foldLeft with appended or prepended

object SecretHandshake {
  val SIGNS = Vector("wink", "double blink", "close your eyes", "jump")
  val REVERSE_SIGNS = 16

  def commands(orders: Int): Seq[String] = {
  
    val stitch =
      if ((orders & REVERSE_SIGNS) == 0)
        (coll: Seq[String], that: String) => coll.:+(that)
      else (coll: Seq[String], that: String) => coll.+:(that)

    (0 until SIGNS.length)
      .foldLeft(Seq(): Seq[String])((output, index) =>
        if ((1 << index & orders) != 0) stitch(output, SIGNS(index))
        else output
      )
  }
}

This approach starts by defining a Vector for holding the signs, and an Int binding for the reverse value.

The command() method starts by defining a val for combining a String with a Seq[String]. The combining is done by a function that is determined by if the input number has a bit set in the same position as binary 16 (10000). If not, the val is set to a function that will append the String argument to the Seq. If so, the val is set to a function that will prepend the String argument to the Seq.

A Range is defined from 0 up to but not including the length of the signs Vector. Each number in the Range is passed to the foldLeft() method for an index value.

The foldLeft() is initialized with an empty Seq[String] for the output value. The lambda in foldLeft uses the bitwise shift left operator to see if the input number has a binary 1 in the same position as 1 shifted left for the index value. If so, then the element for that index in the signs Vector is combined with the output Seq by the function defined for combining a String and Seq[String]. If not, the output Seq is passed as is to the next iteration of foldLeft().

After foldLeft() is done, the commands() method returns the output Seq.