Maximum Length for scala queue

Question

I\'m curious if Scala has some gem hidden in its collection classes that I can use. Basically I\'m looking for something like a FIFO queue, but that has an upper-limit on i

Answer 1

Here is an immutable solution:

class FixedSizeFifo[T](val limit: Int)
( private val out: List[T], private val in: List[T] ) 
extends Traversable[T] {

  override def size = in.size + out.size

  def :+( t: T ) = {
    val (nextOut,nextIn) = if (size == limit) {
      if( out.nonEmpty) {
        ( out.tail, t::in ) 
      } else {
        ( in.reverse.tail, List(t) )
      }
    } else ( out, t::in )
      new FixedSizeFifo( limit )( nextOut, nextIn )
  }

  private lazy val deq = {
    if( out.isEmpty ) {
      val revIn = in.reverse
      ( revIn.head, new FixedSizeFifo( limit )( revIn.tail, List() ) )
    } else {
      ( out.head, new FixedSizeFifo( limit )( out.tail, in ) )
    }
  }
  override lazy val head = deq._1
  override lazy val tail = deq._2

  def foreach[U]( f: T => U ) = ( out ::: in.reverse ) foreach f

}

object FixedSizeFifo {
  def apply[T]( limit: Int ) = new FixedSizeFifo[T]( limit )(List(),List())
}

An example:

val fifo = FixedSizeFifo[Int](3) :+ 1 :+ 2 :+ 3 :+ 4 :+ 5 :+ 6
println( fifo )                //prints: FixedSizeFifo(4, 5, 6)
println( fifo.head )           //prints: 4
println( fifo.tail :+ 7 :+8 )  //prints: FixedSizeFifo(6, 7, 8)