Day25

days/src/main/kotlin/Day25.kt

@@ -0,0 +1,50 @@
+package be.vandewalleh.aoc.days
+
+import be.vandewalleh.aoc.utils.input.Day
+import be.vandewalleh.aoc.utils.input.Input
+import be.vandewalleh.aoc.utils.input.Lines
+import be.vandewalleh.aoc.utils.input.createDay
+
+@Day(25)
+class Day25(@Lines val input: Input<IntArray>) {
+    private val doorPublicKey = input.value[0]
+    private val cardPublicKey = input.value[1]
+
+    private fun encryptionKey(loopSize: Int, publicKey: Int) = transformSubjectNumber(loopSize, publicKey)
+
+    private fun transformSubjectNumber(loopSize: Int, subjectNumber: Int): Int {
+        var number = 1L
+        repeat(loopSize) {
+            number *= subjectNumber
+            number %= 20201227
+        }
+        return number.toInt()
+    }
+
+    private fun transformSubjectNumberStartingWith(currentNumber: Long, subjectNumber: Int): Long {
+        var number = currentNumber
+        number *= subjectNumber
+        number %= 20201227
+        return number
+    }
+
+    private fun findLoopSize(expectedPublicKey: Long): Int {
+        var computedPublicKey = 1L
+        var loopSize = 0
+        do {
+            computedPublicKey = transformSubjectNumberStartingWith(computedPublicKey, 7)
+            loopSize++
+        } while (computedPublicKey != expectedPublicKey)
+        return loopSize
+    }
+
+    fun part1(): Int {
+        val cardLoopSize = findLoopSize(cardPublicKey.toLong())
+        return encryptionKey(cardLoopSize, doorPublicKey)
+    }
+
+}
+
+fun main() = with(createDay<Day25>()) {
+    println(part1())
+}

days/src/main/resources/day25.txt

@@ -0,0 +1,2 @@
+1327981
+2822615