Lab109

Lab109/src/Hashing.java

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+/*
+ * 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;
+    }
+}