https://adventofcode.com/2016/day/2

Solution.kt

import java.lang.Math.*

data class Point(private val x_: Int, private val y_: Int) {
    fun x() = max(0, min(x_, 2))
    fun y() = max(0, min(y_, 2))
    operator fun plus(p: Point) = Point(x() + p.x_, y() + p.y_)
    override fun toString() = "${y() * 3 + x() + 1}"
}

private fun solve(input: String) = input.fold(mutableListOf(Point(1, 1)), { res, c ->
    operator fun MutableList<Point>.plus(p: Point) = removeAt(size - 1) + p
    res.add(when (c) {
        'U' -> res + Point(0, -1)
        'D' -> res + Point(0, 1)
        'R' -> res + Point(1, 0)
        'L' -> res + Point(-1, 0)
        else -> res.last()
    })
    res
})

fun main(args: Array<String>) {
    val input = """|ULL
                   |RRDDD
                   |LURDL
                   |UUUUD""".trimMargin()
    println(solve(input))
}