Day 23 2024

src/main/kotlin/me/peckb/aoc/_2024/calendar/day23/Day23.kt

@@ -0,0 +1,71 @@
+package me.peckb.aoc._2024.calendar.day23
+
+import javax.inject.Inject
+import me.peckb.aoc.generators.InputGenerator.InputGeneratorFactory
+
+class Day23 @Inject constructor(
+  private val generatorFactory: InputGeneratorFactory,
+) {
+  fun partOne(filename: String) = generatorFactory.forFile(filename).readAs(::connection) { input ->
+    val connections = findFriends(input)
+    val triplets = mutableSetOf<Set<String>>()
+
+    findCommonFriend(connections) { me, them, ourFriends ->
+      ourFriends.forEach { ourFriend -> triplets.add(setOf(me, them, ourFriend)) }
+    }
+
+    triplets.count { group -> group.any { it.startsWith('t') } }
+  }
+
+  fun partTwo(filename: String) = generatorFactory.forFile(filename).readAs(::connection) { input ->
+    val connections = findFriends(input)
+    val friends = mutableSetOf<Set<String>>()
+
+    findCommonFriend(connections) { me, them, ourFriends ->
+      if (ourFriends.isNotEmpty()) {
+        val everyFriend = ourFriends.all { maybeMutual ->
+          ourFriends.minus(maybeMutual).all {
+            (maybeMutual in connections[it]!!)
+          }
+        }
+        if (everyFriend) {
+          friends.add(ourFriends.plus(me).plus(them))
+        }
+      }
+    }
+
+    friends.maxBy { it.size }.toList().sorted().joinToString(",")
+  }
+
+  private fun connection(line: String) : Pair<String, String> {
+    return line.split("-").let { (a, b) -> a to b }
+  }
+
+  private fun findFriends(input: Sequence<Pair<String, String>>): MutableMap<String, Set<String>> {
+    val connections = mutableMapOf<String, Set<String>>()
+
+    input.forEach { (c1, c2) ->
+      connections.merge(c1, setOf(c2)) { a, b -> a + b }
+      connections.merge(c2, setOf(c1)) { a, b -> a + b }
+    }
+
+    return connections
+  }
+
+  private fun findCommonFriend(connections: Map<String, Set<String>>, handleFriends: (String, String, Set<String>) -> Unit) {
+    val keys = connections.keys.toList()
+    (0 until keys.size - 2).forEach { i ->
+      val me = keys[i]
+      val myFriends = connections[me]!!
+
+      (i + 1 until keys.size - 1).forEach { j ->
+        val them = keys[j]
+        val theirFriends = connections[them]!!
+
+        if (me in theirFriends) {
+          handleFriends(me, them, myFriends.intersect(theirFriends))
+        }
+      }
+    }
+  }
+}

src/main/kotlin/me/peckb/aoc/_2024/calendar/day23/README.md

@@ -0,0 +1 @@
+## [Day 23: LAN Party](https://adventofcode.com/2024/day/23)

src/test/kotlin/me/peckb/aoc/_2024/TestDayComponent.kt

@@ -22,6 +22,7 @@ import me.peckb.aoc._2024.calendar.day19.Day19Test
 import me.peckb.aoc._2024.calendar.day20.Day20Test
 import me.peckb.aoc._2024.calendar.day21.Day21Test
 import me.peckb.aoc._2024.calendar.day22.Day22Test
+import me.peckb.aoc._2024.calendar.day23.Day23Test
 import javax.inject.Singleton
 import me.peckb.aoc.DayComponent
 import me.peckb.aoc.InputModule
@@ -52,4 +53,5 @@ internal interface TestDayComponent : DayComponent {
   fun inject(day20Test: Day20Test)
   fun inject(day21Test: Day21Test)
   fun inject(day22Test: Day22Test)
+  fun inject(day23Test: Day23Test)
 }

src/test/kotlin/me/peckb/aoc/_2024/calendar/day23/Day23Test.kt

@@ -0,0 +1,32 @@
+package me.peckb.aoc._2024.calendar.day23
+
+import javax.inject.Inject
+
+import me.peckb.aoc._2024.DaggerTestDayComponent
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.BeforeEach
+import org.junit.jupiter.api.Test
+
+internal class Day23Test {
+  @Inject
+  lateinit var day23: Day23
+
+  @BeforeEach
+  fun setup() {
+    DaggerTestDayComponent.create().inject(this)
+  }
+
+  @Test
+  fun testDay23PartOne() {
+    assertEquals(998, day23.partOne(DAY_23))
+  }
+
+  @Test
+  fun testDay23PartTwo() {
+    assertEquals("cc,ff,fh,fr,ny,oa,pl,rg,uj,wd,xn,xs,zw", day23.partTwo(DAY_23))
+  }
+
+  companion object {
+    private const val DAY_23: String = "advent-of-code-input/2024/day23.input"
+  }
+}