computus_gregorian.sql

-- Computus
-- Easter in the Gregorian calendar
-- Copied from https://docs.rs/computus/1.0.0/src/computus/lib.rs.html

CREATE OR REPLACE FUNCTION computus_gregorian(_year INTEGER) 
RETURNS DATE AS $$
DECLARE
 AA INTEGER;
 BB INTEGER;
 CC INTEGER;
 DD INTEGER;
 EE INTEGER;
 FF INTEGER;
 GG INTEGER;
 HH INTEGER;
 II INTEGER;
 KK INTEGER;
 LL INTEGER;
 MM INTEGER;
 MON INTEGER;
 DAY INTEGER;
BEGIN
  IF _year < 1583 OR _year > 9999 THEN
    RAISE EXCEPTION 'Out of range';
  END IF;

  AA := _year % 19;
  BB := _year / 100;
  CC := _year % 100;
  DD := BB / 4;
  EE := BB % 4;
  FF := (BB + 8) / 25;
  GG := (BB - FF + 1) / 3;
  HH := (19 * AA + BB - DD - GG + 15) % 30;
  II := CC / 4;
  KK := CC % 4;
  LL := (32 + 2 * EE + 2 * II - HH - KK) % 7;
  MM := (AA + 11 * HH + 22 * LL) / 451;
  MON := (HH + LL - 7 * MM +  114) / 31;
  DAY := (HH + LL - 7 * MM +  114) % 31 + 1;
  RETURN make_date(_year, MON, DAY);
END;
$$ LANGUAGE plpgsql IMMUTABLE STRICT;