Random-looking id generation in Postgres

pg-feistel-ids.sql

CREATE OR REPLACE FUNCTION base32_encode(n bigint, min_width int DEFAULT 8) RETURNS text AS $$
DECLARE
  alphabet CONSTANT text := '0123456789abcdefghjkmnpqrstvwxyz';
  output text := '';
BEGIN
  IF n < 0 THEN
    RAISE EXCEPTION 'base32_encode called with negative value';
  END IF;

  WHILE n != 0 LOOP
    output := substr(alphabet, 1 + (n % 32)::int, 1) || output;
    n := n / 32;
  END LOOP;

  IF char_length(output) < min_width THEN
    output := lpad(output, min_width, '0');
  END IF;

  RETURN output;
END;
$$ LANGUAGE plpgsql IMMUTABLE STRICT;


CREATE OR REPLACE FUNCTION feistel_cipher(value bigint, key int DEFAULT 1065940451) returns bigint AS $$
DECLARE
  -- Constants that control the size of the input and output.
  num_bits CONSTANT int := 38;
  max_piece_val CONSTANT bigint := power(2, num_bits / 2)::bigint - 1;

  -- Constants for the round function (a linear congruential generator)
  round_f_inc CONSTANT bigint := key % max_piece_val;
  round_f_mul CONSTANT bigint := (power(2, 31) - key)::bigint % max_piece_val;

  l1 bigint;
  l2 bigint;
  r1 bigint;
  r2 bigint;
  round int := 0;
BEGIN
  -- Pull out the n/2 most significant bits by shifting and masking
  l1 := (value >> (num_bits / 2)) & max_piece_val;

  -- Pull out the n/2 least significant bits by masking
  r1 := value & max_piece_val;

  WHILE round < 3 LOOP
    l2 := r1;
    r2 := l1 # ((round_f_mul * r1 + round_f_inc) % max_piece_val)::bigint;
    l1 := l2;
    r1 := r2;
    round := round + 1;
  END LOOP;

  RETURN ((l1::bigint << (num_bits / 2)) + r1);
END;
$$ LANGUAGE plpgsql STRICT IMMUTABLE;

CREATE OR REPLACE FUNCTION generate_id(seqname text) RETURNS text AS $$
DECLARE
  seqval bigint;
BEGIN
  SELECT nextval(seqname) INTO seqval;
  RETURN base32_encode(feistel_cipher(seqval, hashtext(seqname)));
END;
$$ LANGUAGE plpgsql STRICT IMMUTABLE;