Limiting string size in bytes

import scala.annotation.tailrec

object test extends App {

  def cutOff(s: String, maxBytes: Int): String = {
    @tailrec
    def loop(chars: Seq[Char], nchars: Int, bytesSoFar: Int): Int =
      if (chars.isEmpty) nchars
      else {
        val charSize = if (chars.head < 256) 1 else 2
        val newBytesSoFar = bytesSoFar + charSize
        if (newBytesSoFar > maxBytes) nchars
        else loop(chars.tail, nchars + 1, newBytesSoFar)
      }

    val nchars = loop(s.toCharArray, 0, 0)
    s.substring(0, s.length min nchars)
  }

  def dump(s: String, maxBytes: Int) = println(s"$s limited to $maxBytes bytes: ${cutOff(s, maxBytes)}")

  dump("r", 3)
  dump("r", 2)
  dump("r", 1)
  dump("r", 0)
  dump("й", 3)
  dump("й", 2)
  dump("й", 1)
  dump("й", 0)
}

/*
r limited to 3 bytes: r
r limited to 2 bytes: r
r limited to 1 bytes: r
r limited to 0 bytes:
й limited to 3 bytes: й
й limited to 2 bytes: й
й limited to 1 bytes:
й limited to 0 bytes:
*/