suplib/src/Merger.ig


 Copyright 1996-2003 John D. Polstra.
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 * $Id: Merger.ig.html,v 1.3 2010-04-29 17:20:02 wagner Exp $ 

Merger is a generic interface defining an iterator for merging two ordered sequences.

GENERIC INTERFACE Merger(Elem);
Where Elem.T is a reference type and Elem contains

      PROCEDURE Compare(a, b: Elem.T): [-1..1];
Compare must be a total order. It may be declared with a parameter mode of either VALUE or READONLY, but not VAR.

TYPE
  T <: Public;

  Public = OBJECT METHODS
    next(VAR a, b: Elem.T): BOOLEAN RAISES ANY;

    compare(READONLY a, b: Elem.T): [-1..1];

    getA(): Elem.T RAISES ANY := NIL;
    getB(): Elem.T RAISES ANY := NIL;
  END;
A Merger.T(Elem), or merger, is an iterator which yields successive elements from a merge of two sorted input sequences. The merger gets elements from the two input sequences by calling its methods m.getA() and m.getB(). These methods are undefined by default, and must be appropriately overridden through subtyping. Each method must return the next element of its input sequence, or NIL if the sequence is exhausted. The elements so returned must be ordered as defined by m.compare(a, b).

The default implementation of m.compare(a, b) calls Elem.Compare(a, b). It may be overridden if desired.

The elements of the merged sequence are obtained by calls to m.next(a, b). This method returns FALSE when the merged sequence is exhausted; otherwise, it returns TRUE. The VAR parameters a and/or b are set to the next element of the merged sequence, depending on which input sequence it came from. If the elements from the two input sequences are equal according to m.compare(a, b), then both a and b are set. Otherwise, one of them is set to NIL.

The RAISES set for the next method is the union of the RAISES sets of getA and getB.

END Merger.