## cit_util/src/OpenArrayTable.ig

$Id: OpenArrayTable.ig,v 1.2 2001-09-19 14:07:43 wagner Exp$

Copyright 1992 Digital Equipment Corporation. Distributed only by permission. See the file COPYRIGHT for a full description. Last modified on Thu Sep 22 11:05:54 PDT 1994 by heydon modified on Mon Nov 8 15:14:32 PST 1993 by mcjones modified on Sun Jan 19 18:07:07 PST 1992 by gnelson

Table is a generic interface defining partial maps that support update and iteration. \index{map!updatable} This version supports only open arrays as the key.

GENERIC INTERFACE OpenArrayTable(Key, Value);

Where Key.T is an open array type, Value.T is not an open array type, both Key and Value contain

      CONST Brand = <text-constant>;

and Key additionally contains

      PROCEDURE Equal(k1, k2: Key.T): BOOLEAN;
PROCEDURE Hash(k: Key.T): Word.T;
PROCEDURE Clone(k : Key.T) : REF Key.T;

Brand must be a text constant. It will be used to construct a brand for the opaque type Table.Default and any generic types instantiated with the Table interface. For a non-generic interface, we recommend choosing the name of the interface.

Equal must be an equivalence relation and Hash must respect that equivalence relation, in other words, if Equal(k1, k2), then Hash(k1)=Hash(k2).

Hash and Equal may be declared with a parameter mode of either VALUE or READONLY, but not VAR.

Clone must return a reference to an object of the same size as k. This object must not be referenced later.


IMPORT Word;

CONST
Brand = "(Table " & Key.Brand & " " & Value.Brand & ")";
DefaultBrand = "(Default " & Brand & ")";
(* A "Table.Default" is revealed to have the brand "DefaultBrand". *)

TYPE
T = OBJECT METHODS
get(READONLY k: Key.T; VAR v: Value.T): BOOLEAN;
delete(READONLY k: Key.T; VAR v: Value.T): BOOLEAN;
size(): CARDINAL;
iterate(): Iterator
END;
Iterator = OBJECT METHODS
next(VAR k: Key.T; VAR v: Value.T): BOOLEAN
END;
Default <: T OBJECT METHODS
init(sizeHint: CARDINAL := 0): Default;
END;
END OpenArrayTable.

A Table(Key, Value).T, or table, is a partial map from Key.Ts to Value.Ts. Actually, it turns out to be useful for a table to treat two different keys as if they are the same whenever they are equivalent according to some specified equivalence relation. For example, if you are creating a table with a Key.T of TEXT, you are likely to want Text.Equal as the equivalence relation.

Formally, a table tbl has the components:

      canon(tbl) a map on elements of Key.T
map(tbl)   a map from elements of Key.T to elements of Value.T

canon(tbl) represents an equivalence relation: canon(tbl)(k) is the {\em canonical representative} of all the keys that are equivalent to k. The domain of map(tbl) includes only canonical representatives, that is, elements in the range of canon(tbl). The equivalence relation underlying canon(tbl) must be time-invariant. For example, it can't depend on the values of particular references since some garbage collectors move REF values.

The methods of an object tbl of type Table.T have the following specifications:

The call tbl.get(k, v) sets v to map(tbl)(canon(tbl)(k)) and returns TRUE if canon(tbl)(k) is in dom(map(tbl)). Otherwise, it returns FALSE without changing v.

The call tbl.put(k, v) changes map(tbl)(canon(tbl)(k)) to v and returns TRUE if canon(k) is in dom(map(tbl)). Otherwise, it sets the value of map(tbl)(canon(tbl)(k)) to v, and returns FALSE.

The call tbl.delete(k, v) sets v to map(tbl)(canon(tbl)(k)), removes (canon(tbl)(k), v) from map(tbl), and returns TRUE if canon(tbl)(k) is in dom(map(tbl)). Otherwise, it returns FALSE without changing v.

The call tbl.size() returns the size of dom(map(tbl)), that is, the number of entries in tbl.

The call tbl.iterate() returns an iterator, which is an object that can be used to iterate over the key-value pairs in tbl. See the definition of the type Iterator below.

If i is the result of the call tbl.iterate(), then the call it.next(k, v) selects an entry from tbl that has not already been returned by i, sets k and v to its key and value, and returns TRUE. If no entries remain, the call returns FALSE without setting k or v. It is a checked runtime error to call next after it has returned FALSE. The client must ensure that while an iterator is in use, the parent table is not modified.

The type Default is an implementation of T using chained hashing. The methods specific to an object dflt of type Default have the following specifications:

The call dflt.init(sizeHint) returns dflt after initializing it to a table with an empty map(dflt). If sizeHint is greater than 0, init assumes that put will subsequently be called with at least sizeHint different keys; these calls on put may execute somewhat faster than if sizeHint was 0. The init method has side-effects on the table.

The call dflt.keyEqual(k1, k2) returns Key.Equal(k1, k2) and the call dflt.keyHash(k) returns Key.Hash(k). The other methods call keyEqual and keyHash whenever they need to consult the table's equivalence relation. This means a subtype of Default can determine the equivalence relation by overriding keyEqual and keyHash, providing keyEqual implements an equivalence relation and keyHash respects that relation.

For efficiency, tables and their iterators are not monitored, so a client accessing a table from multiple threads must ensure that if two operations are active concurrently, then neither of them has side effects on the same table or iterator. The T.put, T.delete, and Default.init methods are the only ones with side effects on the table. An iterator's next method has side-effects on the iterator.