From 150322aa1a07495c01115c057fdc55b6ad401ac7 Mon Sep 17 00:00:00 2001
From: Oscar Molnar <oscar@tymscar.com>
Date: Wed, 18 Dec 2024 12:17:27 +0000
Subject: [PATCH] Finish 2024 day 18 both parts

---
 2024/kotlin/src/main/kotlin/Main.kt           |  2 +
 .../kotlin/com/tymscar/day18/part1/part1.kt   | 60 +++++++++++++++-
 .../kotlin/com/tymscar/day18/part2/part2.kt   | 69 ++++++++++++++++++-
 3 files changed, 129 insertions(+), 2 deletions(-)

diff --git a/2024/kotlin/src/main/kotlin/Main.kt b/2024/kotlin/src/main/kotlin/Main.kt
index 6715f5b..0e39d8f 100644
--- a/2024/kotlin/src/main/kotlin/Main.kt
+++ b/2024/kotlin/src/main/kotlin/Main.kt
@@ -17,6 +17,7 @@ import com.tymscar.day14.solve as day14
 import com.tymscar.day15.solve as day15
 import com.tymscar.day16.solve as day16
 import com.tymscar.day17.solve as day17
+import com.tymscar.day18.solve as day18
 
 fun main() {
     day01()
@@ -36,4 +37,5 @@ fun main() {
     day15()
     day16()
     day17()
+    day18()
 }
\ No newline at end of file
diff --git a/2024/kotlin/src/main/kotlin/com/tymscar/day18/part1/part1.kt b/2024/kotlin/src/main/kotlin/com/tymscar/day18/part1/part1.kt
index 710e1b9..9e596d8 100644
--- a/2024/kotlin/src/main/kotlin/com/tymscar/day18/part1/part1.kt
+++ b/2024/kotlin/src/main/kotlin/com/tymscar/day18/part1/part1.kt
@@ -1,5 +1,63 @@
 package com.tymscar.day18.part1
 
+import java.util.PriorityQueue
+
+const val MAP_SIZE = 71
+const val NUM_BYTES = 1024
+
+private typealias Map = HashMap<Vec2, MemoryType>
+
+private data class Vec2(var x: Int, var y: Int) {
+    constructor(list: List<Int>) : this(list[0], list[1])
+
+    fun getValidNeighbours(): List<Vec2> {
+        return listOf(
+            Vec2(x + 1, y),
+            Vec2(x - 1, y),
+            Vec2(x, y + 1),
+            Vec2(x, y - 1)
+        ).filter { it.x in 0 until MAP_SIZE && it.y in 0 until MAP_SIZE }
+    }
+}
+
+private enum class MemoryType { CORRUPTED, SAFE }
+
+private fun dijkstra(start: Vec2, end: Vec2, map: Map): Int {
+    val visited = HashSet<Vec2>()
+    val costs = HashMap<Vec2, Int>()
+    val queue = PriorityQueue<Pair<Vec2, Int>>(compareBy { it.second })
+
+    queue.add(start to 0)
+    while (queue.isNotEmpty()) {
+        val (currPos, currCost) = queue.poll()
+        if (currPos == end) return currCost
+
+        if (visited.contains(currPos)) continue
+        visited.add(currPos)
+
+        val neighbours = currPos.getValidNeighbours()
+        for (neighbour in neighbours) {
+            if (map.getOrDefault(
+                    neighbour,
+                    MemoryType.SAFE
+                ) == MemoryType.CORRUPTED || visited.contains(neighbour)
+            ) continue
+            val nextCost = currCost + 1
+            if (nextCost < costs.getOrDefault(neighbour, Int.MAX_VALUE)) {
+                costs[neighbour] = nextCost
+                queue.add(neighbour to nextCost)
+            }
+        }
+    }
+    return Int.MAX_VALUE
+}
+
 fun solve(input: String): String {
-    return input
+    val bytes = input.lines().map { Vec2(it.split(",").map { it.toInt() }) }
+    val map = (0 until NUM_BYTES).associate { bytes[it] to MemoryType.CORRUPTED }.toMap()
+
+    val start = Vec2(0, 0)
+    val end = Vec2(MAP_SIZE - 1, MAP_SIZE - 1)
+
+    return dijkstra(start, end, map as Map).toString()
 }
diff --git a/2024/kotlin/src/main/kotlin/com/tymscar/day18/part2/part2.kt b/2024/kotlin/src/main/kotlin/com/tymscar/day18/part2/part2.kt
index 3a67638..a041de9 100644
--- a/2024/kotlin/src/main/kotlin/com/tymscar/day18/part2/part2.kt
+++ b/2024/kotlin/src/main/kotlin/com/tymscar/day18/part2/part2.kt
@@ -1,5 +1,72 @@
 package com.tymscar.day18.part2
 
+import java.util.PriorityQueue
+
+const val MAP_SIZE = 71
+const val NUM_BYTES = 1024
+
+private typealias Map = HashMap<Vec2, MemoryType>
+
+private data class Vec2(var x: Int, var y: Int) {
+    constructor(list: List<Int>) : this(list[0], list[1])
+
+    fun getValidNeighbours(): List<Vec2> {
+        return listOf(
+            Vec2(x + 1, y),
+            Vec2(x - 1, y),
+            Vec2(x, y + 1),
+            Vec2(x, y - 1)
+        ).filter { it.x in 0 until MAP_SIZE && it.y in 0 until MAP_SIZE }
+    }
+
+    override fun toString(): String = "$x,$y"
+}
+
+private enum class MemoryType { CORRUPTED, SAFE }
+
+private fun canFindPath(start: Vec2, end: Vec2, map: Map): Boolean {
+    val visited = HashSet<Vec2>()
+    val costs = HashMap<Vec2, Int>()
+    val queue = PriorityQueue<Pair<Vec2, Int>>(compareBy { it.second })
+
+    queue.add(start to 0)
+    while (queue.isNotEmpty()) {
+        val (currPos, currCost) = queue.poll()
+        if (currPos == end) return true
+
+        if (visited.contains(currPos)) continue
+        visited.add(currPos)
+
+        val neighbours = currPos.getValidNeighbours()
+        for (neighbour in neighbours) {
+            if (map.getOrDefault(
+                    neighbour,
+                    MemoryType.SAFE
+                ) == MemoryType.CORRUPTED || visited.contains(neighbour)
+            ) continue
+            val nextCost = currCost + 1
+            if (nextCost < costs.getOrDefault(neighbour, Int.MAX_VALUE)) {
+                costs[neighbour] = nextCost
+                queue.add(neighbour to nextCost)
+            }
+        }
+    }
+    return false
+}
+
 fun solve(input: String): String {
-    return input
+    val bytes = input.lines().map { Vec2(it.split(",").map { it.toInt() }) }
+    val map = (0 until NUM_BYTES).associate { bytes[it] to MemoryType.CORRUPTED }.toMutableMap()
+
+    val start = Vec2(0, 0)
+    val end = Vec2(MAP_SIZE - 1, MAP_SIZE - 1)
+
+    var currentByte = NUM_BYTES - 1
+
+    while (canFindPath(start, end, map as Map)) {
+        map[bytes[currentByte]] = MemoryType.CORRUPTED
+        currentByte++
+    }
+
+    return bytes[currentByte - 1].toString()
 }