2024 - Day 16 - part 2

src/main/kotlin/no/rodland/advent_2024/Day16.kt

@@ -5,49 +5,79 @@ import java.util.*
 
 // template generated: 15/12/2024
 // Fredrik Rødland 2024
-data class State(val pos: Pos, val dir: Direction, val cost: Int)
+data class State(val pos: Pos, val dir: Direction, val cost: Int, val positions: Set<Pos>)
 
 class Day16(val input: List<String>) : Day<Int, Int, Cave> {
 
     private val parsed = input.parse()
     private val maze = parsed
 
     override fun partOne(): Int {
-        val shortestPath = dijkstra(maze)
-        return shortestPath
+        return dijkstraWithAllMinimalPaths(maze).first
     }
 
     override fun partTwo(): Int {
-        return 2
+        return dijkstraWithAllMinimalPaths(maze).second.flatMap { it.positions }.toSet().size
     }
 
-    private fun dijkstra(maze: Cave): Int {
+    private fun dijkstraWithAllMinimalPaths(maze: Cave): Pair<Int, MutableList<State>> {
         val start = Pos(1, maze.size - 2) // Starting position (S)
         val end = Pos(maze[0].size - 2, 1) // Ending position (E)
 
         val pq = PriorityQueue<State>(compareBy { it.cost })
-        val visited = mutableSetOf<Pair<Pos, Direction>>() // Track visited states by Pos and Direction
-
-        pq.add(State(start, Direction.EAST, 0))
+        val costMap = mutableMapOf<Pair<Pos, Direction>, Int>()
+        val endStates = mutableListOf<State>() // Collect all end states
 
+        var minCost = Int.MAX_VALUE
+        pq.add(State(start, Direction.EAST, 0, setOf(start)))
         while (pq.isNotEmpty()) {
             val current = pq.poll()
-            if (!visited.add(Pair(current.pos, current.dir))) continue
-            if (current.pos == end) return current.cost
 
-            val move = current.pos.next(current.dir)
-            if (move in maze && maze[move] != '#') {
-                pq.add(State(move, current.dir, current.cost + 1))
+            // If this state exceeds the known minimum cost, skip it
+            if (current.cost > minCost) continue
+
+            // Allow revisiting if the cost is equal to the minimum
+            val key = current.pos to current.dir
+
+            // if we have an entry in the cache for this (pos,dir) which is STRICTLY lower we don't have to
+            // keep adding it.  If it's equal or null - we should handle it.
+            if (costMap[key]?.let { it < current.cost } == true) continue
+            costMap[key] = current.cost
+
+            // Add positions to current state's path
+            val newPositions = current.positions + current.pos
+
+            // If we reached the end, check and update the minimum cost
+            if (current.pos == end) {
+                minCost = current.cost
+                endStates.add(current.copy(positions = newPositions))
+                continue
+            }
+
+            // Generate next states
+            val nextPos = current.pos.next(current.dir)
+            val left = current.dir.left()
+            val nextLeft = current.pos.next(left)
+            val right = current.dir.right()
+            val nextRight = current.pos.next(right)
+
+            if (inMazeAndNotWall(nextPos, maze)) {
+                pq.add(State(nextPos, current.dir, current.cost + 1, newPositions))
+            }
+            if (inMazeAndNotWall(nextLeft, maze)) {
+                pq.add(State(nextLeft, left, current.cost + 1001, newPositions))
+            }
+            if (inMazeAndNotWall(nextRight, maze)) {
+                pq.add(State(nextRight, right, current.cost + 1001, newPositions))
             }
-            pq.add(State(current.pos, current.dir.left(), current.cost + 1000))
-            pq.add(State(current.pos, current.dir.right(), current.cost + 1000))
         }
-        return -1 // If no path is found
+        return minCost to endStates
     }
 
-    override fun List<String>.parse(): Cave {
-        return this.toCave()
-    }
+    private fun inMazeAndNotWall(nextPos: Pos, maze: Cave) = nextPos in maze && maze[nextPos] != '#'
+
+
+    override fun List<String>.parse() = this.toCave()
 
     override val day = "16".toInt()
 }

src/test/kotlin/no/rodland/advent_2024/Day16Test.kt

@@ -16,9 +16,9 @@ internal class Day16Test {
     private val test16 = "2024/input_16_test.txt".readFile()
 
     private val resultTestOne = 7036
-    private val resultTestTwo = 2
+    private val resultTestTwo = 45
     private val resultOne = 99460
-    private val resultTwo = 2
+    private val resultTwo = 500
 
     val test = defaultTestSuiteParseOnInit(
         Day16(data16),
@@ -64,6 +64,7 @@ internal class Day16Test {
         }
 
         @Test
+        @Slow(450)
         fun `16,1,live,1`() {
             report(test.livePart1)
         }
@@ -77,6 +78,7 @@ internal class Day16Test {
         }
 
         @Test
+        @Slow(400)
         fun `16,2,live,1`() {
             report(test.livePart2)
         }