MongoDB: How can I add a new hash field directly from the console? - mongodb

Question

I have objects like:
{ "_id" : ObjectId( "4e00e83608146e71e6edba81" ),
....
"text" : "Text now exists in the database"}
and I can add hash fields through java using the com.mongodb.util.Hash.longHash method to create
{ "_id" : ObjectId( "4e00e83608146e71e6edba81" ),
....
"text" : "Text now exists in the database",
"tHash" : -4375633875013353634 }
But this is quite slow. I would like to be able to do something within the database like:
db.foo.find( {} ).forEach( function (x) {
x.tHash = someFunction(x.text); // create a long hash compatible with com.mongodb.util.Hash.longHash
db.foo.save(x);
});
Does anyone know how I can call this long hash within the Javascript function?

Answer 1

First define a nice hashCode function to use. JavaScript does not have a hashCode function by default on all objects so you will need to write one. Or just use this one:
var hashCode = function(s) {
if (s == null) return 0;
if (s.length == 0) return 1;
var hash = 0;
for (var i = 0; i < s.length; i++) {
hash = ((hash << 5) - hash) + s.charCodeAt(i);
hash = hash & hash; // Convert to 32bit integer
}
return hash;
};
Alternatively use another hash function like MD5 - there are scripts that can generate them for you.

Answer 2

I gave up trying to replicate the Mongo Java driver Hash.longHash method in Javascript, since JS treats everything as a float and doesn't handle the overflow like Java does. I found some examples of replicating the Java hashCode function in JS and so I did this:
longHash = function(s){
var hash = 0;
if (s.length == 0) return hash;
for (i = 0; i < s.length; i++) {
char = s.charCodeAt(i);
hash = ((hash<<5)-hash)+char;
hash = hash & hash; // Convert to 32bit integer
}
return NumberInt(hash);
};
db.foo.find( {} ).forEach( function (x) {
x.cHash = longHash(x.c);
db.foo.save(x);
});
which at least let me do a integer level hash code on the existing data. This will be enough to narrow down data for indexing.
Update: I just updated with by returning a NumberInt type instead. By default the hash was a Javascript number and was stored in Mongo as a Double taking much more space than necessary. The NumberInt is a 32-bit signed integer, and NumberLong is a 64-bit version.