Difference between revisions of "A simple implementation of the Mersenne twister"
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Revision as of 22:57, 11 March 2017
A simple implementation of the Mersenne twister
When you look at the sourcecode that implements Freepascal's random it may look overwelmingly complex.
The algorithm itself, though, is very understandable. Here is simple implementation according to MT19937.
Enjoy, Thaddy.
It is basically taken from the C version in wikipedia. Of course I added this one there too.
unit mersenne;
{$mode objfpc}
interface
type
TMersenne = class sealed
strict private
const
// Define MT19937 constants (32-bit RNG)
N = 624;M = 397;R = 31;A = $9908B0DF;F = 1812433253;
U = 11;S = 7;B = $9D2C5680;T = 15;C = $EFC60000;L = 18;
MASK_LOWER = (QWORD(1) << R) - 1;
MASK_UPPER = QWORD(1) << R;
class var mt:array[0..N-1] of dword;
class var index:word;
class procedure twist;inline;static;
public
class constructor create;
class procedure initialize(const seed:dword);inline;static;
class function Extractu32:dword;inline;static;
end;
implementation
class constructor TMersenne.Create;
begin
initialize(12345678);
end;
class procedure TMersenne.Initialize(const seed:dword);inline;static;
var
i:dword;
begin
mt[0] := seed;
for i := 1 to pred(N) do
mt[i] := F * (mt[i - 1] xor (mt[i - 1] >> 30)) + i;
index := N;
end;
class procedure TMersenne.Twist;inline;static;
var
i,x,xA:dword;
begin
for i := 0 to pred(N) do
begin
x := (mt[i] and MASK_UPPER) + (mt[(i + 1) mod N] and MASK_LOWER);
xA := x >> 1;
if ( x and $1 ) <> 0 then
xA := xA xor A;
mt[i] := mt[(i + M) mod N] xor xA;
end;
index := 0;
end;
class function TMersenne.ExtractU32:dword;inline;static;
var
y:dword;
i:integer;
begin
i := index;
if index >= N then
begin
Twist;
i := index;
end;
y := mt[i];
index := i + 1;
y := y xor (mt[i] >> U);
y := y xor (y << S) and B;
y := y xor (y << T) and C;
y := y xor(y >> L);
Result :=y;
end;
end.