Delphi compatible LCG Random
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Many Freepascal programmers also maintain sourcecode in Delphi.
Even if you have moved to Freepascal from Delphi you may have data that relies on Delphi's Random.
Here are cross-platform functions that generate Delphi-identical pseudo-random numbers given the same RandSeed:
unit lcg_random;
// Delphi compatible LCG random number generator routines for Freepascal.
// (c)2017, Thaddy de Koning. Use as you like
// Algorithm, Delphi multiplier and increment taken from:
// https://en.wikipedia.org/wiki/Linear_congruential_generator
// The default Delphi RandomSeed is determined as zero.
{$ifdef fpc}{$mode objfpc}{$endif}
interface
function LCGRandom: extended; overload;inline;
function LCGRandom(const range:longint):longint;overload;inline;
implementation
function IM:cardinal;inline;
begin
RandSeed := RandSeed * 134775813 + 1;
Result := RandSeed;
end;
function LCGRandom: extended; overload;inline;
begin
Result := IM * 2.32830643653870e-10;
end;
function LCGRandom(const range:longint):longint;overload;inline;
begin
Result := IM * range shr 32;
end;
end.
A small demo
program lcgdemo;
// Compile and run in both Delphi and FPC and compare the output.
// It is exactly the same
{$ifdef fpc}{$mode objfpc}
{$Macro on}{$define random := LCGRandom}
{$endif}
{$ifdef mswindows}{$apptype console}{$endif}
uses lcg_random;
var i:integer;
begin
RandSeed := 999; // default delphi randseed is zero
for i := 1 to 20 do
begin
write(Random(100):4); // Delphi: Random FPC:LCGRandom. See macro
if i mod 5 = 0 then writeln;
end;
RandSeed := 999; // default delphi randseed is zero
for i := 1 to 20 do
begin
write(Random:4:8,' ');// Delphi: Random FPC:LCGRandom
if i mod 5 = 0 then writeln;
end;
Readln;
end.