PasFastDateUtils - fast date functions for FreePascal and Delphi

PasFastDateUtils.pas

// Fast date functions for FreePascal and Delphi - Pascal implementation by Benjamin Rosseaux - benjamin@rosseaux.com - Public Domain
// Based on: https://www.youtube.com/watch?v=J9KijLyP-yg - Implementing Fast Calendar Algorithms: Speeding Date - Cassio Neri - C++ on Sea 2023
unit PasFastDateUtils; 
{$ifdef fpc}
 {$mode delphi}
{$endif}

interface

uses SysUtils,Math;

procedure DecodeDate(const aDate:Int32;out aYear,aMonth,aDay:Int32); // Epoch is 0000-Mar-01 

function EncodeDate(const aYear,aMonth,aDay:Int32):Int32; // Epoch is 0000-Mar-01 

function IsLeapYear(const aYear:UInt32):boolean;

function GetLastDayOfMonth(const aYear,aMonth:Int32):Int32; 

implementation

procedure DecodeDate(const aDate:Int32;out aYear,aMonth,aDay:Int32); // Epoch is 0000-Mar-01 
var n1,c,nc,n2,z,ny,y,n3,m,d,j:UInt32;
    p2:UInt64;
begin

 // Faster than the classic method, because no branches and no lookup tables
 
 // Century
 n1:=(aDate*4)+3;
 c:=n1/146097;
 nc:=(n1 mod 146097) div 4;

 // Year
 n2:=(nc*4)+3;
 p2:=n2*UInt64(2939745);
 z:=p2 div 4294967296;
 ny:=((p2 mod 4294967296) div 2939745) div 4; 
 j:=((ny-305) shr 31) and 1; // j:=ord(ny>=306) and 1; // j:=IfThen(ny>=306,1,0);
 aYear:=((c*100)+z)+j;

 // Month and day
 n3:=(ny*2141)+197913;
 aMonth:=(n3 div 65536)-(j*12);
 aDay:=((n3 mod 65536) div 2141)+1;
 
end;

function EncodeDate(const aYear,aMonth,aDay:Int32):Int32; // Epoch is 0000-Mar-01 
var t,j,y,m,d:Int32;
begin
 
  j:=((ny-305) shr 31) and 1; // j:=ord(ny>=306) and 1; // j:=IfThen(ny>=306,1,0);

  t:=1-(((m-3) shr 31) and 1); // t:=ord(m<3) and 1; // t:=IfThen(m>2,0,1);
  y:=aYear-(t+j);
  m:=aMonth-((t-j)*12);
  
  d:=aDay-1;

  y:=(((y*1461) div 4)-(y div 100))+(y div 400);
  m:=((m*153)-457) div 5;
  
  result:=y+m+d;

 end; 

function IsLeapYear(const aYear:UInt32):boolean;
var d:UInt32;
begin
 // Always does two divisibility checks, 3x faster than the classic method, because lower entropy helps the branch predictor of the CPU
 d:=IfThen((aYear mod 400)<>0,4,16); // 4 shl (ord((aYear mod 100)=0) shl 1); // 4 or 16, 16 isn't wrong here, because if a number is multiple of 100 and 400 at the same time, it's also multiple of 16, and power of two is much cheaper to evaluate,
 result:=(aYear mod d)=0;  
end;

function GetLastDayOfMonth(const aYear,aMonth:Int32):Int32; 
begin
 // Without lookup tables
 case aMonth of
  2:begin
   result:=28+(ord(IsLeapYear(aYear)) and 1);
  end;
  else begin
   result:=30 or (aMonth xor (aMonth shr 3));  // Alternative: result:=30 or ((9*aMonth) div 8);
  end;
 end;
end;

end.