Day 21 2024

src/main/kotlin/me/peckb/aoc/_2024/calendar/day21/Day21.kt

@@ -0,0 +1,112 @@
+package me.peckb.aoc._2024.calendar.day21
+
+import javax.inject.Inject
+import me.peckb.aoc.generators.InputGenerator.InputGeneratorFactory
+import java.util.LinkedList
+import kotlin.math.sign
+
+class Day21 @Inject constructor(
+  private val generatorFactory: InputGeneratorFactory,
+) {
+  fun partOne(filename: String) = generatorFactory.forFile(filename).read { input ->
+    solve(input, indirections = 3)
+  }
+
+  fun partTwo(filename: String) = generatorFactory.forFile(filename).read { input ->
+    solve(input, indirections = 26)
+  }
+
+  private fun solve(input: Sequence<String>, indirections: Int) : Long {
+    return input.sumOf { code ->
+      val length = getLength(NUMERIC_PAD, code, indirections)
+      val size = code.take(3).toLong()
+
+      length * size
+    }
+  }
+
+  private fun getLength(keyPad: Map<Char, Pair<Int, Int>>, code: String, indirections: Int): Long {
+    val lengthKey = LengthKey(keyPad.size, code, indirections)
+
+    if (LENGTH_CACHE.containsKey(lengthKey)) {
+      return LENGTH_CACHE[lengthKey]!!
+    }
+
+    if (indirections == 0) {
+      return code.length.toLong().also { LENGTH_CACHE[lengthKey] = it }
+    }
+
+    val minLength = "A${code}".toList()
+      .windowed(2)
+      .sumOf { (s, e) ->
+        paths(keyPad, s, e).minOf { getLength(DIRECTION_PAD, "${it}A", indirections - 1) }
+      }
+
+    return minLength.also { LENGTH_CACHE[lengthKey] = it }
+  }
+
+  private fun paths(keyPad: Map<Char, Pair<Int, Int>>, start: Char, end: Char): List<String> {
+    val pathKey = PathKey(keyPad.size, start, end)
+
+    if (PATHS_CACHE.containsKey(pathKey)) {
+      return PATHS_CACHE[pathKey]!!
+    }
+
+    val paths = mutableListOf<String>()
+    val partialPaths = LinkedList<Pair<Pair<Int, Int>, String>>().apply {
+      add(keyPad[start]!! to "")
+    }
+
+    pathGeneration@ while(partialPaths.isNotEmpty()) {
+      val (position, path) = partialPaths.poll()
+
+      val target = keyPad[end]!!
+      if (position == target) {
+        paths.add(path)
+        continue@pathGeneration
+      }
+
+      val yDelta = target.second - position.second
+      if (yDelta != 0) {
+        val newPoint = position.first to position.second + yDelta.sign
+        if (newPoint in keyPad.values) {
+          val dir = if (yDelta > 0) { ">" } else { "<" }
+          partialPaths.add(newPoint to "$path$dir")
+        }
+      }
+
+      val xDelta = target.first - position.first
+      if (xDelta != 0) {
+        val newPoint = position.first + xDelta.sign to position.second
+        if (newPoint in keyPad.values) {
+          val dir = if (xDelta > 0) { "v" } else { "^" }
+          partialPaths.add(newPoint to "$path$dir")
+        }
+      }
+    }
+
+    return paths.also { PATHS_CACHE[pathKey] = it }
+  }
+
+  companion object {
+    val NUMERIC_PAD = mapOf(
+      '7' to (0 to 0), '8' to (0 to 1), '9' to (0 to 2),
+      '4' to (1 to 0), '5' to (1 to 1), '6' to (1 to 2),
+      '1' to (2 to 0), '2' to (2 to 1), '3' to (2 to 2),
+                       '0' to (3 to 1), 'A' to (3 to 2),
+    )
+
+    val DIRECTION_PAD = mapOf(
+                       '^' to (0 to 1), 'A' to (0 to 2),
+      '<' to (1 to 0), 'v' to (1 to 1), '>' to (1 to 2)
+    )
+
+    val LENGTH_CACHE = mutableMapOf<LengthKey, Long>()
+
+    val PATHS_CACHE = mutableMapOf<PathKey, List<String>>()
+  }
+}
+
+data class LengthKey(val padIndicator: Int, val pattern: String, val robots: Int)
+
+data class PathKey(val padIndicator: Int, val start: Char, val end: Char)

src/main/kotlin/me/peckb/aoc/_2024/calendar/day21/README.md

@@ -0,0 +1 @@
+## [Day 21: Keypad Conundrum](https://adventofcode.com/2024/day/21)

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

@@ -20,6 +20,7 @@ import me.peckb.aoc._2024.calendar.day17.Day17Test
 import me.peckb.aoc._2024.calendar.day18.Day18Test
 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 javax.inject.Singleton
 import me.peckb.aoc.DayComponent
 import me.peckb.aoc.InputModule
@@ -48,4 +49,5 @@ internal interface TestDayComponent : DayComponent {
   fun inject(day18Test: Day18Test)
   fun inject(day19Test: Day19Test)
   fun inject(day20Test: Day20Test)
+  fun inject(day21Test: Day21Test)
 }

src/test/kotlin/me/peckb/aoc/_2024/calendar/day21/Day21Test.kt

@@ -0,0 +1,32 @@
+package me.peckb.aoc._2024.calendar.day21
+
+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 Day21Test {
+  @Inject
+  lateinit var day21: Day21
+
+  @BeforeEach
+  fun setup() {
+    DaggerTestDayComponent.create().inject(this)
+  }
+
+  @Test
+  fun testDay21PartOne() {
+    assertEquals(246_990, day21.partOne(DAY_21))
+  }
+
+  @Test
+  fun testDay21PartTwo() {
+    assertEquals(306_335_137_543_664, day21.partTwo(DAY_21))
+  }
+
+  companion object {
+    private const val DAY_21: String = "advent-of-code-input/2024/day21.input"
+  }
+}