Middle square method (PRNG)

{
    middle_square_method.pas computes pseudorandom numbers starting with a number
    given at the command line. (Outdated method, for educational purposes only.
    Try 9319 or 12314.) To compile run "fpc middle_square_method.pas".
    Copyright (C) <February 29, 2016> Henning Polzer,
    send comments and error reports to: h underscore polzer at gmx dot de.

    This program is free software; you can redistribute it and/or
    modify it under the terms of the GNU General Public License
    as published by the Free Software Foundation; either version 2
    of the License, or (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
}

PROGRAM quadratmittengenerator;
CONST obergrenze = 50; { als sichere Abbruchbedingung }
TYPE Tgz = cardinal;    { cardinal von 0 bis 2^32-1, Standard: integer }

VAR fehlercode: integer;    { muss als integer vereinbart werden }
    erg, i, z: Tgz;
    verlassen: boolean;


FUNCTION qmg (zahl: Tgz): Tgz;
VAR diff, links, quadrat: Tgz;

  FUNCTION zp (exp: Tgz): Tgz; { Zehnerpotenz berechnen }
  VAR h, i: Tgz;

  BEGIN
    h := 1;
    FOR i := 1 TO exp DO
      h := h * 10;
    zp := h
  END; { zp }

  FUNCTION stellen (wert: Tgz): Tgz; { Stellenzahl ermitteln }
  BEGIN
    IF z <> 0 THEN stellen := trunc ((abs (ln (wert) / ln (10)))) + 1 ELSE stellen := 1
  END; { stellen }

BEGIN { qmg }
  quadrat := zahl * zahl;
  diff := stellen (quadrat) - stellen (zahl);
  links := trunc (diff / 2);
  quadrat := trunc (quadrat / zp (diff - links));
  IF zahl < 4 THEN qmg := 0 ELSE
    qmg := quadrat MOD zp (stellen (quadrat) - links)
END; { qmg }


BEGIN { Hauptprogramm }
  val (paramstr (1), z, fehlercode);                { Standard: read (z) }
  IF (paramcount = 1) AND (fehlercode = 0) AND (z > 0) THEN { genau eine Zahl > 0 eingeben }
  BEGIN
    i := 1;
    verlassen := false;
    REPEAT
      erg := qmg (z);
      verlassen := erg = z;
      z := erg;
      write (erg, ' ');
      i := i + 1 { Obergrenze sichere Abbruchbedingung }
    UNTIL verlassen OR (erg = 0) OR (i > obergrenze); { Groesse willkuerlich gesetzt }
    writeln
  END ELSE writeln ('Aufruf: qmg eine_zahl_groesser_null')
END.