--
-- Convert hex value to a decimal one.  It's possible to do this using
-- undocumented features of the bit type, such as:
--
--     "SELECT 'xff'::text::bit(8)::int;"
--
-- This function relies on that only to convert single hex digits, meaning
-- it handles abitrarily large numbers too.  The code is inspired by the hex
-- to decimal examples at http://postgres.cz and is not case sensitive.
--
-- Sample tests:
--
-- SELECT hex_to_dec('FF');
-- SELECT hex_to_dec('ffff');
-- SELECT hex_to_dec('FFff');
-- SELECT hex_to_dec('FFFFFFFFFFFFFFFF');
--
CREATE OR REPLACE FUNCTION hex_to_dec (text)
RETURNS numeric AS
$$
DECLARE 
    r numeric;
    i int;
    digit int;
BEGIN
    r := 0;
    FOR i in 1..length($1) LOOP 
        EXECUTE E'SELECT x\''||substring($1 from i for 1)|| E'\'::integer' INTO digit;
        r := r * 16 + digit;
        END LOOP;
    RETURN r;
END
;
$$ LANGUAGE plpgsql IMMUTABLE
;

--
-- Process the output from pg_current_xlog_location() or
-- pg_current_xlog_insert_location() and return a WAL Logical Serial Number
-- from that information.  That represents an always incrementing offset
-- within the WAL stream, proportional to how much data has been written
-- there.  The input will look like '2/13BDE690'.
--
-- Sample use:
--
-- SELECT wal_lsn(pg_current_xlog_location());
-- SELECT wal_lsn(pg_current_xlog_insert_location());
--
-- There's no error checking here.  If you input a hex string without a "/"
-- in it, the function will process it without complaint, returning a large
-- number as if that were the left hand side of a valid pair.
--
CREATE OR REPLACE FUNCTION wal_lsn (text)
RETURNS numeric AS $$
SELECT hex_to_dec(split_part($1,'/',1)) * 16 * 1024 * 1024 * 255
    + hex_to_dec(split_part($1,'/',2));
$$ language sql
;