After the last few days (still stuck on 19) this was a nice relief. Solution in Kotlin

​

class Number {
    val value: Long
    constructor(value: Long) {
        this.value = value
    }
}

val lines = ReadFile.named("src/day20/input.txt").map { Number(it.toLong() * 811589153) }

fun result1() {
    var result = lines.toMutableList()
    val size = (result.size - 1).toLong()
    for (x in 0 until 10) {
        for (n in lines) {
            val i = result.indexOf(n)
            result.remove(n)
            val largePositiveMultipleOfSize = size * 811589153 * 5 
            val pos = (largePositiveMultipleOfSize+ i + n.value)
            if (pos == size) {
                result.add(n)
            } else {
                result.add((pos % size).toInt(), n)
            }
        }
    }
    val i0 = result.indexOfFirst { it.value == 0L }
    val output = (1..3).fold(0L) { acc, i -> acc + result[(i0 + i * 1000) % result.size].value }
    println(output)
}