day 20

src/main/kotlin/me/gergo/Aoc20.kt

@@ -0,0 +1,100 @@
+package me.gergo
+
+import java.io.File
+
+fun main() {
+    val mixedNumbers = File("src/main/resources/input20.txt").readLines()
+        .map(String::toLong)
+        .map { it * 811589153 } // Part Two - apply decryption key
+        .map { NumberList(it) }
+
+    mixedNumbers.reduce(NumberList::add)
+    mixedNumbers[0].loopWith(mixedNumbers[mixedNumbers.size - 1])
+
+    for (i in 1..10) // Part Two - mix 10 times
+        for (n in mixedNumbers) {
+            // Doing a `mod size-1` to speed things up
+            n.moveBy(Math.floorMod(n.value, mixedNumbers.size - 1L))
+        }
+
+    val zero = mixedNumbers.first { it.value == 0L }
+    val result1 = zero.next(1000).value + zero.next(2000).value + zero.next(3000).value
+    println("Sum of coordinates: $result1")
+}
+
+private class NumberList(val value: Long) {
+    private var prev: NumberList? = null
+    private var next: NumberList? = null
+
+    fun add(other: NumberList): NumberList {
+        next = other
+        other.prev = this
+        return other
+    }
+
+    fun next(steps: Long): NumberList {
+        var result = this
+        for (i in 1..steps) result = result.next!!
+        return result
+    }
+
+    fun prev(steps: Long): NumberList {
+        var result = this
+        for (i in 1..steps) result = result.prev!!
+        return result
+    }
+
+    fun loopWith(tail: NumberList) {
+        this.prev = tail
+        tail.next = this
+    }
+
+    fun moveBy(steps: Long) {
+        if (steps > 0) {
+            moveAfter(next(steps))
+        } else {
+            moveBefore(prev(-steps))
+        }
+    }
+
+    private fun moveBefore(other: NumberList) {
+        if (other == this) return
+        remove()
+        val prev = other.prev!!
+        prev.next = this
+        other.prev = this
+        this.prev = prev
+        this.next = other
+    }
+
+    private fun moveAfter(other: NumberList) {
+        if (other == this) return
+        remove()
+        val next = other.next!!
+        other.next = this
+        next.prev = this
+        this.prev = other
+        this.next = next
+    }
+
+    private fun remove() {
+        val prev = this.prev!!
+        val next = this.next!!
+        prev.next = next
+        next.prev = prev
+        this.prev = null
+        this.next = null
+    }
+
+    override fun toString(): String {
+        val buf = StringBuilder()
+        var current: NumberList? = this
+        while (current != null) {
+            if (current != this) buf.append(',')
+            buf.append(current.value)
+            current = current.next
+            if (current == this) break
+        }
+        return buf.toString()
+    }
+}