Day 02 2025 (#294)

Author: peckb1

src/main/kotlin/me/peckb/aoc/_2025/calendar/day02/Day02.kt

@@ -0,0 +1,56 @@
+package me.peckb.aoc._2025.calendar.day02
+
+import javax.inject.Inject
+import me.peckb.aoc.generators.InputGenerator.InputGeneratorFactory
+
+class Day02 @Inject constructor(
+  private val generatorFactory: InputGeneratorFactory,
+) {
+  fun partOne(filename: String) = generatorFactory.forFile(filename).readOne { idRangeList ->
+    idRangeList.countInvalidIds(::isIdInvalid)
+  }
+
+  fun partTwo(filename: String) = generatorFactory.forFile(filename).readOne { idRangeList ->
+    idRangeList.countInvalidIds(::isIdActuallyInvalid)
+  }
+
+  fun String.countInvalidIds(validation: (Long) -> Boolean): Long {
+    return split(",")
+      .map { it.split("-").let { (s, e) -> IdRange(s.toLong(), e.toLong()) } }
+      .sumOf { idRange -> (idRange.start..idRange.end).filter { id -> validation(id) }.sum() }
+  }
+
+  private fun isIdInvalid(id: Long): Boolean {
+    val idString = id.toString()
+
+    // odd numbers cannot be the same pattern repeated
+    if (idString.length %2 != 0) {
+      return false
+    }
+
+    // so we have an even length ID
+    val halfLength = idString.length / 2
+    return idString.take(halfLength) == idString.drop(halfLength)
+  }
+
+  private fun isIdActuallyInvalid(id: Long): Boolean {
+    val idString = id.toString()
+    val fullLength = idString.length
+    val halfLength = idString.length / 2
+
+    return (1 .. halfLength).any { lengthToRepeat ->
+      // if we can't form a full pattern of string length no sense trying
+      if (fullLength % lengthToRepeat != 0) { return@any false }
+
+      val data = idString.take(lengthToRepeat)
+      var repeatedCount = 1
+      while(repeatedCount * lengthToRepeat <= fullLength) {
+        if (data.repeat(repeatedCount) == idString) { return@any true }
+        repeatedCount++
+      }
+      return@any false
+    }
+  }
+}
+
+data class IdRange(val start: Long, val end: Long)

src/main/kotlin/me/peckb/aoc/_2025/calendar/day02/README.md

@@ -0,0 +1 @@
+## [Day 2: Gift Shop](https://adventofcode.com/2025/day/2)

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

@@ -1,6 +1,7 @@
 package me.peckb.aoc._2025
 
 import dagger.Component
+import me.peckb.aoc._2025.calendar.day02.Day02Test
 import javax.inject.Singleton
 import me.peckb.aoc.DayComponent
 import me.peckb.aoc.InputModule
@@ -10,4 +11,5 @@ import me.peckb.aoc._2025.calendar.day01.Day01Test
 @Component(modules = [InputModule::class])
 internal interface TestDayComponent : DayComponent {
   fun inject(day01Test: Day01Test)
+  fun inject(day02Test: Day02Test)
 }

src/test/kotlin/me/peckb/aoc/_2025/calendar/day02/Day02Test.kt

@@ -0,0 +1,32 @@
+package me.peckb.aoc._2025.calendar.day02
+
+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 Day02Test {
+  @Inject
+  lateinit var day02: Day02
+
+  @BeforeEach
+  fun setup() {
+    DaggerTestDayComponent.create().inject(this)
+  }
+
+  @Test
+  fun testDay02PartOne() {
+    assertEquals(28846518423, day02.partOne(DAY_02))
+  }
+
+  @Test
+  fun testDay02PartTwo() {
+    assertEquals(31578210022, day02.partTwo(DAY_02))
+  }
+
+  companion object {
+    private const val DAY_02: String = "advent-of-code-input/2025/day02.input"
+  }
+}