Day 25 2020

src/main/kotlin/me/peckb/aoc/_2020/calendar/day25/Day25.kt

@@ -0,0 +1,34 @@
+package me.peckb.aoc._2020.calendar.day25
+
+import javax.inject.Inject
+
+import me.peckb.aoc.generators.InputGenerator.InputGeneratorFactory
+
+class Day25 @Inject constructor(
+  private val generatorFactory: InputGeneratorFactory,
+) {
+  fun partOne(filename: String) = generatorFactory.forFile(filename).read {
+    val (cardPublicKey, doorPublicKey) = it.toList().map { it.toLong() }
+    val cardLoopSize = findLoopSize(cardPublicKey)
+
+    findEncryptionKey(doorPublicKey, cardLoopSize)
+  }
+
+  private fun findLoopSize(publicKey: Long): Int {
+    var value = 1L
+    var loopSize = 0
+
+    while(value != publicKey) {
+      loopSize++
+      value = (value * 7L) % 2020_12_27L
+    }
+
+    return loopSize
+  }
+
+  private fun findEncryptionKey(publicKey: Long, loopSize: Int): Long {
+    var value = 1L
+    repeat(loopSize) { value = (value * publicKey) % 2020_12_27L }
+    return value
+  }
+}

src/main/kotlin/me/peckb/aoc/_2020/calendar/day25/README.md

@@ -0,0 +1 @@
+## [Day 25: Combo Breaker](https://adventofcode.com/2020/day/25)

src/test/kotlin/me/peckb/aoc/_2020/TestDayComponent.kt

@@ -24,6 +24,7 @@ import me.peckb.aoc._2020.calendar.day21.Day21Test
 import me.peckb.aoc._2020.calendar.day22.Day22Test
 import me.peckb.aoc._2020.calendar.day23.Day23Test
 import me.peckb.aoc._2020.calendar.day24.Day24Test
+import me.peckb.aoc._2020.calendar.day25.Day25Test
 import javax.inject.Singleton
 
 import me.peckb.aoc.DayComponent
@@ -57,4 +58,5 @@ internal interface TestDayComponent : DayComponent {
   fun inject(day22Test: Day22Test)
   fun inject(day23Test: Day23Test)
   fun inject(day24Test: Day24Test)
+  fun inject(day25Test: Day25Test)
 }

src/test/kotlin/me/peckb/aoc/_2020/calendar/day25/Day25Test.kt

@@ -0,0 +1,28 @@
+package me.peckb.aoc._2020.calendar.day25
+
+import javax.inject.Inject
+
+
+import me.peckb.aoc._2020.DaggerTestDayComponent
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.BeforeEach
+import org.junit.jupiter.api.Test
+
+internal class Day25Test {
+  @Inject
+  lateinit var day25: Day25
+
+  @BeforeEach
+  fun setup() {
+    DaggerTestDayComponent.create().inject(this)
+  }
+
+  @Test
+  fun testDay25PartOne() {
+    assertEquals(290487, day25.partOne(DAY_25))
+  }
+
+  companion object {
+    private const val DAY_25: String = "advent-of-code-input/2020/day25.input"
+  }
+}