Fibonacci (advanced) {5 different ways/procedures}

MODULE fibonacci;
FROM InOut IMPORT WriteString, WriteLn, WriteInt, ReadInt, Read;

VAR
      proc : PROCEDURE (INTEGER, VAR BOOLEAN): INTEGER;
      result, n : INTEGER;
      ok, again, repeat, ft : BOOLEAN;
      option : CHAR;



PROCEDURE fibonaccislow1(n: INTEGER;VAR ok: BOOLEAN): INTEGER;
CONST
     gr = 35;
VAR
   f: ARRAY[0..2] OF INTEGER;
   i: INTEGER;

BEGIN
     IF (0 < n) AND (n < gr) THEN
       ok := TRUE;
       f[0] := 0;
       f[1] := 1;
       f[2] := f[0] + f[1];
       IF n <= 2 THEN
         RETURN f[n]
       ELSE
           i := 2;
           REPEAT
                 f[0] := f[1];
                 f[1] := f[2];
                 ok := MAX(INTEGER) - f[2] > f[1];
                 IF ok THEN
                   f[2] := f[1] + f[0];
                   i := i+ 1
                 END
           UNTIL (i = n) OR NOT ok;
           IF i = n THEN
             ok := TRUE;
             RETURN f[2]
           END
       END
     ELSE ok := FALSE;
     RETURN 0
     END
END fibonaccislow1;

PROCEDURE fibonaccislow2(n: INTEGER;VAR ok : BOOLEAN): INTEGER;
VAR
   f0, f1, i : INTEGER;
BEGIN
     IF 0 <= n THEN
       ok := TRUE;
       f0 := 0;
       f1 := 1;
       i := 1;
       WHILE ok AND (i < n) DO
            ok := MAX(INTEGER) - f1 - f1 > f0;
            IF ok THEN
              f0 := f0 + f1;
              f1 := f0 + f1;
              i := i+ 2
            END
       END;
       IF ok THEN
         IF ODD(n) THEN
           RETURN f1
         ELSE
             RETURN f0
         END
       END
     ELSE
         ok := FALSE;
           RETURN 0
     END;
END fibonaccislow2;

PROCEDURE fibonaccirecs(n: INTEGER;VAR ok : BOOLEAN): INTEGER;
         PROCEDURE Fib(n: INTEGER): INTEGER;
         BEGIN
              IF n = 1 THEN
                RETURN n
              ELSE
                RETURN Fib(n-1) + Fib(n-2)
              END
         END Fib;
BEGIN
     IF n >= 0 THEN
       ok := TRUE;
       RETURN Fib(n)
     ELSE
        ok := FALSE;
        RETURN 0
     END
END fibonaccirecs;

PROCEDURE fibonaccirecf(n: INTEGER;VAR ok: BOOLEAN): INTEGER;
CONST
     dg = 0;
     gg = 20;

     PROCEDURE Fib(f1,f0,n: INTEGER): INTEGER;
     BEGIN
          IF n = 0 THEN
            RETURN f0
          ELSE
              RETURN Fib(f0+f1,f1,n-1)
          END
     END Fib;

BEGIN
     IF (dg <= n) AND (gg >= n) THEN
       ok := TRUE;
       RETURN Fib(1,0,n);
     ELSE
         ok := FALSE;
         RETURN 0
     END
END fibonaccirecf;

PROCEDURE fibonacciff(n: INTEGER;VAR ok: BOOLEAN): INTEGER;
VAR
   p1, p2, g1, g2, temp: INTEGER;
   chose: BOOLEAN;
BEGIN
     IF 0 <= n THEN
       ok := TRUE;
       p1 := 0;
       p2 := 1;
       g1 := 1;
       g2 := 2;
       chose := ODD(n);                         (*stack overflow*)
       n := n DIV 2;                            (*check later why!*)
       WHILE n > 0 DO
            IF ODD(n) THEN
              temp := p1*g1;
              p1 := p1*g2 + p2*g1 - temp;
              p2 := p2*g2 + temp
            END;
            n := n DIV 2;
            IF n > 0 THEN
              temp := g1*g1;
              g1 := 2*g1*g2 - temp;
              g2 := g2*g2 + temp
            END
       END;
       IF chose THEN
         RETURN p2
       ELSE
           RETURN p1
       END
     ELSE
         ok := FALSE
     END;
     RETURN 0
END fibonacciff;





BEGIN
     ft := TRUE;
     REPEAT
           repeat := FALSE;
           WriteString('Enter number, n: ');
           ReadInt(n); WriteLn;
           IF ft THEN
              WriteString('Select procedure, For  '); WriteLn;
              WriteString('iterative, slow.. select 1;');WriteLn;
              WriteString('iterative, fast.. select 2;');WriteLn;
              WriteString('recursive,slow.. select 3;');WriteLn;
              WriteString('recursive,fast.. select 4;');WriteLn;
              WriteString('complex, via the matrix.. select 5;');WriteLn;    (*stackoverflow with this one*)
              WriteString('waiting on input..: ');
              again := FALSE;
              REPEAT
                    Read(option); WriteLn;
                    IF again THEN
                      WriteString('Re-enter your chose, must be (nmb >=1 AND <= 5);');
                    END;
                    again := TRUE;
              UNTIL (option = '1') OR (option = '2') OR (option = '3') OR
              (option = '4') OR (option = '5'); WriteLn;
              CASE option OF
                  '1' : proc := fibonaccislow1|
                  '2' : proc := fibonaccislow2|
                  '3' : proc :=  fibonaccirecs|
                  '4' : proc :=  fibonaccirecf|
                  '5' : proc := fibonacciff
               ELSE
                   WriteString('subtle eR!')
               END;
               ft := FALSE;
           END;
           result := proc(n,ok);
           IF ok THEN
             WriteString('The Resutl is: ');
             WriteInt(result,0);
           ELSE
               WriteString('You have to re-enter number n!');
               repeat := TRUE;
           END;
      UNTIL repeat = FALSE;

END fibonacci.