boxtec
/
yacy_search_server
mirror of https://github.com/yacy/yacy_search_server
4
1
Fork 0
Code Issues Projects Releases Wiki Activity

added kelondroHashtable (not finished yet)

Browse Source 
git-svn-id: https://svn.berlios.de/svnroot/repos/yacy/trunk@321 6c8d7289-2bf4-0310-a012-ef5d649a1542
pull/1/head
orbiter 20 years ago
parent 56d28a16f0
commit 4afcf10158
1 changed files with 219 additions and 0 deletions
Show Stats Download Patch File Download Diff File

219
source/de/anomic/kelondro/kelondroHashtable.java
Unescape View File

@ -0,0 +1,219 @@
// kelondroArray.java
// ------------------
// part of the Kelondro Database
// (C) by Michael Peter Christen; mc@anomic.de
// first published on http://www.anomic.de
// Frankfurt, Germany, 2005
// last major change: 21.06.2005
//
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation; either version 2 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
//
// Using this software in any meaning (reading, learning, copying, compiling,
// running) means that you agree that the Author(s) is (are) not responsible
// for cost, loss of data or any harm that may be caused directly or indirectly
// by usage of this softare or this documentation. The usage of this software
// is on your own risk. The installation and usage (starting/running) of this
// software may allow other people or application to access your computer and
// any attached devices and is highly dependent on the configuration of the
// software which must be done by the user of the software; the author(s) is
// (are) also not responsible for proper configuration and usage of the
// software, even if provoked by documentation provided together with
// the software.
//
// Any changes to this file according to the GPL as documented in the file
// gpl.txt aside this file in the shipment you received can be done to the
// lines that follows this copyright notice here, but changes must not be
// done inside the copyright notive above. A re-distribution must contain
// the intact and unchanged copyright notice.
// Contributions and changes to the program code must be marked as such.
/*
we implement a hashtable based on folded binary trees
each hight in these binary trees represents one step of rehasing
the re-hashing is realised by extending the number of relevant bits in the given hash
We construct the binary tree as follows
- there exists no root node
- at height-1 are 2 nodes, and can be accessed by using only the least significant bit of the hash
- at height-2 are 4 nodes, addresses by (hash & 3) - mapping the 2 lsb of the hash
- at height-3 are 8 nodes, addresses by (hash & 7)
- .. and so on.
The number of nodes N(k) that are needed for a tree of height-k is
N(k) = 2**k + N(k-1) = 2**(k + 1) - 2 [where k > 0]
We fold this tree by putting all heights of the tree in a sequence
Computation of the position (the index) of a node:
given:
hash h, with k significant bits (representing a height-k): h|k
then the position of a node node(h,k) is
node(h,k) = N(k - 1) + h|k [where k > 0]
We use these nodes to sequentially store a hash h at position node(h, 1), and
if that fails on node(h, 2), node(h, 3) and so on.
This is highly inefficient for the most heights k = 1, ..., (?)
The 'inefficient-border' depends on the number of elements that we want to store.
We therefore introduce an offset o which is the number of bits that are not used
at the beginning of (re-)hashing. But even if these o re-hasing steps are not done,
all bits of the hash are relevant.
Now the number of nodes N(k) that are needed is computed by N(k,o):
N(k,o) = N(k) - N(o) = 2**(k + 1) - 2**(o + 1) [where k > o, o >= 0]
When o=0 then this is equivalent to N(k).
The node-formula must be adopted as well
node(h,k,o) = N(k - 1, o) + h|k [where k > o, o >= 0]
So if you set an offset o, this leads to a minimum number of nodes
at level k=o+1: node(0,o + 1,o) = N(o, o) = 0 (position of the first entry)
Computatiion of the maxlen 'maxk', the maximum height of the tree for a given
number of maximum entries 'maxsize' in the hashtable:
maxk shall be computed in such a way, that N(k,o) <= maxsize, for any o or k
This means paricualary, that
node(h,k,o) < maxsize
for h|k we must consider the worst case:
h|k (by maxk) = 2**k - 1
therefore
node(h,maxk,o) < maxsize
N(maxk - 1, o) + h|maxk < maxsize [where maxk > o, o >= 0]
2**maxk - 2**(o + 1) + 2**maxk - 1 < maxsize [where maxk > o, o >= 0]
2**maxk - 2**(o + 1) + 2**maxk < maxsize + 1 [where maxk > o, o >= 0]
2**maxk + 2**maxk < maxsize + 2**(o + 1) + 1 [where maxk > o, o >= 0]
2**(maxk+1) < maxsize + 2**(o + 1) + 1 [where maxk > o, o >= 0]
maxk < log2(maxsize + 2**(o + 1) + 1) [where maxk > o, o >= 0]
setting maxk to
maxk = log2(maxsize)
will make this relation true in any case, even if maxk = log2(maxsize) + 1
would also be correct in some cases
Now we can use the following formula to create the folded binary hash tree:
node(h,k,o) = 2**k - 2**(o + 1) + h|k
to compute the node index and
maxk = log2(maxsize)
to compute the upper limit of re-hashing
*/
package de.anomic.kelondro;
import java.io.File;
import java.io.IOException;
public class kelondroHashtable {
kelondroArray hashArray;
int offset;
int maxk;
int maxrehash;
public kelondroHashtable(File file, int[] columns, int offset, int maxsize, int maxrehash) throws IOException {
// this creates a new hashtable
// the key element is not part of the columns array
// this is unlike the kelondroTree, where the key is part of a row
// the offset is a number of bits that is omitted in the folded tree hierarchy
// a good number for offset is 8
// the maxsize number is the maximum number of elements in the hashtable
// this number is needed to omit grow of the table in case of re-hashing
// the maxsize is re-computed to a virtual folding height and will result in a tablesize
// less than the given maxsize. The actual maxsize can be retrieved by maxsize()
hashArray = new kelondroArray(file, extCol(columns), 6);
this.offset = offset;
this.maxk = kelondroMSetTools.log2a(maxsize); // equal to log2(maxsize) + 1
if (this.maxk >= kelondroMSetTools.log2a(maxsize + power2(offset + 1) + 1) - 1) this.maxk--;
hashArray.seti(0, this.offset);
hashArray.seti(1, this.maxk);
hashArray.seti(1, this.maxk);
}
public kelondroHashtable(File file) throws IOException{
// this opens a file with an existing hashtable
hashArray = new kelondroArray(file);
}
private int[] extCol(int[] columns) {
int[] newCol = new int[columns.length + 1];
newCol[0] = 4;
System.arraycopy(columns, 0, newCol, 1, columns.length);
return newCol;
}
public static int power2(int x) {
int p = 1;
while (x > 0) {p = p << 1; x--;}
return p;
}
/*
public synchronized byte[][] get(int key) throws IOException {
}
public synchronized byte[][] put(int key, byte[][] newrow) throws IOException {
}
*/
private class Hash {
int key;
int hash;
int depth;
public Hash(int key) {
this.key = key;
this.hash = key;
this.depth = offset + 1;
}
public int key() {
return key;
}
private int hash() {
return hash & (power2(depth) - 1); // apply mask
}
public int depth() {
return depth;
}
public void rehash() {
depth++;
if (depth > maxk) {
depth = offset + 1;
hash = (int) ((5 * (long) hash - 7) / 3 + 13);
}
}
public int node() {
// node(h,k,o) = 2**k - 2**(o + 1) + h|k
return power2(depth) - power2(offset + 1) + hash();
}
}
}
Write Preview
Loading…
Cancel
Save