Main.scala

import scala.annotation.tailrec
import scala.collection.immutable.HashSet
import scala.io.Source

object Main {
  def main(args: Array[String]): Unit = {
    val lines = Source.fromFile("input.txt").getLines().toList
    val (part1,_) = execute(lines, 0, 0, HashSet[Integer]())

    println("part1=" + part1)
    
    lines.zipWithIndex.foreach(
      (code, number) => {
        if (code.startsWith("jmp") || code.startsWith("nop")) {
          val (inst, param) = code.splitAt(3)
          val newInst = if(inst == "jmp") "nop" else "jmp"
          val trialLines = lines.updated(number, newInst + param)

          val (acc, terminated) = execute(trialLines, 0, 0, HashSet[Integer]())
          
          if(terminated) {
            println("part2=" + acc)
          }
        } 
      }
    )
  }
  
  @tailrec
  def execute(codes: List[String], pos:Integer, acc:Integer, visited:Set[Integer]) : (Integer, Boolean) = {
    val code = codes(pos)
    
    val (inst, op) = parse(code)

    if(pos >= codes.size - 1) (acc, true)
    else if (visited.contains(pos)) (acc, false)
    else {
      inst match {
        case "acc"  => execute(codes, pos + 1, acc + op, visited + pos)
        case "jmp"  => execute(codes, pos + op, acc, visited + pos)
        case _  => execute(codes, pos + 1, acc,  visited + pos)
      }
    }
  }
  
  def parse(code:String) : (String, Integer) = {
    val (inst, op) = code.splitAt(4)
    (inst.trim, parseOp(op))
  }

  def parseOp(op:String): Integer = {
    val (sign, amountStr) = op.splitAt(1)
    val amount = Integer.parseInt(amountStr)
    
    sign match {
      case "-" => -amount
      case _ => amount
    }
  }
}