-- Collision-resistant ids optimized for horizontal scaling and performance, for PL/PgSQL.
-- Based on https://github.com/ericelliott/cuid
-- Version 1.0.0

-- Usage: SELECT cuid();

-- BEGIN CONFIG ---

-- Put a unique host ID (int) here per server instance.
-- Once set, this value should not be changed.
SET plpgsql.cuid_hostid = "897892";

-- Create the sequence used to track ID counts. Used in _safeCounter()
CREATE SEQUENCE IF NOT EXISTS "cuid_seq";

-- END OF CONFIG ---

-- Get the next ID in counter. The sequence will reset after max_id is reached.
CREATE OR REPLACE FUNCTION _safeCounter() RETURNS VARCHAR AS $$
DECLARE
  seq_id bigint;
  max_id int := 1679616;
BEGIN
  SELECT nextval('cuid_seq') INTO seq_id;

  IF seq_id > max_id THEN
    SELECT setval('cuid_seq', 0) INTO seq_id;
  END IF;

  RETURN pad(base36enc(seq_id), 4);
END;
$$ LANGUAGE PLPGSQL;

-- Uses the backend PID and a unique host ID to generate a fingerprint.
-- Unfortunately getting the hostname of a Pg server is rather expensive so we opt for
-- a static unique host ID.
CREATE OR REPLACE FUNCTION _fingerprint() RETURNS VARCHAR AS $$
DECLARE
  padding int := 2;
  ret varchar;
BEGIN
  ret := pad(base36enc(pg_backend_pid()), padding);
  ret := ret || pad(base36enc(current_setting('plpgsql.cuid_hostid')::bigint), padding);
  RETURN ret;
END;
$$ LANGUAGE PLPGSQL;

-- Encodes a base-10 bigint to a base-36 string representation.
CREATE OR REPLACE FUNCTION base36enc(num bigint) RETURNS VARCHAR AS $$
DECLARE
  base36 char[];
  ret varchar;
  val bigint;
  tmp bigint;
BEGIN
  base36 := ARRAY[
    '0','1','2','3','4','5','6','7','8','9','a','b',
    'c','d','e','f','g','h','i','j','k','l','m','n',
    'o','p','q','r','s','t','u','v','w','x','y','z'
  ];

  IF num ISNULL THEN
    RETURN NULL;
  END IF;

  val := num;
  ret := '';
  LOOP
      tmp := val % 36 + 1;
      ret := base36[tmp] || ret;
      val := val / 36;
      IF val = 0 THEN
          exit;
      END IF;
  END LOOP;

  IF num < 0 THEN
      ret := '-' || ret;
  END IF;

  RETURN ret;
END;
$$ LANGUAGE PLPGSQL;

-- Adds zero-padding of a size to a num string. If the string is longer than size
-- it gets truncated to the left.
CREATE OR REPLACE FUNCTION pad(num varchar, size int) RETURNS VARCHAR AS $$
BEGIN
  RETURN lpad(right(num, size), size, '0');
END;
$$ LANGUAGE PLPGSQL;

-- Get value of current timestamp encoded as base-36
CREATE OR REPLACE FUNCTION _timestamp() RETURNS VARCHAR AS $$
DECLARE
  now_ts bigint;
BEGIN
  SELECT floor(extract(EPOCH FROM clock_timestamp()))
    INTO now_ts;

  RETURN base36enc(now_ts);
END;
$$ LANGUAGE PLPGSQL;

-- Get a random base-36 valid string of size block_size.
CREATE OR REPLACE FUNCTION _randomBlock() RETURNS VARCHAR AS $$
DECLARE
  base36 varchar := '0123456789abcdefghijklmnopqrstuvwxyz';
  block_size int := 4;
  val varchar;
BEGIN
  SELECT array_to_string(ARRAY(
    SELECT substring(base36 FROM (random() * 36)::int FOR 1)
      FROM generate_series(1, block_size)), '', '0')
    INTO val;

  RETURN val;
END;
$$ LANGUAGE PLPGSQL;

-- Generates a new Collision-resistant ID (CUID)
CREATE OR REPLACE FUNCTION cuid() RETURNS VARCHAR AS $$
DECLARE
  ret varchar;
BEGIN
  ret := 'c';
  ret := ret || _timestamp();
  ret := ret || _safeCounter();
  ret := ret || _fingerprint();
  ret := ret || _randomBlock();
  ret := ret || _randomBlock();
  RETURN ret;
END;
$$ LANGUAGE PLPGSQL;