The Damm algorithm produces a check digit for a particular integer. The Damm algorithm is better than all the other algorithms we have and simpler to implement to boot. It can detect every single-digit error or transposition error.
The main use is to define a Damm code, which is an integer whose last digit is the check digit. I wanted to be able to use this from within PostgreSQL to define my own primary keys with this nice built-in validity property.
Check out the source and run the following command:
$ sudo make install
You should see output like this:
/bin/sh /opt/local/lib/postgresql93/pgxs/src/makefiles/../../config/install-sh -c -d '/opt/local/share/postgresql93/extension'
/bin/sh /opt/local/lib/postgresql93/pgxs/src/makefiles/../../config/install-sh -c -d '/opt/local/share/postgresql93/extension'
/usr/bin/install -c -m 644 ./damm.control '/opt/local/share/postgresql93/extension/'
/usr/bin/install -c -m 644 ./damm--1.0.sql '/opt/local/share/postgresql93/extension/'
This extension is pure SQL with no C component, so it may be possible
to install unprivileged. The extension can be loaded with the WITH SCHEMA option, but it is not dynamically relocatable due to the
lookup table.
This library offers two "levels" of API. The high-level API is to use
nextdamm to produce the next Damm code from some underlying
sequence:
CREATE SEQUENCE thing_id_seq;
CREATE TABLE things (
id damm_code DEFAULT nextdamm('thing_id_seq') PRIMARY KEY,
…
);
This is not much different from using SERIAL, which is just
shorthand for bigint DEFAULT nextval('thing_id_seq') combined with
automatically creating the sequence. (If anyone knows how to create a
type in Postgres that could do that automatically, please let me know,
as I'd like to make the above code simpler.)
The low-level API is to use generate_damm to create a Damm code from
a number by synthesizing the check digit and appending it. It sounds
worse than it is. For instance, suppose you want to use a date with a
Damm code. Interactively, it's going to look something like this:
damm=# select generate_damm(to_char(current_timestamp, 'YYYYMMDD')::bigint);
generate_damm
---------------
201404265
(1 row)
The 5 is a check digit appended to the original value (20140426). This might be an appropriate way to produce user-visible IDs from dates.
If you don't want to use the damm_code type, you may want to use the
function valid_damm_code to validate a number.
damm=# select valid_damm_code(201404265);
valid_damm_code
-----------------
t
(1 row)
This software is made available under the terms of the MIT license. If you use the software and find it useful, please let me know, I'd love to hear from you.