How can i generate a long hash of a String?

Question

I have a java applciation in which I want to generate long ids for strings (in order to store those strings in neo4j). In order to avoid data duplication, I would like to generate an id for each string stored in a long integer, which should be unique for each string. How can I do that ?

Answer 1

This code will calculate pretty good hash:

String s = "some string";
long hash = UUID.nameUUIDFromBytes(s.getBytes()).getMostSignificantBits();

Answer 2

Why don't you have a look a the hashcode() function of String, and just adopt it to using long values instead?

Btw. if there was a way to create a unique ID for each String, then you would have found a compression algorithm that would be able to pack every String into 8 bytes (not possible by definition).

Answer 3

long has 64 bits. A String of length 9 has 72 bits. from pigeon hole principle - you cannot get a unique hashing for 9 chars long strings to a long.

If you still want a long hash: You can just take two standard [different!] hash functions for String->int, hash1() and hash2() and calculate: hash(s) = 2^32* hash1(s) + hash2(s)

Answer 4

A simple 64 bits hash can be implement by combining CRC32 with Adler32.

Example in Java:

package com.example;

import java.util.zip.Adler32;
import java.util.zip.CRC32;

public class MySimpleHash {

    /**
     * Calculate a 64 bits hash by combining CRC32 with Adler32.
     * 
     * @param bytes a byte array
     * @return a hash number
     */
    public static long getHash(byte[] bytes) {

        CRC32 crc32 = new CRC32();
        Adler32 adl32 = new Adler32();

        crc32.update(bytes);
        adl32.update(bytes);

        long crc = crc32.getValue();
        long adl = adl32.getValue();

        return (crc << 32) | adl;
    }

    public static void main(String[] args) {
        String string = "This is a test string";
        long hash = getHash(string.getBytes());
        System.out.println("output: " + hash);
    }
}

output: 7732385261082445741

Example in Python:

#!/usr/bin/python3

import zlib

def get_hash(bytes):
    return zlib.crc32(bytes) << 32 | zlib.adler32(bytes)

string = "This is a test string"
hash = get_hash(string.encode())
print("output:", hash)

output: 7732385261082445741

This Gist compares some hash methods: https://gist.github.com/fabiolimace/507eac3d35900050eeb9772e5b1871ba

Answer 5

There are many answers, try the following:

• http://stackoverflow.com/questions/415953/generate-md5-hash-in-java EDIT: removed, I've missed the long requirement. Mea culpa.
• http://en.wikipedia.org/wiki/Perfect_hash_function

Or, as suggested before, check out the sources.

PS. One more technique is to maintain a dictionary of strings: since you're unlikely to get 2⁶⁴ strings any time soon, you can have perfect mapping. Note though that that mapping may as well become a major bottleneck.