Checksum/Hash Functions

Does PostgreSQL have any built-in check sum or hash functions.

I want to be able to compute a checksum of a string as I display a record to
the user and when the user updates the record compare the checksum in the
update statement.  I can do the checksum on the client side but it would
save the overhead of an additional select statement at update time if the
checksum could be computed on the database backend.

I know I could write my own function to do this but I first wanted to check
if there was a function already setup to do it.  I have looked at the
documentation but did not find anything.

---------
Bryan White
This email represents the consensus opinion
of the many voices in my head.