Day 03 2025 (#297)

* Day 03 2025

* Optimize by not recursing n=0

* remove extraneous wrapper class

* remove extraneous drop call

* use shifting start index instead of sublists

* formatting

* Optimization, you only need the first index of the max value

Author: peckb1

src/main/kotlin/me/peckb/aoc/_2025/calendar/day03/Day03.kt

@@ -0,0 +1,33 @@
+package me.peckb.aoc._2025.calendar.day03
+
+import javax.inject.Inject
+import me.peckb.aoc.generators.InputGenerator.InputGeneratorFactory
+
+class Day03 @Inject constructor(
+  private val generatorFactory: InputGeneratorFactory,
+) {
+  fun partOne(filename: String) = generatorFactory.forFile(filename).readAs(::batteryArray) { batteryArrays ->
+    batteryArrays.sumOf { batteryArray -> maxJoltage(batteryArray, numToEnable = 2) }
+  }
+
+  fun partTwo(filename: String) = generatorFactory.forFile(filename).readAs(::batteryArray) { batteryArrays ->
+    batteryArrays.sumOf { batteryArray -> maxJoltage(batteryArray, numToEnable = 12) }
+  }
+
+  private fun maxJoltage(batteryArray: List<Long>, startIndex: Int = 0, numToEnable: Int) : Long {
+    val indicesOfFlippableBatteries = startIndex .. ((batteryArray.size - 1) - (numToEnable - 1))
+    val largestStartValue = indicesOfFlippableBatteries.maxOf { i -> batteryArray[i] }
+    val firstIndexOfValue = indicesOfFlippableBatteries.first { i -> batteryArray[i] == largestStartValue }
+
+    // we can bottom out if our next recurse would enable zero batteries
+    if (numToEnable == 1) { return largestStartValue }
+
+    // if we have more batteries to flip - recurse down!
+    val maxSubJoltage = maxJoltage(batteryArray, startIndex = firstIndexOfValue + 1, numToEnable = numToEnable - 1)
+
+    // then add it to our value and return!
+    return "$largestStartValue$maxSubJoltage".toLong()
+  }
+
+  private fun batteryArray(line: String) = line.map { it.digitToInt().toLong() }
+}

src/main/kotlin/me/peckb/aoc/_2025/calendar/day03/README.md

@@ -0,0 +1 @@
+## [Day 3: Lobby](https://adventofcode.com/2025/day/3)

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

@@ -2,6 +2,7 @@ package me.peckb.aoc._2025
 
 import dagger.Component
 import me.peckb.aoc._2025.calendar.day02.Day02Test
+import me.peckb.aoc._2025.calendar.day03.Day03Test
 import javax.inject.Singleton
 import me.peckb.aoc.DayComponent
 import me.peckb.aoc.InputModule
@@ -12,4 +13,5 @@ import me.peckb.aoc._2025.calendar.day01.Day01Test
 internal interface TestDayComponent : DayComponent {
   fun inject(day01Test: Day01Test)
   fun inject(day02Test: Day02Test)
+  fun inject(day03Test: Day03Test)
 }

src/test/kotlin/me/peckb/aoc/_2025/calendar/day03/Day03Test.kt

@@ -0,0 +1,32 @@
+package me.peckb.aoc._2025.calendar.day03
+
+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 Day03Test {
+  @Inject
+  lateinit var day03: Day03
+
+  @BeforeEach
+  fun setup() {
+    DaggerTestDayComponent.create().inject(this)
+  }
+
+  @Test
+  fun testDay03PartOne() {
+    assertEquals(17443, day03.partOne(DAY_03))
+  }
+
+  @Test
+  fun testDay03PartTwo() {
+    assertEquals(172167155440541, day03.partTwo(DAY_03))
+  }
+
+  companion object {
+    private const val DAY_03: String = "advent-of-code-input/2025/day03.input"
+  }
+}