Day 01 2025

Author: peckb1

src/main/kotlin/me/peckb/aoc/_2025/calendar/day01/Day01.kt

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+package me.peckb.aoc._2025.calendar.day01
+
+import me.peckb.aoc._2025.calendar.day01.Day01.Direction.LEFT
+import me.peckb.aoc._2025.calendar.day01.Day01.Direction.RIGHT
+import javax.inject.Inject
+import me.peckb.aoc.generators.InputGenerator.InputGeneratorFactory
+import kotlin.sequences.forEach
+
+class Day01 @Inject constructor(private val generatorFactory: InputGeneratorFactory) {
+  companion object {
+    const val DIAL_NUMBERS = 100
+    const val START_LOCATION = 50
+  }
+
+  fun partOne(filename: String) = generatorFactory.forFile(filename).readAs(::rotation) { rotations ->
+    var dialLocation = START_LOCATION
+    var countAtZeroAfterMovement = 0
+
+    rotations.forEach { rotation ->
+      when (rotation.direction) {
+        LEFT  -> dialLocation -= rotation.count
+        RIGHT -> dialLocation += rotation.count
+      }
+
+      while (dialLocation !in 0 until DIAL_NUMBERS) {
+        if (dialLocation < 0) {
+          dialLocation += DIAL_NUMBERS
+        } else { // if (dialLocation >= DIAL_NUMBERS)
+          dialLocation -= DIAL_NUMBERS
+        }
+      }
+
+      if (dialLocation == 0) { countAtZeroAfterMovement++ }
+    }
+
+    countAtZeroAfterMovement
+  }
+
+  fun partTwo(filename: String) = generatorFactory.forFile(filename).readAs(::rotation) { rotations ->
+    var dialLocation = START_LOCATION
+    var countAtZeroAfterMovement = 0
+
+    rotations.forEach { rotation ->
+      val fullLoops = rotation.count / DIAL_NUMBERS
+      val remainingTurns = rotation.count % DIAL_NUMBERS
+
+      // always increment a full loop
+      countAtZeroAfterMovement += fullLoops
+
+      val dialLocationBeforeChanging = dialLocation
+      if (remainingTurns != 0) {
+        when (rotation.direction) {
+          RIGHT -> {
+            dialLocation += remainingTurns
+            // if we pass over 99, increment again
+            if (dialLocation >= DIAL_NUMBERS) {
+              dialLocation -= DIAL_NUMBERS
+              countAtZeroAfterMovement++
+            }
+          }
+          LEFT -> {
+            dialLocation -= remainingTurns
+            // if we go under 0 ... maybe increment
+            if (dialLocation <= 0) {
+              // going under zero but starting at zero shouldn't double count
+              if (dialLocationBeforeChanging != 0) { countAtZeroAfterMovement++ }
+              // but either way we're under zero and need to fix our dial
+              if (dialLocation < 0)                { dialLocation += DIAL_NUMBERS }
+            }
+          }
+        }
+      }
+    }
+
+    countAtZeroAfterMovement
+  }
+
+  private fun rotation(line: String) : Rotation {
+    val direction = when(val dc = line[0]) {
+      'L' -> LEFT
+      'R' -> RIGHT
+      else -> throw IllegalStateException("Unknown direction $dc")
+    }
+    val count = line.drop(1).toInt()
+
+    return Rotation(direction, count)
+  }
+
+  enum class Direction(val d: String) {
+    LEFT("L"), RIGHT("R");
+
+    override fun toString(): String = d
+  }
+
+  data class Rotation(val direction: Direction, val count: Int) {
+    override fun toString(): String = "$direction$count"
+  }
+}

src/main/kotlin/me/peckb/aoc/_2025/calendar/day01/README.md

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+## [Day 1: Secret Entrance](https://adventofcode.com/2025/day/1)

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

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+package me.peckb.aoc._2025
+
+import dagger.Component
+import javax.inject.Singleton
+import me.peckb.aoc.DayComponent
+import me.peckb.aoc.InputModule
+import me.peckb.aoc._2025.calendar.day01.Day01Test
+
+@Singleton
+@Component(modules = [InputModule::class])
+internal interface TestDayComponent : DayComponent {
+  fun inject(day01Test: Day01Test)
+}

src/test/kotlin/me/peckb/aoc/_2025/calendar/day01/Day01Test.kt

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+package me.peckb.aoc._2025.calendar.day01
+
+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 Day01Test {
+  @Inject
+  lateinit var day01: Day01
+
+  @BeforeEach
+  fun setup() {
+    DaggerTestDayComponent.create().inject(this)
+  }
+
+  @Test
+  fun testDay01PartOne() {
+    assertEquals(962, day01.partOne(DAY_01))
+  }
+
+  @Test
+  fun testDay01PartTwo() {
+    assertEquals(5782, day01.partTwo(DAY_01))
+  }
+
+  companion object {
+    private const val DAY_01: String = "advent-of-code-input/2025/day01.input"
+  }
+}