Day 18.

src/Utils.kt

@@ -75,11 +75,11 @@ class CharGrid {
     val data: Array<CharArray>
     val bChar: Char
 
-    constructor(width: Int, height: Int, borderChar: Char = ' ') {
+    constructor(width: Int, height: Int, borderChar: Char = ' ', fillChar: Char = borderChar) {
         bChar = borderChar
         this.width = width
         this.height = height
-        data = Array(height) { CharArray(width) { bChar } }
+        data = Array(height) { CharArray(width) { fillChar } }
     }
 
     constructor(input: List<String>, borderChar: Char = ' ') {

src/aoc2024/Day18.kt

@@ -0,0 +1,127 @@
+package aoc2024
+
+import CharGrid
+import RelPos
+import println
+import readInput
+import splitInts
+import java.util.*
+
+/*
+--- Day 18: RAM Run ---
+https://adventofcode.com/2024/day/18
+*/
+fun main() {
+
+    val inlineTestInput = """
+5,4
+4,2
+4,5
+3,0
+2,1
+6,3
+2,4
+1,5
+0,6
+3,3
+2,6
+5,1
+1,2
+5,5
+2,5
+6,5
+1,4
+0,4
+6,4
+1,1
+6,1
+1,0
+0,5
+1,6
+2,0
+"""
+
+    data class Node(val pos: RelPos, val relLen: Int)
+
+    fun part1(input: List<String>, steps: Int, wh: Int): Int {
+        val posList = input.map { it.splitInts(",").toIntArray() }
+        val grid = CharGrid(wh, wh, '#', fillChar = '.')
+        posList.take(steps).forEach { grid[it[0], it[1]] = '#' }
+        val queue = PriorityQueue<Node>(Comparator.comparing { -it.pos.dc - it.pos.dr })
+        queue.add(Node(RelPos(0, 0), 0))
+        val bestCosts = HashMap<RelPos, Int>()
+        var minCost = Int.MAX_VALUE
+        while (queue.isNotEmpty()) {
+            val node = queue.remove()
+            val pos = node.pos
+            val cost = node.relLen
+            if (pos.dc == wh - 1 && pos.dr == wh - 1) {
+                minCost = minCost.coerceAtMost(cost)
+                continue
+            }
+            if ((bestCosts[pos] ?: Int.MAX_VALUE) < cost) {
+                continue
+            }
+            bestCosts[pos] = cost
+            queue.addAll(CharGrid.PLUS_POS
+                .map { pos.translate(it) }
+                .filter { grid[it] == '.' && (bestCosts[it] ?: Int.MAX_VALUE) > cost + 1 }
+                .map { Node(it, cost + 1) })
+        }
+        return minCost
+    }
+
+    fun part2(input: List<String>, wh: Int): RelPos {
+        val posList = input.map { it.splitInts(",").toIntArray() }
+        var lowerBound = 0
+        var higherBound = posList.size
+        do {
+            val grid = CharGrid(wh, wh, '#', fillChar = '.')
+            val boulderPos = (lowerBound + higherBound) / 2
+            posList.take(boulderPos).forEach { grid[it[0], it[1]] = '#' }
+
+            var foundPath = false
+            val queue = PriorityQueue<Node>(Comparator.comparing { -it.pos.dc - it.pos.dr })
+            queue.add(Node(RelPos(0, 0), 0))
+            val bestCosts = HashMap<RelPos, Int>()
+            while (queue.isNotEmpty()) {
+                val node = queue.remove()
+                val pos = node.pos
+                val cost = node.relLen
+                if (pos.dc == wh - 1 && pos.dr == wh - 1) {
+                    foundPath = true
+                    break
+                }
+                if ((bestCosts[pos] ?: Int.MAX_VALUE) < cost) {
+                    continue
+                }
+                bestCosts[pos] = cost
+                queue.addAll(CharGrid.PLUS_POS
+                    .map { pos.translate(it) }
+                    .filter { grid[it] == '.' && (bestCosts[it] ?: Int.MAX_VALUE) > cost + 1 }
+                    .map { Node(it, cost + 1) })
+            }
+            if (foundPath) {
+                lowerBound = boulderPos + 1
+            } else {
+                higherBound = boulderPos
+            }
+        } while (lowerBound < higherBound)
+        println("$lowerBound $higherBound")
+        return RelPos(posList[lowerBound - 1][0], posList[lowerBound - 1][1])
+    }
+
+    // test if implementation meets criteria from the description, like:
+    val testInput = inlineTestInput.trim().reader().readLines()
+    //val testInput = readInput("aoc2024/Day18_test")
+    val testInputPart1Result = part1(testInput, 12, 7)
+    println("Part 1 Test: $testInputPart1Result")
+    val testInputPart2Result = part2(testInput, 7)
+    println("Part 2 Test: $testInputPart2Result")
+    check(testInputPart1Result == 22)
+    check(testInputPart2Result == RelPos(6, 1))
+
+    val input = readInput("aoc2024/Day18")
+    part1(input, 1024, 71).println()
+    part2(input, 71).println()
+}