matching against a PushbackInputStream

extr.scala

object extr {

trait Message[T] {

  def read(in: PushbackInputStream): Option[T]

  def tag(in: PushbackInputStream) = {
    val byte = in.read
    in unread byte
    byte
  }

  def peek(in: PushbackInputStream): Array[Byte] = {
    val data = new Array[Byte](in.available)
    in read data; in unread data
    data
  }

  def unapply(in: PushbackInputStream): Option[T] = {
    val data = peek(in)
    val rest = if (data.isEmpty) new Array[Byte](0) else data.tail
    println(getClass.getSimpleName.split("\\$").last + ".unapply(" +
      data.headOption.getOrElse("empty") + " ... " + rest.mkString(" ") + ")")
    read(in)
  }
}

class Number(value: Int) {
  override def toString = "Number(%s)" format value
}
object Number extends Message[Number] {
  def read(in: PushbackInputStream): Option[Number] = {
    if (tag(in) != 0) return None else in.read
    val data = new Array[Byte](4)
    in read data
    Some(new Number(java.nio.ByteBuffer.wrap(data).getInt))
  }
}

def receive[R](input: PushbackInputStream)(reader: PartialFunction[PushbackInputStream, R]) = {
  val fallback: PartialFunction[PushbackInputStream, R] = {
    case in => throw new RuntimeException("Unknown Message")
    // actually respond to client with error, too
  }
  (reader orElse fallback) apply input
}

def createInputStream(data: Array[Byte]): PushbackInputStream =
  new PushbackInputStream(new ByteArrayInputStream(data), data.size)

def text = createInputStream(1.toByte +: 6.toByte +: "Hallo!".getBytes)
def number = createInputStream(Array(0, 0, 0, 0, 42).map(_.toByte))

def fun1(input: PushbackInputStream) = receive(input) {
  case Number(n) => "Number: " + n
}
def fun2(input: PushbackInputStream) = receive(input) {
  case Number(n) => "Number: " + n
  case _ => "nahahaaaa"
}

}

import extr._

scala-repl

scala> fun1(number)
Number.unapply(0 ... 0 0 0 42)
Number.unapply(empty ... )
scala.MatchError: java.io.PushbackInputStream@72e1c560 (of class java.io.PushbackInputStream)
	at ...

scala> fun2(number)
Number.unapply(0 ... 0 0 0 42)
res4: java.lang.String = Number: Number(42)

scala>