PROGRAM ULAM

!$DOUBLE

PROCEDURE HAILSTONE(X,PRT%->COUNT)
   COUNT=1
   IF PRT% THEN PRINT(X,) END IF
   REPEAT
      IF X/2<>INT(X/2) THEN
          X=X*3+1
        ELSE
          X=X/2
      END IF
      IF PRT% THEN PRINT(X,) END IF
      COUNT=COUNT+1
   UNTIL X=1
   IF PRT% THEN PRINT END IF
END PROCEDURE

BEGIN
   HAILSTONE(27,TRUE->COUNT)
   PRINT("Sequence length for 27:";COUNT)
   MAX_COUNT=2
   NMAX=2
   FOR I=3 TO 100000 DO
      HAILSTONE(I,FALSE->COUNT)
      IF COUNT>MAX_COUNT THEN NMAX=I MAX_COUNT=COUNT END IF
   END FOR
   PRINT("Max. number is";NMAX;" with";MAX_COUNT;"elements")
END PROGRAM