Psuedo-Ramdom Number Generator

17 Dec 2003


      Don't know if its proper to post some trivial code snippets on the 
list. I felt some one just might find is useful.

Vipul

{*********************************************************************

                 LFSR-based Psuedo Random Number Generator

 LFSR is a random number generator based on a Linear Feedback Shift 
 Register The _tap_ sequence is 31, 6, 4, 2, 1 and 0. The primitive
 polynomial  mod  2 formed  from the tap sequence is x^32 + x^7 + x^5
 + x^3 + x^2 + x +  1.  The function  LFSRRandom  generates  a 16-bit
 unsigned  word.  Function  SetInit defines the initial parameters to
 run the generator with. It should be noted that  a particular set of
 Initial parameters will always generate  the  same set of 
 psuedo-random numbers.

 Vipul Ved Prakash <vipul@pobox.com>
 25th Feb '95 
**********************************************************************}

uses crt;

var lShiftReg : longint;
    wLastRandom : word;


procedure LFSRRandom(var wGeneratedRandom : word);
var
   i : integer;
   wNewRandom : word;
   lFnResult : longint;
begin
     wNewRandom := 0;
     for i := random(3) to random(3) + 13 do
     begin
          lFnResult := ((lShiftReg shr 31) xor (lShiftReg shr 06) xor
          (lShiftReg shr 04) xor (lShiftReg shr 02) xor (lShiftReg shr 01) xor
                        (lShiftReg)) and $1;
          lShiftReg := (lFnResult shl 31) or (lShiftReg shr 1);
          wNewRandom := wNewRandom or (lFnResult shl i)
     end;
     wGeneratedRandom := wLastRandom xor wNewRandom;
     wLastRandom := wNewRandom
end;

procedure paramError;
begin
     writeln;
     writeln;
     writeln(#07,'Syntax : RGen <random number file> <number of
     randoms to be generated> '); writeln; halt;
end;

procedure setInit(x, y : longint; z : integer);
begin
     if x <> 0 then lShiftReg :=  x else lShiftReg := 1; { Anything
     but 0 } wLastRandom := y; randseed := z;
end;


var
   i : longint;
   ch : char;
   wGeneratedRandom : word;
   ranfile : text;
   S : string;
   number : longint;
   code : integer;
   dummy : integer;
   count : integer;
begin
     count := 0;
     clrscr;
     setInit(4,5,10);
     if (paramcount < 2) then paramerror else assign(ranfile,
     paramstr(1)); rewrite(ranfile); val(paramstr(2), number, code);
     if code <> 0 then paramerror; for i := 1 to number do begin
          count := count + 1;
          LFSRRandom(wGeneratedRandom);
          str(wGeneratedRandom, S);
          writeln(ranfile, S);
          if count > 100 then
          begin
               count := 0;
               write('.'); {Write a dot after every 100 numbers}
          end;
     end;
     close(ranfile);
     writeln;
     writeln('Done. Press a key to exit...');
     ch := readkey;
     writeln;
end.
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   '88888888:::888888%'   Vipul Ved Prakash        Fax : +91-11-3328849
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