Difference between revisions of "A simple implementation of the Mersenne twister"
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Jump to navigationJump to search| Line 7: | Line 7: | ||
It is basically taken from the C version in wikipedia and gererates the same output.<br> | It is basically taken from the C version in wikipedia and gererates the same output.<br> | ||
Of course I added this one to wikipedia too. You don't have to create this class. it is fully static.<br> | Of course I added this one to wikipedia too. You don't have to create this class. it is fully static.<br> | ||
| − | Simply call Mer32. | + | Simply call Mer32.Random to obtain a random value. Cast to integer for signed output. |
| Line 15: | Line 15: | ||
interface | interface | ||
type | type | ||
| − | + | TMersenne32 = class sealed | |
strict private | strict private | ||
const | const | ||
| Line 36: | Line 36: | ||
implementation | implementation | ||
| − | class constructor | + | class constructor TMersenne32.Create; |
begin | begin | ||
initialize(12345678); | initialize(12345678); | ||
end; | end; | ||
| − | class procedure | + | class procedure TMersenne32.Initialize(const seed:dword);inline;static; |
var | var | ||
i:dword; | i:dword; | ||
| Line 51: | Line 51: | ||
end; | end; | ||
| − | class procedure | + | class procedure TMersenne32.Twist;inline;static; |
var | var | ||
i,x,xA:dword; | i,x,xA:dword; | ||
| Line 66: | Line 66: | ||
end; | end; | ||
| − | class function | + | class function TMersenne32.Random:dword;inline;static; |
var | var | ||
y:dword; | y:dword; | ||
Revision as of 13:42, 12 March 2017
When you look at the source code that implements Free Pascal's random it may look overwelmingly complex.
The algorithm itself, though, is very understandable. Here is simple 32 bit implementation according to MT19937.
Enjoy,
Thaddy.
It is basically taken from the C version in wikipedia and gererates the same output.
Of course I added this one to wikipedia too. You don't have to create this class. it is fully static.
Simply call Mer32.Random to obtain a random value. Cast to integer for signed output.
unit mersenne;
{$mode objfpc}
interface
type
TMersenne32 = 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;
mer32 = type TMersenne;
implementation
class constructor TMersenne32.Create;
begin
initialize(12345678);
end;
class procedure TMersenne32.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 TMersenne32.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 TMersenne32.Random: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.