yap-6.3/library/splay.yap

/*************************************************************************
*									 *
*	 YAP Prolog 							 *
*									 *
*	Yap Prolog was developed at NCCUP - Universidade do Porto	 *
*									 *
* Copyright L.Damas, V.S.Costa and Universidade do Porto 1985-1997	 *
*									 *
**************************************************************************
*									 *
* File:		splay.yap						 *
* Last rev:	5/12/99							 *
* mods:									 *
* comments:	Vijay Saraswat's implementation of Splay trees		 *
*									 *
*************************************************************************/

/** @defgroup Splay_Trees Splay Trees
@ingroup YAPLibrary
@{

Splay trees are explained in the paper "Self-adjusting Binary Search
Trees", by D.D. Sleator and R.E. Tarjan, JACM, vol. 32, No.3, July 1985,
p. 668. They are designed to support fast insertions, deletions and
removals in binary search trees without the complexity of traditional
balanced trees. The key idea is to allow the tree to become
unbalanced. To make up for this, whenever we find a node, we move it up
to the top. We use code by Vijay Saraswat originally posted to the Prolog
mailing-list.



 @pred splay_access(- _Return_,+ _Key_,? _Val_,+ _Tree_,- _NewTree_) 


If item  _Key_ is in tree  _Tree_, return its  _Val_ and
unify  _Return_ with `true`. Otherwise unify  _Return_ with
`null`. The variable  _NewTree_ unifies with the new tree.

 
*/


/** @pred splay_del(+ _Item_,+ _Tree_,- _NewTree_) 


Delete item  _Key_ from tree  _Tree_, assuming that it is present
already. The variable  _Val_ unifies with a value for key  _Key_,
and the variable  _NewTree_ unifies with the new tree. The predicate
will fail if  _Key_ is not present.

 
*/
/** @pred splay_init(- _NewTree_) 


Initialize a new splay tree.

 
*/
/** @pred splay_insert(+ _Key_,? _Val_,+ _Tree_,- _NewTree_) 


Insert item  _Key_ in tree  _Tree_, assuming that it is not
there already. The variable  _Val_ unifies with a value for key
 _Key_, and the variable  _NewTree_ unifies with the new
tree. In our implementation,  _Key_ is not inserted if it is
already there: rather it is unified with the item already in the tree.

 
*/
/** @pred splay_join(+ _LeftTree_,+ _RighTree_,- _NewTree_) 


Combine trees  _LeftTree_ and  _RighTree_ into a single
tree _NewTree_ containing all items from both trees. This operation
assumes that all items in  _LeftTree_ are less than all those in
 _RighTree_ and destroys both  _LeftTree_ and  _RighTree_.

 
*/
/** @pred splay_split(+ _Key_,? _Val_,+ _Tree_,- _LeftTree_,- _RightTree_) 


Construct and return two trees  _LeftTree_ and  _RightTree_,
where  _LeftTree_ contains all items in  _Tree_ less than
 _Key_, and  _RightTree_ contains all items in  _Tree_
greater than  _Key_. This operations destroys  _Tree_.




 */
:- module(splay,[
	splay_access/5,
	splay_insert/4,
	splay_del/3,
	splay_init/1,
	splay_join/3,
	splay_split/5]).

% Date: Sun 22 Mar 87 03:40:22-EST
% >From: vijay <Vijay.Saraswat@C.CS.CMU.EDU>
% Subject: Splay trees in LP languages.
 
% There have hardly been any interesting programs in this Digest for a
% long while now. Here is something which may stir the slothful among
% you!  I present Prolog programs for implementing self-adjusting binary
% search trees, using splaying. These programs should be among the most
% efficient Prolog programs for maintaining binary search trees, with
% dynamic insertion and deletion.
 
% The algorithm is taken from: "Self-adjusting Binary Search Trees",
% D.D. Sleator and R.E. Tarjan, JACM, vol. 32, No.3, July 1985, p. 668.
% (See Tarjan's Turing Award lecture in this month's CACM for a more
% informal introduction).  
% -----------------------------------------
 
% The operations provided by the program are:
 
% 1. access(i,t):  (implemented by the call access(V, I, T, New))
%   "If item i is in tree t, return a pointer to its location;
%   otherwise return a pointer to the null node."
%   In our implementation, in the call access(V, I, T, New),
%   V is unifies with `null' if the item is not there, else
%   with  `true' if it is there, in which case I is also
%   unified with that item.
 
% 2. insert(i,t):  (implemented by the call insert(I, T, New))
%   "Insert item i in tree t, assuming that it is not there already."
%   (In our implementation, i is not inserted if it is already
%   there: rather it is unified with the item already in the tree.)
 
% 3. delete(i,t):  (implemented by the call del(I, T, New))
%   "Delete item i from tree t, assuming that it is present."
%   (In our implementation, the call fails if the item is not in
%   the tree.)
 
% 4. join(t1,t2):  (Implemented by the call join(T1, T2, New))
%    "Combine trees t1 and t2 into a single tree containing
%     all items from both trees, and return the resulting
%     tree. This operation assumes that all items in t1 are
%     less than all those in t2 and destroys both t1 and t2."
 
% 5. split(i,t): (implemented by the call split(I, T, Left, Right))
%    "Construct and return two trees t1 and t2, where t1
%     contains all items in t less than i, and t2 contains all
%     items in t greater than i. This operations destroys t."
 
% The basic workhorse is the routine bst(Op, Item, Tree, NewTree), which
% returns in NewTree a binary search tree obtained by searching for Item
% in< Tree and splaying. OP controls what must happen if Item is not
% found in the Tree.  If Op = access(V), then V is unified with null if
% the item is not found in the tree, and with true if it is; in the
% latter case Item is also unified with the item found in the tree. In
% the first case, splaying is done at the node at which the discovery
% was made that Item was not in the tree, and in the second case
% splaying is done at the node at which Item is found. If Op=insert,
% then Item is inserted in the tree if it is not found, and splaying is
% done at the new node; if the item is found, then splaying is done at
% the node at which it is found.
 
% A node is simply an n/3 structure: n(NodeValue, LeftSon, RightSon).
% NodeValue could be as simple as an integer, or it could be a (Key,
% Value) pair.


% A node is simply an n/3 structure: n(NodeValue, LeftSon, RightSon).
% NodeValue could be as simple as an integer, or it could be a (Key,
% Value) pair.
 
% Here are the top-level axioms. The algorithm for del/3 is the first
% algorithm mentioned in the JACM paper: namely, first access the
% element to be deleted, thus bringing it to the root, and then join its
% sons. (join/4 is discussed later.)
 
splay_access(V, Item, Val, Tree, NewTree):-
	bst(access(V), Item, Val, Tree, NewTree).
splay_insert(Item, Val,Tree, NewTree):-
	bst(insert, Item, Val, Tree, NewTree).
splay_del(Item, Tree, NewTree):-
	bst(access(true), Item, Val, Tree, n(Item, Val, Left, Right)),
	splay_join(Left, Right, NewTree).
splay_join(Left, Right, New):-
	join(L-L, Left, Right, New).
splay_split(Item, Val, Tree, Left, Right):-
	bst(access(true), Item, Val, Tree, n(Item, Val, Left, Right)).

% We now consider the definition of bst. We use the notion of
% `difference-bsts'. There are two types of difference-bsts, a left one
% and a right one. The left one is of the form T-L where T is a bst and
% L is the *right* son of the node with the largest value in T. The
% right one is of the form T-R where T is a binary search tree and R is
% the *left* son of the node with the smallest value in T. An empty bst
% is denoted by a variable. Hence L-L denotes the empty left (as well as
% right) difference bst.
 
% As discussed in the JACM paper, we start with a notion of a left
% fragment and a right fragment of the new bst to be constructed.
% Intially, the two fragments are empty.
 
bst(Op, Item, Val, Tree, NewTree):-
	bst(Op, Item, Val, L-L, Tree, R-R, NewTree).

% We now consider the base cases. The empty tree is a variable: hence it
% will unify with the atom null. (A non-empty tree is a n/3 structure,
% which will not unify with null). If Item was being *access*ed, then it
% was not found in the tree, and hence Null=null. On the other hand, if
% the Item is found at the root, then the call terminates, with the New
% Tree being set up appropriately.
 
% The base clauses are:
 
bst(access(Null), _Item, _, _L, null, _R, _Tree):- !, Null = null.
bst(access(true), Item, Val, Left-A, n(Item0, Val0, A, B), Right-B, n(Item, Val, Left, Right)) :- Item == Item0, !, Val = Val0.
bst(insert, Item, Val, Left-A, T, Right-B, n(Item0, Val, Left, Right)) :-
	(var(T) ; T = n(Item0, _Val0, A, B), Item == Item0), !, Item = Item0.
% We now consider the zig case, namely that we have reached a node such
% that the required Item is either to the left of the current node and
% the current node is a leaf, or the required item is the left son of
% the current node. Depending upon the Op, the appropriate action is
% taken:
bst(access(Null), Item, _, Left-L, n(X, VX, null, B), Right-B, n(X, VX, Left, Right)) :-
	Item @< X, !, Null = null.
bst(Op, Item, Val, Left, n(X, VX, n(Item, Val, A1, A2), B), R-n(X, VX, NR,B), New):-
	Item @< X, !,
	bst(Op, Item, Val, Left, n(Item, Val, A1, A2), R-NR, New).
% The recursive cases are straightforward:
% Zig-Zig:
bst(Op, Item, Val, Left, n(X, VX, n(Y, VY, Z, B), C), R-n(Y, VY, NR, n(X, VX, B, C)), New):-
	Item @< X, Item @< Y, !,
	bst(Op, Item, Val, Left, Z, R-NR, New).
% Zig-Zag:
bst(Op, Item, Val, L-n(Y, VY, A, NL), n(X, _VX, n(Y, VY, A, Z), C), R-n(X, _NX, NR, C), New):-
	Item @< X, Y @< Item,!,
	bst(Op, Item, Val, L-NL, Z, R-NR, New).
% The symmetric cases for the right sons of the current node
% are straightforward too:

% Zag
bst(access(Null), Item, _, Left-B, n(X, VX, B, null), Right-_R, n(X, VX, Left, Right)):-
	X @< Item, !, Null = null.  % end of the road.
bst(Op, Item, Val, L-n(X, VX, B, NL), n(X, VX, B, n(Item, Val, A1, A2)), Right, New):-
	X @< Item, !,
	bst(Op, Item, Val, L-NL, n(Item, Val, A1, A2), Right, New).
% Zag-Zag
bst(Op, Item, Val, L-n(Y, VY, n(X, VX, C, B), NL), n(X, VX, C, n(Y, VY, B, Z)), Right, New):-
	X @< Item, Y @<Item,!,
	bst(Op, Item, Val, L-NL, Z, Right, New).
% Zag-Zig
bst(Op, Item, Val, L-n(X, VX, A, NL), n(X, VX, A, n(Y, VY, Z, C)), R-n(Y, VY, NR, C), New):-
	X @< Item, Item @< Y,!,
	bst(Op, Item, Val, L-NL, Z, R-NR, New).

% We now consider the definition of join.  To join Left to Right, it is
% sufficient to splay at the rightmost vertex in Left, and make Right
% its Right son. To build NewTree, we initially start with an empty left
join(Left-A, n(X, VX, A, var), Right, n(X, VX, Left, Right)):-!.
join(Left-n(X, VX, A, B), n(X, VX, A, n(Y, VY, B, var)), Right,	n(Y, VY, Left, Right)):- !.
join(Left-n(Y, VY, n(X, VX, C, B), NL), n(X, VX, C, n(Y, VY, B, n(Z, VZ, A1, A2))), Right, New):-
	join(Left-NL, n(Z, VZ,A1, A2), Right, New).
 

splay_init(_).