Javascript Implementation of Jenkins' Hash

Posted 28 Dec 2010 to javascript, hashing, memcache and has Comments

Libmemcached can use a cluster of servers as a backend. Each key is stored/read on only one server. The client determines which server to store/read each key based on a hash of the key. The hash value is a 32-bit integer whose modulus with the number of servers results in an integer in the range (0, number of servers). This is a pretty simple, but thoroughly useful, way to (roughly and likely mostly) evenly bucketize strings.

My needs required getting the same hash value in Javascript for a node.js project I’m working on. There were a few questionable attempts at some of the Jenkins hash functions, but all looked somewhat dubious. I decided to implement the simplest, the “one-at-a-time” hash function, which seemed to be sufficient for my hashing needs (and which is the default hash function in libmemcached).

So here’s the function. It accepts a string key and the interval size to use as a modulus for the integer hash value.

function jenkins_hash(key, interval_size) {
   var hash = 0;
   for (var i=0; i<key.length; ++i) {
      hash += key.charCodeAt(i);
      hash += (hash << 10);
      hash ^= (hash >> 6);
   hash += (hash << 3);
   hash ^= (hash >> 11);
   hash += (hash << 15);
   // make unsigned and modulo interval_size
   return (hash >>> 0) % interval_size;