57 lines
2.4 KiB
Java
57 lines
2.4 KiB
Java
/*
|
|
* Copyright (C) 2021 Gabriel Venberg
|
|
*
|
|
* 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 3 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, see <http://www.gnu.org/licenses/>.
|
|
*/
|
|
|
|
/**
|
|
* an implementation of polynomial hash code and MAD compression functions
|
|
* @author Gabriel Venberg
|
|
*/
|
|
public class Hashing {
|
|
|
|
/**
|
|
* hashes a given string (x0,x1,...,xn-1) by x0a^n-1+x1a^n-2+...+xn-2a+xn-1. using a given value of a
|
|
* @param value the string to hash
|
|
* @param a the value of a to use
|
|
* @return an int representing the resulting hash.
|
|
*/
|
|
public static int polynomialHashCode(String value, int a){
|
|
//due to twos compliment math, we dont actually need to use longs to detect rollover, and can just use a mask to ignore the sign bit!
|
|
//also, wasted 2 hours because I had set this to MIN_VALUE....
|
|
final int mask = Integer.MAX_VALUE;
|
|
int hash=0;
|
|
//using horners rule iteratively
|
|
for(int i=0; i<value.length(); i++){
|
|
hash = (hash * a) + (int) value.charAt(i);
|
|
}
|
|
// im *pretty sure* we only need to zero the sign bit once... will test
|
|
return hash&mask;
|
|
}
|
|
|
|
/**
|
|
* compresses a hash code with by multiply add divide method with given constants
|
|
* @param value the hash code to compress
|
|
* @param N the given value of N (should be the size of your bucket array if using to make hash table)
|
|
* @param p should be a prime number larger than N
|
|
* @param a random value between 1 and p-1
|
|
* @param b random value between 0 and p-1
|
|
* @return the compressed hash.
|
|
*/
|
|
public static int madCompression(int value, int N, int p, int a, int b) throws IllegalArgumentException{
|
|
if(0>b||b>p-1){throw new IllegalArgumentException("b needs to be between 0 and p-1, inclusive.");}
|
|
if(0>=a||a>p-1){throw new IllegalArgumentException("a needs to be betwwen 1 and p-1, inclusive.");}
|
|
return ((a*value+b)%p)%N;
|
|
}
|
|
}
|