Day 18: RAM Run

README.md

@@ -14,6 +14,7 @@ Advent of Code is an Advent calendar of small programming puzzles by [Eric Wastl
 - Day 8: [Resonant Collinearity](https://adventofcode.com/2024/day/8) -- [Day8.kt](https://github.com/andilau/advent-of-code-2024/blob/main/src/main/kotlin/days/Day8.kt)
 - Day 12: [Garden Groups](https://adventofcode.com/2024/day/12) -- [Day12.kt](https://github.com/andilau/advent-of-code-2024/blob/main/src/main/kotlin/days/Day12.kt)
 - Day 14: [Restroom Redoubt](https://adventofcode.com/2024/day/14) -- [Day14.kt](https://github.com/andilau/advent-of-code-2024/blob/main/src/main/kotlin/days/Day14.kt)
+- Day 18: [RAM Run](https://adventofcode.com/2024/day/18) -- [Day18.kt](https://github.com/andilau/advent-of-code-2024/blob/main/src/main/kotlin/days/Day18.kt)
 
 ### Features
 

src/main/kotlin/days/Day18.kt

@@ -0,0 +1,76 @@
+package days
+
+import java.util.*
+
+@AdventOfCodePuzzle(
+    name = "RAM Run",
+    url = "https://adventofcode.com/2024/day/18",
+    date = Date(day = 18, year = 2024)
+)
+class Day18(val input: List<String>) : Puzzle {
+
+    private val elements = if (input.size < 100) 12 else 1024
+    private val max = if (input.size < 100) 6 else 70
+
+    private val bytes: List<Point> = input.take(elements).map { line -> Point.from(line) }
+    private val range = 0..max
+    private val start = Point(range.start, range.start)
+    private val exit = Point(range.last, range.last)
+
+    override fun partOne() = path(bytes)
+
+    override fun partTwo(): String = (elements..input.lastIndex).firstNotNullOf {
+        val b = input.take(it).map { line -> Point.from(line) }
+        try {
+            path(b)
+            null
+        } catch (e: Exception) {
+            b.last().let { (x, y) -> "${x},${y}" }
+        }
+    }
+
+    private fun draw(bytes: List<Point>) {
+        (range).forEach { y ->
+            (range).map { x ->
+                if (bytes.any { it == Point(x, y) }) "#" else "."
+            }.joinToString("").also { println(it) }
+        }
+    }
+
+    data class Point(val x: Int, val y: Int) {
+        operator fun plus(other: Point) = Point(x + other.x, y + other.y)
+        operator fun minus(other: Point) = Point(x - other.x, y - other.y)
+        operator fun times(o: Int) = Point(x * o, y * o)
+
+        fun neighbours() = setOf(
+            copy(x = x + 1),
+            copy(y = y + 1),
+            copy(x = x - 1),
+            copy(y = y - 1),
+        )
+
+        companion object {
+            fun from(line: String) = line.split(',').map { it.toInt() }.let { (x, y) -> Point(x, y) }
+        }
+    }
+
+    private fun path(bytes: List<Point>): Int {
+        val queue = PriorityQueue<Pair<Point, Int>> { a, b -> a.second.compareTo(b.second) }
+        queue.add(start to 0)
+        val seen = mutableSetOf<Point>()
+
+        while (queue.isNotEmpty()) {
+            val (next, cost) = queue.poll()
+
+            if (next in seen) continue
+            seen += next
+            if (next == exit) return cost
+            next.neighbours()
+                .filter { it.x in range && it.y in range }
+                .filter { it !in bytes }
+                .forEach { queue.add(it to cost + 1) }
+        }
+        error("No path found!")
+    }
+
+}