Day02.kt

/*
 * Copyright (c) 2019 by Todd Ginsberg
 */

/**
 * Advent of Code 2019, Day 2 - 1202 Program Alarm
 * Problem Description: http://adventofcode.com/2019/day/2
 * Blog Post/Commentary: https://todd.ginsberg.com/post/advent-of-code/2019/day2/
 */
package com.ginsberg.advent2019

class Day02(input: String) {

    private val memory: IntArray = input.split(",").map { it.toInt() }.toIntArray()

    fun solvePart1(noun: Int = memory[1], verb: Int = memory[2]): Int =
        runProgram(memory, noun, verb)

    fun solvePart2(target: Int = 19_690_720): Int {
        (0..99).forEach { noun ->
            (0..99).forEach { verb ->
                if (runProgram(memory, noun, verb) == target) {
                    return (noun * 100) + verb
                }
            }
        }
        throw IllegalStateException("Cannot find starting noun/verb that end up with $target")
    }

    private fun runProgram(memory: IntArray, noun: Int, verb: Int): Int {
        val memoryCopy = memory.copyOf().apply {
            this[1] = noun
            this[2] = verb
        }

        (memoryCopy.indices step 4).forEach { ip ->
            when (memoryCopy[ip]) {
                1 -> memoryCopy.setRef(ip + 3, memoryCopy.getRef(ip + 1) + memoryCopy.getRef(ip + 2))
                2 -> memoryCopy.setRef(ip + 3, memoryCopy.getRef(ip + 1) * memoryCopy.getRef(ip + 2))
                99 -> return memoryCopy[0]
            }
        }
        throw IllegalStateException("Program ran out of instructions")
    }

    private fun IntArray.setRef(at: Int, value: Int) {
        this[this[at]] = value
    }

    private fun IntArray.getRef(at: Int): Int =
        this[this[at]]

}