Day 06 complete.

README.md

@@ -2,4 +2,6 @@
 
 My attempt in Kotlin to complete the [2024 Advent of Code](https://adventofcode.com/2024).
 
-**Current status:** Day 5 complete.
+**Current status:** Day 5 complete.
+
+**Last updated:** 2024-12-05.

src/main/kotlin/day06/day06.kt

@@ -0,0 +1,121 @@
+// Advent of Code 2024, Day 06.
+// By Sebastian Raaphorst, 2024.
+
+// NOTE: Trying to use pure FP in this question made part 2 run extremely slowly.
+// Mutable data structures are needed to avoid having to continuously copy structures.
+
+package day06
+
+import common.day
+import common.readInput
+
+typealias Point = Pair<Int, Int>
+
+/**
+ * The direction that the guard is facing and moves in.
+ */
+enum class Direction(val delta: Point) {
+    NORTH(-1 to 0),
+    SOUTH(1 to 0),
+    EAST(0 to 1),
+    WEST(0 to -1);
+
+    fun clockwise(): Direction = when (this) {
+        NORTH -> EAST
+        EAST -> SOUTH
+        SOUTH -> WEST
+        WEST -> NORTH
+    }
+}
+
+private operator fun Point.plus(other: Point): Point =
+    (first + other.first) to (second + other.second)
+
+typealias Orientation = Pair<Direction, Point>
+
+data class MapGrid(val rows: Int,
+                   val cols: Int,
+                   val boundaries: Set<Point>) {
+    fun isBoundary(point: Point): Boolean =
+        point in boundaries
+
+    fun isOutOfBounds(point: Point): Boolean =
+        point.first !in 0 until rows || point.second !in 0 until cols
+}
+
+data class Guard(val startPosition: Point,
+                 val map: MapGrid) {
+    /**
+     * Simulate the guard's path and return either:
+     * 1. A set of visited points if it escapes.
+     * 2. Null if it enters a loop.
+     */
+    fun move(addedPoint: Point? = null): Set<Point>? {
+        val visitedPoints = mutableSetOf<Point>()
+        val visitedStates = mutableSetOf<Orientation>()
+
+        var currentPosition = startPosition
+        var currentDir = Direction.NORTH
+
+        while (!map.isOutOfBounds(currentPosition)) {
+            visitedPoints.add(currentPosition)
+
+            // If we return to a state we already visited, we have detected a cycle.
+            val state = currentDir to currentPosition
+            if (state in visitedStates) return null
+            visitedStates.add(state)
+
+            // Move forward or turn if hitting a boundary
+            val nextPosition = currentPosition + currentDir.delta
+            if (map.isBoundary(nextPosition) || (addedPoint != null && nextPosition == addedPoint))
+                currentDir = currentDir.clockwise()
+            else
+                currentPosition = nextPosition
+        }
+        return visitedPoints
+    }
+}
+
+/**
+ * Parse the input into a Guard and MapGrid.
+ */
+fun parseProblem(input: String): Guard {
+    var startPosition: Point? = null
+    val barriers = mutableSetOf<Point>()
+
+    input.trim().lines().forEachIndexed { x, row ->
+        row.forEachIndexed { y, ch -> when (ch) {
+            '^' -> startPosition = Point(x, y)
+            '#' -> barriers.add(Point(x, y))
+        } }
+    }
+
+    val rows = input.lines().size
+    val cols = input.lines().first().length
+    val map = MapGrid(rows, cols, barriers)
+    return Guard(startPosition ?: error("No start position found"), map)
+}
+
+fun answer1(guard: Guard): Int =
+    guard.move()?.size ?: error("Could not calculate")
+
+fun answer2(guard: Guard): Int {
+    val originalPath = guard.move() ?: error("Could not calculate")
+
+    // We want the number of nulls, i.e. the number of times the guard gets in a cycle.
+    return originalPath.count { candidatePoint ->
+        guard.move(candidatePoint) == null
+    }
+}
+
+fun main() {
+    val input = parseProblem(readInput({}::class.day()).trim())
+
+    println("--- Day 6: Guard Gallivant ---")
+
+    // Part 1: 5208
+    println("Part 1: ${answer1(input)}")
+
+    // Part 2: 1972
+    println("Part 2: ${answer2(input)}")
+}