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yap-6.3/library/splay.yap
Vítor Santos Costa ce8a19d381 Add documentation
2015-11-18 15:06:25 +00:00

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Prolog

/*************************************************************************
* *
* 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 *
* *
*************************************************************************/
/**
* @file splay.yap
* @author Vijay Saraswat
* @date Wed Nov 18 01:12:49 2015
*
* @brief "Self-adjusting Binary Search Trees
*
*
*/
:- module(splay,[
splay_access/5,
splay_insert/4,
splay_del/3,
splay_init/1,
splay_join/3,
splay_split/5]).
/** @defgroup Splay_Trees Splay Trees
@ingroup library
@{
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.
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.)
*/
/*
@pred splay_access(- _Return_,+ _Key_,? _Val_,+ _Tree_,- _NewTree_)
v
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_.
*/
*/
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(_).