/*
 * MIT License
 *
 * Copyright (c) 2023 Fabio Lima
 * 
 *  Permission is hereby granted, free of charge, to any person obtaining a copy
 *  of this software and associated documentation files (the "Software"), to deal
 *  in the Software without restriction, including without limitation the rights
 *  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 *  copies of the Software, and to permit persons to whom the Software is
 *  furnished to do so, subject to the following conditions:
 * 
 *  The above copyright notice and this permission notice shall be included in
 *  all copies or substantial portions of the Software.
 * 
 *  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 *  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 *  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 *  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 *  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 *  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 *  THE SOFTWARE.
 */
 
/**
 * Returns a time-ordered UUID (UUIDv6).
 *
 * Referencies:
 * - https://github.com/uuid6/uuid6-ietf-draft
 * - https://github.com/ietf-wg-uuidrev/rfc4122bis
 *
 * MIT License.
 *
 * Tags: uuid guid uuid-generator guid-generator generator time order rfc4122 rfc-4122
 */
create or replace function uuid6() returns uuid as $$
declare
	v_time timestamp with time zone:= null;
	v_secs bigint := null;
	v_usec bigint := null;

	v_timestamp bigint := null;
	v_timestamp_hex varchar := null;

	v_clkseq_and_nodeid bigint := null;
	v_clkseq_and_nodeid_hex varchar := null;

	v_bytes bytea;

	c_epoch bigint := -12219292800; -- RFC-4122 epoch: '1582-10-15 00:00:00'
	c_variant bit(64):= x'8000000000000000'; -- RFC-4122 variant: b'10xx...'
begin

	-- Get seconds and micros
	v_time := clock_timestamp();
	v_secs := EXTRACT(EPOCH FROM v_time);
	v_usec := mod(EXTRACT(MICROSECONDS FROM v_time)::numeric, 10^6::numeric);

	-- Generate timestamp hexadecimal (and set version 6)
	v_timestamp := (((v_secs - c_epoch) * 10^6) + v_usec) * 10;
	v_timestamp_hex := lpad(to_hex(v_timestamp), 16, '0');
	v_timestamp_hex := substr(v_timestamp_hex, 2, 12) || '6' || substr(v_timestamp_hex, 14, 3);

	-- Generate clock sequence and node identifier hexadecimal (and set variant b'10xx')
	v_clkseq_and_nodeid := ((random()::numeric * 2^62::numeric)::bigint::bit(64) | c_variant)::bigint;
	v_clkseq_and_nodeid_hex := lpad(to_hex(v_clkseq_and_nodeid), 16, '0');

	-- Concat timestemp, clock sequence and node identifier hexadecimal
	v_bytes := decode(v_timestamp_hex || v_clkseq_and_nodeid_hex, 'hex');

	return encode(v_bytes, 'hex')::uuid;
	
end $$ language plpgsql;

-- EXAMPLE:
-- 
-- select uuid6() uuid, clock_timestamp()-statement_timestamp() time_taken;

-- EXAMPLE OUTPUT:
-- 
-- |uuid                                  |time_taken        |
-- |--------------------------------------|------------------|
-- |1ed58ca7-060a-62a0-aa64-951dd4e5bb8a  |00:00:00.000104   |

-------------------------------------------------------------------
-- FOR TEST: the expected result is an empty result set
-------------------------------------------------------------------
-- with t as (
--     select uuid6() as id from generate_series(1, 1000)
-- )
-- select * from t
-- where (id is null or id::text !~ '^[a-f0-9]{8}-[a-f0-9]{4}-6[a-f0-9]{3}-[89ab][a-f0-9]{3}-[a-f0-9]{12}$');