/* 

  Some date utils in Picat.


  This Picat model was created by Hakan Kjellerstrand, hakank@gmail.com
  See also my Picat page: http://www.hakank.org/picat/

*/

module date_utils.


% import util.
% import cp.


main => go.


go =>
  
  
  nl.



%
% Day of week, Sakamoto's method
% http://en.wikipedia.org/wiki/Weekday_determination#Sakamoto.27s_Method
%
dow(Y, M, D) = R =>
  T = [0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4],
  if M < 3 then
     Y := Y - 1
  end,
  R = (Y + Y // 4 - Y // 100 + Y // 400 + T[M] + D) mod 7.

% max_days(Year,Month) = Days => 
%   if member(Month, [1,3,5,7,8,10,12]) then 
%     Days = 31
%   elseif member(Month,[4,6,9,11]) then
%     Days = 30
%   else
%     if leap_year(Year) then
%      Days = 29
%    else
%      Days = 28
%    end
%   end.

leap_year(Year) => 
  (Year mod 4 == 0, Year mod 100 != 0) 
  ; 
  Year mod 400 == 0.


%
% http://en.wikipedia.org/wiki/Julian_day
% gregorian2julian
%
g2j(Year,Month,Day) = JD =>
  A = floor((14-Month) / 12), % 1 for Jan or Feb, 0 for other months
  Y = Year + 4800 - A,
  M = Month + 12*A - 3, % 0 for Mars, 11 for Feb
  JD = Day + floor( (153*M + 2) / 5) + 365*Y + floor(Y/4) -
       floor(Y / 100) + floor(Y / 400) - 32045.


%
% julian2gregorian
%
j2g(JD) = Date =>
  Y=4716,
  V=3,
  J=1401,
  U=5,
  M=2,
  S=153,
  N=12,
  W=2,
  R=4,
  B=274277,
  P=1461,
  C= -38,
  F = JD + J + (((4 * JD + B) div 146097) * 3) div 4 + C,
  E = R * F + V,
  G = mod(E, P) div R,
  H = U * G + W,
  Day = (mod(H, S)) div U + 1,
  Month = mod(H div S + M, N) + 1,
  Year = (E div P) - Y + (N + M - Month) div N,
  Date = [Year,Month,Day].


% for the alternative approach
max_days_in_month(Year,Month) = Days => 
  if member(Month, [1,3,5,7,8,10,12]) then 
    Days = 31
  elseif member(Month,[4,6,9,11]) then
    Days = 30
  else
    if leap_year(Year) then
     Days = 29
   else
     Days = 28
   end
  end.