Solution 2025-01 (Secret Entrance)

Author: rhaendel

src/main/kotlin/de/ronny_h/aoc/year2025/day01/SecretEntrance.kt

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+package de.ronny_h.aoc.year2025.day01
+
+import de.ronny_h.aoc.AdventOfCode
+import de.ronny_h.aoc.year2025.day01.RotationDirection.LEFT
+import de.ronny_h.aoc.year2025.day01.RotationDirection.RIGHT
+import kotlin.math.abs
+
+fun main() = SecretEntrance().run(1177, 6768)
+
+class SecretEntrance : AdventOfCode<Int>(2025, 1) {
+
+    override fun part1(input: List<String>): Int {
+        var count = 0
+        var position = 50
+        input.parse().forEach {
+            val rotated = position.rotate(it)
+            val newPosition = rotated.mod(100)
+            if (newPosition == 0) {
+                count++
+            }
+            position = newPosition
+        }
+        return count
+    }
+
+    override fun part2(input: List<String>): Int {
+        var count = 0
+        var position = 50
+        input.parse().forEach {
+            val rotated = position.rotate(it)
+            val newPosition = rotated.mod(100)
+            val fullRounds = abs(rotated).div(100)
+            if (newPosition == 0 && fullRounds == 0) {
+                count++
+            }
+            count += fullRounds
+            if (position != 0 && rotated < 0) {
+                count++
+            }
+            position = newPosition
+        }
+        return count
+    }
+
+    private fun Int.rotate(rotation: Rotation): Int = when (rotation.direction) {
+        LEFT -> this - rotation.amount
+        RIGHT -> this + rotation.amount
+    }
+}
+
+fun List<String>.parse(): List<Rotation> = map {
+    val direction = when (it.first()) {
+        'L' -> LEFT
+        'R' -> RIGHT
+        else -> error("unknown direction: $it")
+    }
+    Rotation(direction, it.substring(1).toInt())
+}
+
+data class Rotation(val direction: RotationDirection, val amount: Int)
+
+enum class RotationDirection { LEFT, RIGHT }

src/test/kotlin/de/ronny_h/aoc/year2025/day01/SecretEntranceTest.kt

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+package de.ronny_h.aoc.year2025.day01
+
+import de.ronny_h.aoc.extensions.asList
+import de.ronny_h.aoc.year2025.day01.RotationDirection.LEFT
+import de.ronny_h.aoc.year2025.day01.RotationDirection.RIGHT
+import io.kotest.core.spec.style.StringSpec
+import io.kotest.matchers.shouldBe
+
+class SecretEntranceTest : StringSpec({
+
+    "input can be parsed" {
+        val input = """
+            L68
+            L30
+            R48
+        """.asList()
+
+        input.parse() shouldBe listOf(
+            Rotation(LEFT, 68),
+            Rotation(LEFT, 30),
+            Rotation(RIGHT, 48),
+        )
+    }
+
+    val input = """
+            L68
+            L30
+            R48
+            L5
+            R60
+            L55
+            L1
+            L99
+            R14
+            L82
+        """.asList()
+
+    "part 1: the password to open the door" {
+        SecretEntrance().part1(input) shouldBe 3
+    }
+
+    "part 2: the password to open the door with method 0x434C49434B" {
+        SecretEntrance().part2(input) shouldBe 6
+    }
+
+    "part 2 with 2x overflow" {
+        SecretEntrance().part2(listOf("L200")) shouldBe 2
+    }
+
+    "part 2 with 2x underflow" {
+        SecretEntrance().part2(listOf("R200")) shouldBe 2
+    }
+
+    "part 2 with an exact 0 and 2x overflow" {
+        SecretEntrance().part2(listOf("L50", "L200")) shouldBe 3
+    }
+
+    "part 2 with an exact 0 and 2x underflow" {
+        SecretEntrance().part2(listOf("L50", "R200")) shouldBe 3
+    }
+
+    "part 2 with an exact 0 and 2x overflow not landing on 0" {
+        SecretEntrance().part2(listOf("L50", "L199")) shouldBe 2
+    }
+
+    "part 2 with an exact 0 and 2x underflow not landing on 0" {
+        SecretEntrance().part2(listOf("L50", "R199")) shouldBe 2
+    }
+})