39/50-orange)
-![20/25 days](https://img.shields.io/badge/📅%20days-20/25-blue)
+![41/50 stars](https://img.shields.io/badge/🌟%20stars-41/50-orange)
+![21/25 days](https://img.shields.io/badge/📅%20days-21/25-blue)
 
 [aoc]: https://adventofcode.com
 
diff --git a/aoc-2021-kotlin/README.md b/aoc-2021-kotlin/README.md
index c8c777a..e2188a4 100644
--- a/aoc-2021-kotlin/README.md
+++ b/aoc-2021-kotlin/README.md
@@ -1,7 +1,7 @@
 # Advent of Code 2021 in Kotlin
 ![Kotlin](https://img.shields.io/badge/Kotlin-grey?logo=Kotlin)
-![39/50 stars](https://img.shields.io/badge/🌟%20stars-39/50-orange)
-![20/25 days](https://img.shields.io/badge/📅%20days-20/25-blue)
+![41/50 stars](https://img.shields.io/badge/🌟%20stars-41/50-orange)
+![21/25 days](https://img.shields.io/badge/📅%20days-21/25-blue)
 
 Welcome to the Advent of Code Kotlin project created by [tchojnacki][github] using the
 [Advent of Code Kotlin Template][template] delivered by JetBrains.
@@ -28,7 +28,7 @@ Welcome to the Advent of Code Kotlin project created by [tchojnacki][github] usi
 | Day 16: Packet Decoder          |   🌟   |   🌟   |
 | Day 17: Trick Shot              |   🌟   |   🌟   |
 | Day 18: Snailfish               |   🌟   |   🌟   |
-| Day 19: Beacon Scanner          |        |        |
+| Day 19: Beacon Scanner          |   🌟   |   🌟   |
 | Day 20: Trench Map              |        |        |
 | Day 21: Dirac Dice              |        |        |
 | Day 22: Reactor Reboot          |        |        |
diff --git a/aoc-2021-kotlin/src/Day19.kt b/aoc-2021-kotlin/src/Day19.kt
new file mode 100644
index 0000000..8bc403a
--- /dev/null
+++ b/aoc-2021-kotlin/src/Day19.kt
@@ -0,0 +1,234 @@
+import kotlin.math.absoluteValue
+
+object Day19 {
+    private class Scanner private constructor(val beacons: Set<Pos3D>) {
+        companion object {
+            private const val THRESHOLD = 12
+
+            fun withCenteredBeacon() = Scanner(setOf(Pos3D(0, 0, 0)))
+
+            fun parseScanners(lines: List<String>): List<Scanner> {
+                val strippedLines = lines.filter { it.isNotBlank() }
+
+                val groupStartIndices = strippedLines
+                    .asSequence()
+                    .withIndex()
+                    .filter { it.value.startsWith("---") }
+                    .map { it.index }
+                    .toList()
+
+                return strippedLines
+                    .withIndex()
+                    .groupBy {
+                        groupStartIndices.last { idx -> idx <= it.index }
+                    }
+                    .values
+                    .map { entry ->
+                        Scanner(
+                            entry
+                                .asSequence()
+                                .filter { !it.value.startsWith("---") }
+                                .map { Pos3D.fromString(it.value) }
+                                .toSet()
+                        )
+                    }
+            }
+
+            private fun findOffset(from: Scanner, to: Scanner, axis: (Pos3D) -> Int) =
+                to.beacons
+                    .flatMap { t -> from.beacons.map { f -> axis(t) - axis(f) } }
+                    .toSet()
+                    .find { offset ->
+                        intersection(
+                            from.beacons.map { axis(it) + offset },
+                            to.beacons.map(axis)
+                        ).size >= THRESHOLD
+                    }
+
+            fun findTransform(from: Scanner, to: Scanner): Transform? {
+                if (!from.canOverlapWith(to)) return null
+
+                val targetOffsets = uniquePairs(to.beacons)
+                    .flatMap { (a, b) -> listOf(a - b, b - a) }
+
+                val rotation = Transform.Rotation.allRotations.find { rotation ->
+                    intersection(
+                        targetOffsets,
+                        uniquePairs(from.withTransform(rotation).beacons).map { (a, b) -> a - b }
+                    ).count() >= THRESHOLD * (THRESHOLD - 1) / 2
+                } ?: return null
+
+                val rotatedScanner = from.withTransform(rotation)
+
+                val translation = Transform.Translation(
+                    Pos3D(
+                        findOffset(rotatedScanner, to) { it.x }!!,
+                        findOffset(rotatedScanner, to) { it.y }!!,
+                        findOffset(rotatedScanner, to) { it.z }!!
+                    )
+                )
+
+                return rotation + translation
+            }
+        }
+
+        private fun beaconOffsetIds() = uniquePairs(beacons).map { (a, b) ->
+            (b - a).let {
+                setOf(
+                    it.x.absoluteValue,
+                    it.y.absoluteValue,
+                    it.z.absoluteValue
+                )
+            }
+        }
+
+        private fun canOverlapWith(other: Scanner) =
+            intersection(beaconOffsetIds(), other.beaconOffsetIds())
+                .count() >= THRESHOLD * (THRESHOLD - 1) / 2
+
+        fun withTransform(transform: Transform) = Scanner(beacons.map { transform.apply(it) }.toSet())
+    }
+
+    private sealed class Transform {
+        companion object {
+            val identity get() = Composite(listOf())
+        }
+
+        abstract fun apply(pos: Pos3D): Pos3D
+        abstract fun inverted(): Transform
+        protected abstract val list: List<Transform>
+        abstract override fun toString(): String
+
+        operator fun plus(other: Transform) = Composite(list + other.list)
+
+        class Translation(private val offset: Pos3D) : Transform() {
+            override fun apply(pos: Pos3D) = pos + offset
+            override fun inverted() = Translation(-offset)
+            override val list get() = listOf(this)
+            override fun toString() = offset.let { (x, y, z) -> "T($x,$y,$z)" }
+        }
+
+        class Rotation private constructor(private val index: Int) : Transform() {
+            companion object {
+                val allRotations = (0 until 24).map { Rotation(it) }
+            }
+
+            override fun apply(pos: Pos3D) = pos.let { (x, y, z) ->
+                listOf(
+                    Pos3D(x, y, z), Pos3D(x, z, -y), Pos3D(x, -y, -z), Pos3D(x, -z, y),
+                    Pos3D(-x, -y, z), Pos3D(-x, z, y), Pos3D(-x, y, -z), Pos3D(-x, -z, -y),
+                    Pos3D(y, z, x), Pos3D(y, x, -z), Pos3D(y, -z, -x), Pos3D(y, -x, z),
+                    Pos3D(-y, -z, x), Pos3D(-y, x, z), Pos3D(-y, z, -x), Pos3D(-y, -x, -z),
+                    Pos3D(z, x, y), Pos3D(z, y, -x), Pos3D(z, -x, -y), Pos3D(z, -y, x),
+                    Pos3D(-z, -x, y), Pos3D(-z, y, x), Pos3D(-z, x, -y), Pos3D(-z, -y, -x)
+                )
+            }[index]
+
+            override fun inverted() = Pos3D(1, 2, 3).let {
+                allRotations.find { inverse ->
+                    inverse.apply(this.apply(it)) == it
+                }!!
+            }
+
+            override val list get() = listOf(this)
+
+            override fun toString() = "R($index)"
+        }
+
+        class Composite(private val queue: List<Transform>) : Transform() {
+            override fun apply(pos: Pos3D) = queue.fold(pos) { acc, transform -> transform.apply(acc) }
+            override fun inverted() = Composite(queue.reversed().map { it.inverted() })
+            override val list get() = queue
+            override fun toString() = if (list.isEmpty()) "I" else "C(${queue.joinToString("->")})"
+        }
+    }
+
+    private data class Pos3D(val x: Int, val y: Int, val z: Int) {
+        companion object {
+            fun fromString(string: String) = string
+                .split(",")
+                .map(String::toInt)
+                .let { Pos3D(it[0], it[1], it[2]) }
+        }
+
+        operator fun unaryMinus() = Pos3D(-x, -y, -z)
+
+        operator fun plus(other: Pos3D) = Pos3D(x + other.x, y + other.y, z + other.z)
+
+        operator fun minus(other: Pos3D) = Pos3D(x - other.x, y - other.y, z - other.z)
+    }
+
+    private class Graph(private val adjacencyList: List<Set<Edge>>) {
+        data class Edge(val destination: Int, val transform: Transform)
+
+        private fun getEdge(from: Int, to: Int) = adjacencyList[from].find { it.destination == to }
+
+        fun findTransformsToZero(): List<Transform> {
+            val predecessors = mutableMapOf(0 to 0)
+            val queue = mutableListOf(0)
+
+            while (queue.isNotEmpty()) {
+                val current = queue.removeFirst()
+                for (edge in adjacencyList[current].filter { !predecessors.contains(it.destination) }) {
+                    predecessors[edge.destination] = current
+                    queue.add(edge.destination)
+                }
+            }
+
+            return adjacencyList.indices.map { i ->
+                generateSequence(i) { if (it == 0) null else predecessors[it] }
+                    .zipWithNext()
+                    .map { (a, b) -> getEdge(a, b)!!.transform }
+                    .fold(Transform.identity) { a, b -> a + b }
+            }
+        }
+
+        override fun toString() = adjacencyList.withIndex().joinToString("\n") { "${it.index}: ${it.value}" }
+    }
+
+    private fun <T> uniquePairs(iterable: Iterable<T>) =
+        iterable.withIndex().flatMap {
+            iterable.drop(it.index + 1).map { second -> it.value to second }
+        }
+
+    private fun <T> intersection(first: Iterable<T>, second: Iterable<T>): List<T> {
+        val remaining = first.toMutableList()
+        return second.filter { remaining.remove(it) }
+    }
+
+    fun bothParts(input: List<String>) = Scanner.parseScanners(input).let { scanners ->
+        val edges = uniquePairs(scanners.withIndex())
+            .mapNotNull { (a, b) ->
+                Scanner.findTransform(a.value, b.value)?.let { a.index to Graph.Edge(b.index, it) }
+            }
+            .flatMap { listOf(it, it.second.destination to Graph.Edge(it.first, it.second.transform.inverted())) }
+
+        val transforms = Graph(scanners.indices.map { i ->
+            edges.asSequence().filter { it.first == i }.map { it.second }.toSet()
+        }).findTransformsToZero()
+
+        val firstPart = scanners
+            .zip(transforms)
+            .map { it.first.withTransform(it.second).beacons }
+            .reduce { a, b -> a + b }.size
+
+        val scannerPositions = transforms.map { Scanner.withCenteredBeacon().withTransform(it).beacons.first() }
+        val secondPart = uniquePairs(scannerPositions).maxOfOrNull {
+            (it.second - it.first).let { (x, y, z) -> x.absoluteValue + y.absoluteValue + z.absoluteValue }
+        }
+
+        return@let firstPart to secondPart
+    }
+}
+
+fun main() {
+    val testInput = readInputAsLines("Day19_test")
+    val testOutput = Day19.bothParts(testInput)
+    check(testOutput.first == 79)
+    check(testOutput.second == 3621)
+
+    val input = readInputAsLines("Day19")
+    val output = Day19.bothParts(input)
+    println(output.first)
+    println(output.second)
+}
diff --git a/aoc-2021-kotlin/src/Utils.kt b/aoc-2021-kotlin/src/Utils.kt
index f0a420b..8fe3f75 100644
--- a/aoc-2021-kotlin/src/Utils.kt
+++ b/aoc-2021-kotlin/src/Utils.kt
@@ -1,8 +1,8 @@
 import java.io.File
 
-fun readInputAsLines(name: String): List<String> = File("src", "$name.txt").readLines()
+fun readInputAsLines(name: String): List<String> = File("aoc-2021-kotlin/src", "$name.txt").readLines()
 
-fun readInputAsString(name: String): String = File("src", "$name.txt").readText()
+fun readInputAsString(name: String): String = File("aoc-2021-kotlin/src", "$name.txt").readText()
 
 fun readInputAsNumbers(name: String): List<Int> = readInputAsLines(name).map(String::toInt)