Re: [Gcl-devel] RE: GCL bugs (decode-float, rationalize, expt)

[Camm Maguire]

Greetings!  Just a quick note here to thank you for working on this!  

1) Would you like to be added to the developer list?

2) I could add si::floor et. al. to access the floor function in libm
   without problem if you'd like.

3) Will try to look into the printing precision discrepancies soon.

4) As you already realize, I'm most happy to put in a better
   rationalize if (eq (float (rationalize x)) x) 

Take care,

"Stavros Macrakis" <address@hidden> writes:

> Camm--
> 
> Here is code for rationalize which returns the simplest rational.  I
> have done some testing, but of course haven't been able to test against
> the new integer-decode-float (see comment in code).
> 
>       -s
> 
> (defun float-rationalize (x)
> ;;; Given x, returns n/d such that float(n/d)=x and d is minimal.
> ;;; Assuming integer-decode-float works as stated below, automatically
> ;;; takes into account the precision of the input float.
>   (cond ((= x 0) 0)
>         ((< x 0) (- (float-rationalize (- x))))
>         (t (multiple-value-bind
>                (mantissa exponent sign)
>                (integer-decode-float x)
> ;;; We assume that integer-decode-float reflects the internal
> ;;; representation exactly.  In particular, the number of digits of
> ;;; precision in mantissa is exactly the number of digits of
> ;;; precision in the floating-point number, and not reduced to lowest
> ;;; terms e.g. (integer-decode-float 1.0) => 2^52 -52 1, and NOT 1 0 1.
> ;;; Similarly for denormalized numbers: (integer-decode-float
> ;;; least-positive-double-float) should be 1 -1074 1, and not 2^52
> ;;; -1126 1.  The CL spec is not clear about all this.
>              (simplest-in-range
> ;;; A float represents a half-open interval, but using the closed
> ;;; interval is OK because the midpoint is always simpler than
> ;;; the endpoints.  Similarly, it never matters that the interval for
> ;;; exact powers of 2 (except least-positive-float) is assymmetric.
>               (/ (- (* mantissa 2) 1)
>                  (expt 2 (- 1 exponent)))
>               (/ (+ (* mantissa 2) 1)
>                  (expt 2 (- 1 exponent))) )))))
> 
> ;;; From Appendix C4 of the IEEE Scheme standard
> ;;; Alan Bawden's implementation of Hardy and Wright's algorithm.
> ;;; Converted to CL by Stavros Macrakis
> (defun simplest-in-range (bottom top)
>   ;; 0 < bottom < top
>   (multiple-value-bind (bottom-integer bottom-fraction)
>       (truncate bottom)
>     (if (= bottom-fraction 0)
>         bottom
>       (multiple-value-bind (top-integer top-fraction)
>           (truncate top)
>         (if (= bottom-integer top-integer)
>             (+ bottom-integer
>                (/ 1 (simplest-in-range
>                      (/ 1 top-fraction)
>                      (/ 1 bottom-fraction))))
>           (+ 1 bottom-integer))))))
> 
> 
> 

-- 
Camm Maguire                                            address@hidden
==========================================================================
"The earth is but one country, and mankind its citizens."  --  Baha'u'llah