package examples
 
/** Illustrate the use of pattern matching in Scala.
 *  Like patterns.scala, but uses extractors for representation independence
 */
object extractorPatterns {
   
  /** We need an abstract base class for trees. Subclasses with
   *  the 'case' modifier can be used in pattern matching expressions
   *  to deconstruct trees.
   *
   *  Here, we replaced case classes of patterns.scala with objects
   *  that hide the actual implementation of Branch and Leaf. Note
   *  that the remaining code does not change. In this way, we
   *  can change the implementation later without affecting clients,
   *  which is called representation independence.
   */
  abstract class Tree
 
  object Branch {
    /* method to contruct branches @see extractorPatterns.tree1 */
    def apply(left: Tree, right: Tree): Tree = new BranchImpl(left, right)
    /* extractor method referenced in match expressions @see extractorPatterns.sumLeaves */
    def unapply(x:Tree): Option[(Tree,Tree)] = x match {
      case y:BranchImpl => Some(y.left, y.right)
      case _            => None
    }
    private class BranchImpl(val left:Tree, val right:Tree) extends Tree
  }
  object Leaf {
    /* method to contruct leaves @see tree1 */
    def apply(x:Int): Tree = new LeafImpl(x);
    /* extractor method referenced in match expressions @see extractorPatterns.sumLeaves */
    def unapply(x:Tree): Option[Int] = x match {
      case y:LeafImpl => Some(y.x)
      case _          => None
    }
    private class LeafImpl(val x: Int) extends Tree
  }
 
 
  /** Case classes have an implicit constructor methods which allows
   *  to create objects withouth the 'new' keyword. It saves some typing
   *  and makes code clearer.
   *
   *  Here, the task of the case class constructor is performed by the
   *  method Branch.apply - the singleton Branch is treated as if it
   *  were a function value. This trick works with any value that has
   *  an apply method.
   */
  val tree1 = Branch(Branch(Leaf(1), Leaf(2)), Branch(Leaf(3), Leaf(4)))
 
  /** Return the sum of numbers found in leaves.
   *  'match' is a generalization of 'switch' in C-like languages
   *
   *  Patterns consist of case class constructors (which can
   *  be nested), and lower case variables which are
   *  bound to the values with which the class has been constructed.
   *
   *  For extractors, it is not the name of a case class, but the name of
   *  the singleton object Branch which is used to refer to its extractor method
   *  Branch.unapply - the pattern is the 'reverse' of a method
   *  call, with the result being matched in the subpatterns. This works
   *  for any value that has an appropriate extractor method.
   */
  def sumLeaves(t: Tree): Int = t match {
    case Branch(l, r) => sumLeaves(l) + sumLeaves(r)
    case Leaf(x) => x
  }
 
  /** This illustrates the use of Option types. Since the
   *  method is not known in advance to find 'x', the
   *  return type is an Option. Options have two possible
   *  values, either 'Some' or 'None'. It is a type-safe
   *  way around 'null' values.
   */
  def find[A, B](it: Iterator[Pair[A, B]], x: A): Option[B] = {
    var result: Option[B] = None
    while (it.hasNext && result == None) {
      val Pair(x1, y) = it.next;
      if (x == x1) result = Some(y)
    }
    result
  }
 
  def printFinds[A](xs: List[Pair[A, String]], x: A) =
    find(xs.elements, x) match {
      case Some(y) => println(y)
      case None => println("no match")
    }
 
  def main(args: Array[String]) {
    println("sum of leafs=" + sumLeaves(tree1));
    printFinds(List(Pair(3, "three"), Pair(4, "four")), 4)
  }
}