Day8.kt

import java.io.File

var largestSeenValue = 0

fun main(args: Array<String>) {
    val instructions = File("input.txt").readLines()
    println("Part 1: largest value = ${getLargestValue(instructions)}")
    println("Part 2: largest seen value = $largestSeenValue")
}

fun getLargestValue(instructions: List<String>) : Int {
    val registers = mutableListOf<Register>()
    instructions.forEach {
        val command = it.substringBefore(" if").split(" ")
        val condition = it.substringAfter("if ").split(" ")
        if (hasMetCondition(condition, registers)) {
            executeCommand(command, registers)
        }
    }
    registers.sortByDescending { it.value }
    return registers[0].value
}

private fun hasMetCondition(condition: List<String>, registers: MutableList<Register>) : Boolean {
    registers.addRegister(Register(condition[0]))
    val register = registers.getRegister(condition[0])
    return when(condition[1]) {
        "==" -> register.value == condition[2].toInt()
        "!=" -> register.value != condition[2].toInt()
        ">=" -> register.value >= condition[2].toInt()
        "<=" -> register.value <= condition[2].toInt()
        ">" -> register.value > condition[2].toInt()
        "<" -> register.value < condition[2].toInt()
        else -> false
    }
}

private fun executeCommand(command: List<String>, registers: MutableList<Register>) {
    registers.addRegister(Register(command[0]))
    val register = registers.getRegister(command[0])
    when(command[1]) {
        "inc" -> register.inc(command[2].toInt())
        "dec" -> register.dec(command[2].toInt())
    }
    if (register.value > largestSeenValue) largestSeenValue = register.value
}

private fun List<Register>.getRegister(name: String) = this.first { it == Register(name) }

private fun MutableList<Register>.addRegister(register: Register) {
    if (register !in this) add(register)
}

private data class Register(val name: String, var value: Int) {
    constructor(name: String) : this(name, 0)

    fun dec(x: Int) {
        value -= x
    }

    fun inc(x: Int) {
        value += x
    }

    override fun equals(other: Any?): Boolean {
        if (other is Register) {
            return this.name == other.name
        }
        return false
    }

    override fun hashCode(): Int {
        return name.hashCode()
    }
}