Day 11 2025 (#306)

* Part One

* Part two

* remove extraneous default

* readme

* remove debug

* associateBy

* oneline

Author: peckb1

src/main/kotlin/me/peckb/aoc/_2025/calendar/day11/Day11.kt

@@ -0,0 +1,89 @@
+package me.peckb.aoc._2025.calendar.day11
+
+import javax.inject.Inject
+import me.peckb.aoc.generators.InputGenerator.InputGeneratorFactory
+
+class Day11 @Inject constructor(
+  private val generatorFactory: InputGeneratorFactory,
+) {
+  fun partOne(filename: String) = generatorFactory.forFile(filename).readAs(::server) { input ->
+    val serverMap = input.plus(Server("out")).associateBy { it.id }
+
+    countAllPaths(serverMap, serverMap["you"]!!)
+  }
+
+  fun partTwo(filename: String) = generatorFactory.forFile(filename).readAs(::server) { input ->
+    val serverMap = input.plus(Server("out")).associateBy { it.id }
+
+    countAllPaths(
+      serverMap = serverMap,
+      start = serverMap["svr"]!!,
+      mandatoryStops = setOf(serverMap["dac"]!!, serverMap["fft"]!!)
+    )
+  }
+
+  fun countAllPaths(
+    serverMap: Map<String, Server>,
+    start: Server,
+    end: Server = serverMap["out"]!!,
+    mandatoryStops: Set<Server> = emptySet(),
+  ): Long {
+    // Cache needs to include which mandatory stops have been visited
+    val stepsCount = mutableMapOf<Pair<Server, Set<Server>>, Long>()
+    // mutable state of our current path
+    val currentPath = mutableSetOf<Server>()
+
+    fun dfs(current: Server, stopsHit: Set<Server> = emptySet()): Long {
+      // AoC is nice, and we don't have loops but uh ... just in case someone on reddit gets squirrelly with their data
+      if (current in currentPath) return 0
+
+      // Cache check needs to consider which mandatory stops were visited
+      val cacheKey = current to stopsHit
+
+      // if we have already found the steps for when we hit this node after already touching
+      // some / all of the mandatory stops we need to hit then we can bail out
+      // since we only update the cache after we have bottomed out
+      stepsCount[cacheKey]?.let { return it }
+
+      // If this is stop is a mandatory stop, we don't want to update the
+      // steps passed in, so keep it in a new variable
+      val updatedStopsHit = if (current in mandatoryStops) { stopsHit + current } else { stopsHit }
+
+      val steps = when (current) {
+        // Only count if we're at the end AND we've visited all required nodes!
+        end if mandatoryStops.size == updatedStopsHit.size -> 1
+        // At end but missing required nodes
+        end -> 0
+        // not at the end yet, so keep going...
+        else -> {
+          // update our current path so our kids know where we have been
+          currentPath.add(current)
+
+          // find out how many steps we have
+          val steps = serverMap[current.id]!!.nextServers.sumOf { dfs(serverMap[it]!!, updatedStopsHit) }
+
+          // current path is mutable so don't forget to pop ourselves back off the stack!
+          currentPath.remove(current)
+
+          steps
+        }
+      }
+
+      // Cache result with the visited required nodes
+      stepsCount[cacheKey] = steps
+
+      return steps
+    }
+
+    return dfs(start)
+  }
+
+  private fun server(line: String): Server {
+    val (id, serverList) = line.split(": ")
+    return Server(id, (serverList.split(" ")))
+  }
+}
+
+data class Server(val id: String, val nextServers: List<String> = emptyList()) {
+  override fun toString(): String = id
+}

src/main/kotlin/me/peckb/aoc/_2025/calendar/day11/README.md

@@ -0,0 +1 @@
+## [Day 11: Reactor](https://adventofcode.com/2025/day/11)

src/test/kotlin/me/peckb/aoc/_2025/TestDayComponent.kt

@@ -10,6 +10,7 @@ import me.peckb.aoc._2025.calendar.day07.Day07Test
 import me.peckb.aoc._2025.calendar.day08.Day08Test
 import me.peckb.aoc._2025.calendar.day09.Day09Test
 import me.peckb.aoc._2025.calendar.day10.Day10Test
+import me.peckb.aoc._2025.calendar.day11.Day11Test
 import javax.inject.Singleton
 import me.peckb.aoc.DayComponent
 import me.peckb.aoc.InputModule
@@ -28,4 +29,5 @@ internal interface TestDayComponent : DayComponent {
   fun inject(day08Test: Day08Test)
   fun inject(day09Test: Day09Test)
   fun inject(day10Test: Day10Test)
+  fun inject(day11Test: Day11Test)
 }

src/test/kotlin/me/peckb/aoc/_2025/calendar/day11/Day11Test.kt

@@ -0,0 +1,32 @@
+package me.peckb.aoc._2025.calendar.day11
+
+import javax.inject.Inject
+
+import me.peckb.aoc._2025.DaggerTestDayComponent
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.BeforeEach
+import org.junit.jupiter.api.Test
+
+internal class Day11Test {
+  @Inject
+  lateinit var day11: Day11
+
+  @BeforeEach
+  fun setup() {
+    DaggerTestDayComponent.create().inject(this)
+  }
+
+  @Test
+  fun testDay11PartOne() {
+    assertEquals(423, day11.partOne(DAY_11))
+  }
+
+  @Test
+  fun testDay11PartTwo() {
+    assertEquals(333657640517376, day11.partTwo(DAY_11))
+  }
+
+  companion object {
+    private const val DAY_11: String = "advent-of-code-input/2025/day11.input"
+  }
+}