Solution 2025-03 (Lobby)

Author: rhaendel

src/main/kotlin/de/ronny_h/aoc/year2025/day03/Lobby.kt

@@ -0,0 +1,38 @@
+package de.ronny_h.aoc.year2025.day03
+
+import de.ronny_h.aoc.AdventOfCode
+
+fun main() = Lobby().run(17324, 171846613143331)
+
+class Lobby : AdventOfCode<Long>(2025, 3) {
+    override fun part1(input: List<String>): Long = input
+        .parse().sumOf(List<Int>::joltage)
+
+    override fun part2(input: List<String>): Long = input
+        .parse().sumOf(List<Int>::joltageWithSafetyOverride)
+}
+
+fun List<Int>.joltage(): Long {
+    val first = subList(0, lastIndex).max()
+    val firstIndex = indexOf(first)
+    val second = subList(firstIndex + 1, size).max()
+    return "$first$second".toLong()
+}
+
+fun List<Int>.joltageWithSafetyOverride(): Long {
+    var index = -1
+    val batteriesSwitchedOn = mutableListOf<Int>()
+    for (remaining in 11 downTo 0) {
+        val remainingBatteries = subList(index + 1, size - remaining)
+        val chosenBattery = remainingBatteries.max()
+        batteriesSwitchedOn.add(chosenBattery)
+        index += 1 + remainingBatteries.indexOf(chosenBattery)
+    }
+    return batteriesSwitchedOn.joinToString("").toLong()
+}
+
+fun List<String>.parse() = map { bank ->
+    bank.map {
+        it.digitToIntOrNull() ?: error("should never be null")
+    }
+}

src/test/kotlin/de/ronny_h/aoc/year2025/day03/LobbyTest.kt

@@ -0,0 +1,47 @@
+package de.ronny_h.aoc.year2025.day03
+
+import de.ronny_h.aoc.extensions.asList
+import io.kotest.core.spec.style.StringSpec
+import io.kotest.data.forAll
+import io.kotest.data.row
+import io.kotest.matchers.shouldBe
+
+class LobbyTest : StringSpec({
+
+    val input = """
+        987654321111111
+        811111111111119
+        234234234234278
+        818181911112111
+    """.asList()
+
+    "joltage" {
+        forAll(
+            row(listOf(9, 8, 7, 6, 5, 4, 3, 2, 1, 1, 1, 1, 1, 1, 1), 98),
+            row(listOf(8, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 9), 89),
+            row(listOf(2, 3, 4, 2, 3, 4, 2, 3, 4, 2, 3, 4, 2, 7, 8), 78),
+            row(listOf(8, 1, 8, 1, 8, 1, 9, 1, 1, 1, 1, 2, 1, 1, 1), 92),
+        ) { bank, expected ->
+            bank.joltage() shouldBe expected
+        }
+    }
+
+    "joltageWithSafetyOverride" {
+        forAll(
+            row(listOf(9, 8, 7, 6, 5, 4, 3, 2, 1, 1, 1, 1, 1, 1, 1), 987654321111),
+            row(listOf(8, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 9), 811111111119),
+            row(listOf(2, 3, 4, 2, 3, 4, 2, 3, 4, 2, 3, 4, 2, 7, 8), 434234234278),
+            row(listOf(8, 1, 8, 1, 8, 1, 9, 1, 1, 1, 1, 2, 1, 1, 1), 888911112111),
+        ) { bank, expected ->
+            bank.joltageWithSafetyOverride() shouldBe expected
+        }
+    }
+
+    "part 1: the total output joltage" {
+        Lobby().part1(input) shouldBe 357
+    }
+
+    "part 2: the total output joltage with safety override" {
+        Lobby().part2(input) shouldBe 3121910778619
+    }
+})