suplib/src/Merger.ig

 Copyright 1996-2003 John D. Polstra.
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 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.