Copyright (C) 1992, Digital Equipment Corporation                        
 All rights reserved.                                                     
 See the file COPYRIGHT for a full description.                           
 Last modified on Thu Jun 10 13:57:31 PDT 1993 by meehan                  
      modified on Mon Feb  1 00:52:05 PST 1993 by mhb                     
      modified on Tue Jun 16 13:08:35 PDT 1992 by muller                  
An arbitrary VBT is made into a multi by providing a set of methods for maintaining the logical structure. The methods are used for replacing, inserting, traversing, and performing other common operations on the children.

In a language with multiple inheritance, multis would simply inherit different methods from different parent-types. In Modula-3, however, we achieve this effect by creating an instance mc of type MultiClass.T, and attaching mc to a VBT v by way of v's property set. The object mc points back to v via the field mc.vbt.

Clients defining their own multis can make a VBT v ``into'' a multi by calling Be(v,mc) during the initialization of the VBT. They must call BeChild on each new child when it is inserted, and UnChild when a child of a multi is deleted. MultiFilter.Replace, MultiSplit.Replace, and MultiSplit.Insert all do this automatically, and MultiSplit.Insert calls BeChild.



        vbt: VBT.T;              (* READONLY *)
        <* LL = VBT.mu *>
        replace (ch, new: VBT.T);
        insert  (pred, new: VBT.T);
        move    (pred, ch: VBT.T);
        succ    (ch: VBT.T): VBT.T;
        pred    (ch: VBT.T): VBT.T;
        nth     (n: CARDINAL): VBT.T;
        index   (ch: VBT.T): CARDINAL;
\subsubsection{The MultiSplit methods}

The methods implement the behavior in the MultiSplit interface.

The method call mc.replace(ch,new) implements the operation

      MultiSplit.Replace(mc.vbt, ch, new)
and the call mc.replace(ch,NIL) implements
      MultiSplit.Delete(mc.vbt, ch)
Before calling the method, the generic code in the MultiSplit interface checks that ch is a multi-child of mc.vbt, and, if new is not NIL, calls BeChild(mc.vbt, new). After calling the method, the generic code calls UnChild(mc.vbt, ch), if ch was not NIL.

Similarly, the method call mc.insert(pred,new) implements the operation

      MultiSplit.Insert(mc.vbt, pred, new)
Before calling the method, the generic code in MultiSplit checks that pred is a multi-child of mc.vbt and calls BeChild(mc.vbt, new). If new is NIL, MultiSplit.Insert raises a runtime exception.

The default methods for replace and insert are both equal to NIL, so every multi-split needs to override these methods.

The method call mc.move(pred, ch) implements

      MultiSplit.Move(mc.vbt, pred, ch)
Before calling the method, the generic code in MultiSplit verifies that ch and pred are both multi-children of mc.vbt (or NIL, in the case of pred). The call to mc.move is avoided if pred=ch or mc.succ(pred)=ch.

The default move method for a MultiClass.T object mc is simply a call to mc.replace(ch, NIL) followed by a call to mc.insert(pred, ch).

This default method is naive on two fronts. One, it is not particularly efficient since the tree of VBTs is typically being manipulated twice. Two, and more importantly, some multi-splits will take action as part of the replace method (e.g., reallocating the screen layout of its children) that is not ``undone'' by the subsequent call to the insert method.

The method calls

all implement the corresponding operations in the MultiSplit interface. The default pred, nth and index methods are implemented by repeatedly calling the succ method. The default succ method finds the successor of ch for the MultiClass.T object mc by a depth-first walk of mc.vbt's descendants, starting after ch, and stopping at the first VBT w for which IsChild(mc.vbt, w) returns TRUE, or when all of mc.vbt's descendants have been visited, in which case, ch has no successor so NIL is returned. In practice, the default succ method seems to work nearly all of the time; however, there is often a more efficient way to implement a succ method for any particular multi-split.

\subsubsection{The MultiFilter methods}

  Split <: T;
  Filter <: Split;
The default methods for a Filter are the same as for a Split, except that the insert method has a default. Thus, you only need to override the replace method of a multi-filter.

The default method call mc.insert(pred, new) is

      mc.replace (mc.succ(pred), new)
Also, the move method is never run; the generic code in Split.Move ensures this.

\subsubsection{Procedures for creating multis}

PROCEDURE Be (v: VBT.T; mc: T);
<* LL.sup <= VBT.mu *>
Make v into a multi by storing mc on v's property set and setting mc.vbt to v.

PROCEDURE Resolve (v: VBT.T): T;
<* LL.sup < v *>
Return the multiclass of v, that is, the mc for which Be(v,mc) was previously called. Return NIL if there is no such mc.

PROCEDURE BeChild (v: VBT.T; ch: VBT.T);
<* LL.sup < ch *>
Make ch into one of v's children that is exposed to the client via the MultiSplit or MultiFilter interfaces. It is possible for ch to be a child of more than one multi, and it is possible that ch is not related to v in the VBT hierarchy.

PROCEDURE UnChild (v: VBT.T; ch: VBT.T);
<* LL.sup < ch *>
Unmark ch as one of v's children that is exposed to the client via the MultiSplit or MultiFilter interfaces.

<* LL.sup < ch *>
Return TRUE iff BeChild(v,ch) was previously invoked and UnChild(v,ch) has not been subsequently called.

PROCEDURE Parents (ch: VBT.T): RefList.T (* of VBT.T *);
<* LL.sup < ch *>
Return a list of VBTs for which IsChild(v,ch) is TRUE. The list may be NIL.

END MultiClass.