Example of Skip List source code
01.01.2007
{*
Example of Skip List source code for Pascal:
Skip Lists are a probabilistic alternative to balanced trees, as
described in the June 1990 issue of CACM and were invented by
William Pugh in 1987.
This file contains source code to implement a dictionary using
skip lists and a test driver to test the routines.
A couple of comments about this implementation:
The routine randomLevel has been hard-coded to generate random
levels using p=0.25. It can be easily changed.
The insertion routine has been implemented so as to use the
dirty hack described in the CACM paper: if a random level is
generated that is more than the current maximum level, the
current maximum level plus one is used instead.
The code, as presented, allows duplicate entries which act in a FIFO
manner. To disallow duplicates, the insert procedure needs to be modified
as indicated in the insert procedure.
The library includes an adaptation of the Unix C random number generator,
since the standard Pascal random number generator is unsuitable.
An attempt has been made to make sure that no integer overflow occurs
in the random number generator. However, this could not be tested.
randomBits is defined to be the number of bits returned by a call to
random(). For most all machines with 32-bit integers, this is 31 bits
as currently set.
The routines defined in this file are:
init: defines terminator and initializes the random bit source
newList: returns a new, empty list
freeList(l): deallocates the list l
randomLevel: Returns a random level
insert(l,key,value): inserts the binding (key,value) into l. If
allowDuplicates is undefined, returns true if key was newly
inserted into the list, false if key already existed
delete(l,key): deletes any binding of key from the l. Returns
false if key was not defined.
search(l,key,value): Searches for key in l and returns true if found.
If found, the value associated with key is stored in value
*}
program test(input,output);
const
sampleSize = 65536;
(* The following constants are associated with the random number generator *)
differentRandoms = 1073741824; (* 2^30 *)
randomTableSize = 31; (* size of table used *)
(* The follow constants are associated with the routines to procedure
random levels *)
ChunkSize = 4; (* ChunkSize = 1/p *)
randomChucksInAWord = 15; (* = log(base ChunkSize) differentRandoms *)
MaxLevel = 16;
type
randomStuff = 0..maxint;
keyType= 0..maxint;
valueType = 0..maxint;
levelNum = 1..MaxLevel;
node = ^nodeRec;
list = ^listRec;
listRec = record
level : integer;
header : node;
end;
nodeRec = record
key: keyType;
value: valueType;
case levelNum of
1: (fwd1: array [1..1] of node;);
2: (fwd2: array [1..2] of node;);
3: (fwd3: array [1..3] of node;);
4: (fwd4: array [1..4] of node;);
5: (fwd5: array [1..5] of node;);
6: (fwd6: array [1..6] of node;);
7: (fwd7: array [1..7] of node;);
8: (fwd8: array [1..8] of node;);
9: (fwd9: array [1..9] of node;);
10: (fwd10: array [1..10] of node;);
11: (fwd11: array [1..11] of node;);
12: (fwd12: array [1..12] of node;);
13: (fwd13: array [1..13] of node;);
14: (fwd14: array [1..14] of node;);
15: (fwd15: array [1..15] of node;);
16: (fwd: array [1..16] of node;)
end;
var randomChunksLeft : integer;
randomBits : randomStuff;
l : list;
i,k : integer;
v : valueType;
keys : array [1..sampleSize] of keyType;
terminator:node;
head,tail : integer;
randomTable : array[1..31] of randomStuff;
function randomNumber: integer;
begin
randomTable[head] := (randomTable[head] + randomTable[tail])
mod differentRandoms;
head := (head mod randomTableSize) + 1;
tail := (tail mod randomTableSize) + 1;
randomNumber := randomTable[head];
end;
procedure initRandom;
begin
head := 4;
tail := 1;
randomTable[1] := 1005175229;
randomTable[2] := 143302914;
randomTable[3] := 1027100827;
randomTable[4] := 879468478;
randomTable[5] := 755253631;
randomTable[6] := 928858961;
randomTable[7] := 331648406;
randomTable[8] := 45248011;
randomTable[9] := 26209743;
randomTable[10] := 433832350;
randomTable[11] := 944843483;
randomTable[12] := 438263339;
randomTable[13] := 813528929;
randomTable[14] := 629457392;
randomTable[15] := 618906479;
randomTable[16] := 573714703;
randomTable[17] := 766270699;
randomTable[18] := 275680090;
randomTable[19] := 436578616;
randomTable[20] := 509842102;
randomTable[21] := 649659208;
randomTable[22] := 891701505;
randomTable[23] := 24441858;
randomTable[24] := 562763940;
randomTable[25] := 980071615;
randomTable[26] := 1011597961;
randomTable[27] := 643279273;
randomTable[28] := 241719451;
randomTable[29] := 157584038;
randomTable[30] := 1069844923;
randomTable[31] := 471560540;
end;
function randomLevel:integer;
var newLevel : integer;
shiftedBits : randomStuff;
done : boolean;
begin
newLevel := 0;
done := false;
repeat
shiftedBits := randomBits div ChunkSize;
done := (shiftedBits * ChunkSize) <> randomBits;
newLevel := newLevel + 1;
randomBits := shiftedBits;
randomChunksLeft := randomChunksLeft -1;
if randomChunksLeft = 0 then
begin
randomBits := randomNumber;
randomChunksLeft := randomChucksInAWord;
end;
until done;
if newLevel > MaxLevel then newLevel := MaxLevel;
randomLevel := newLevel;
end;
procedure init;
begin
new(terminator,1);
terminator^.key := (256*256*256*64-1)*2+1;
randomBits := randomNumber;
randomChunksLeft := randomChucksInAWord;
end;
function newList:list;
var l : list;
i : integer;
begin
new(l);
l^.level := 1;
new(l^.header,MaxLevel);
for i := 1 to MaxLevel do l^.header^.fwd[i] := terminator;
newList := l;
end;
procedure freeList(l:list);
var
k : levelNum;
done : boolean;
p : node;
begin
p := l^.header^.fwd[1];
while (p <> terminator) do begin
k := 1;
done := false;
while not done do begin
l^.header^.fwd[k] := p^.fwd[k];
k := k+1;
if k > l^.level then done := true
else done := l^.header^.fwd[k] <> p;
end;
case k-1 of
1 : dispose(p,1);
2 : dispose(p,2);
3 : dispose(p,3);
4 : dispose(p,4);
5 : dispose(p,5);
6 : dispose(p,6);
7 : dispose(p,7);
8 : dispose(p,8);
9 : dispose(p,9);
10 : dispose(p,10);
11 : dispose(p,11);
12 : dispose(p,12);
13 : dispose(p,13);
14 : dispose(p,14);
15 : dispose(p,15);
16 : dispose(p,16)
end;
p := l^.header^.fwd[1];
end;
dispose(l^.header,MaxLevel);
dispose(l);
end;
procedure insert(l:list;key:keyType;value:valueType);
var
k : levelNum;
update : array[1..MaxLevel] of node;
p,q : node;
begin
p := l^.header;
for k:= l^.level downto 1 do begin
q := p^.fwd[k];
while q^.key < key do begin
p := q;
q := p^.fwd[k];
end;
update[k] := p;
end;
(* The insert routine currently allows duplicate entries. To make
it update previous entries instead, put in the following code:
if q^.key == key then begin
q^.value = value;
end;
else
*)
begin
k := randomLevel;
if k>l^.level then begin
k := l^.level+1;
l^.level := k;
update[k] := l^.header;
end;
case k of
1 : new(q,1);
2 : new(q,2);
3 : new(q,3);
4 : new(q,4);
5 : new(q,5);
6 : new(q,6);
7 : new(q,7);
8 : new(q,8);
9 : new(q,9);
10 : new(q,10);
11 : new(q,11);
12 : new(q,12);
13 : new(q,13);
14 : new(q,14);
15 : new(q,15);
16 : new(q,16)
end;
q^.key := key;
q^.value := value;
for k := k downto 1 do begin
p := update[k];
q^.fwd[k] := p^.fwd[k];
p^.fwd[k] := q;
end;
end;
end;
procedure delete(l:list;key:keyType);
var
k : levelNum;
update : array[1..MaxLevel] of node;
p,q : node;
done : boolean;
begin
p := l^.header;
for k:= l^.level downto 1 do begin
q := p^.fwd[k];
while q^.key < key do begin
p := q;
q := p^.fwd[k];
end;
update[k] := p;
end;
if (q^.key = key) then begin
k := 1;
done := false;
while not done do begin
update[k]^.fwd[k] := q^.fwd[k];
k := k+1;
if k > l^.level then done := true
else done := update[k]^.fwd[k] <> q;
end;
case k-1 of
1 : dispose(q,1);
2 : dispose(q,2);
3 : dispose(q,3);
4 : dispose(q,4);
5 : dispose(q,5);
6 : dispose(q,6);
7 : dispose(q,7);
8 : dispose(q,8);
9 : dispose(q,9);
10 : dispose(q,10);
11 : dispose(q,11);
12 : dispose(q,12);
13 : dispose(q,13);
14 : dispose(q,14);
15 : dispose(q,15);
16 : dispose(q,16)
end;
end;
end;
function search(l : list; key : keyType; var value : valueType) : boolean;
var k : integer;
p,q : node;
begin
p := l^.header;
for k:= l^.level downto 1 do begin
q := p^.fwd[k];
while q^.key < key do begin
p := q;
q := p^.fwd[k];
end;
end;
if (q^.key <> key) then search := false
else begin
value := q^.value;
search := true;
end;
end;
begin
initRandom;
init;
l := newList;
for k := 1 to sampleSize do begin
keys[k]:=randomNumber;
insert(l,keys[k],keys[k]);
end;
for i := 1 to 4 do begin
for k := 1 to sampleSize do
if not search(l,keys[k],v) then writeln('not found');
for k := 1 to sampleSize do begin
delete(l,keys[k]);
keys[k] := randomNumber;
insert(l,keys[k],keys[k]);
end;
end;
freeList(l);
end.