A solution to Advent of Code 2022 Day 6 without sets, only loops (modeled as nested recursive functions).

Day06Loops.scala

/*
Copyright 2022 Tyler Coles (javadocmd.com)

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

import scala.annotation.tailrec
import scala.io.Source

object Day06Loops:
  final def main(args: Array[String]): Unit =
    val input = Source.fromResource("Day06.input.txt").mkString
    val part1 = findMarker(input, length = 4)
    println(part1)
    val part2 = findMarker(input, length = 14)
    println(part2)

  @tailrec
  def findMarker(signal: String, length: Int, position: Int = 0): Int =
    @tailrec
    def findDuplicate(check: Int, cursor: Int, stop: Int): Int =
      // when check advances to stop, conclude no duplicate found
      if check == stop then -1
      // when cursor advances to stop, advance check and reset cursor
      else if cursor == stop then findDuplicate(check + 1, check + 2, stop)
      // if the characters at check and cursor are equal, duplicate found
      // return the index at which to continue the outer search
      else if signal(check) == signal(cursor) then check + 1
      // otherwise, advance cursor
      else findDuplicate(check, cursor + 1, stop)

    if position + length > signal.length then -1 // stop if there aren't enough characters left to make the marker
    else
      // search for duplicates within `length` of our current position
      findDuplicate(position, position + 1, position + length) match
        case -1   => position + length // no duplicate means we've found the marker! success
        case next => findMarker(signal, length, position = next) // otherwise keep looking
  end findMarker

Scala 3.2.1