xlisponly/lsp/rational.lsp

   1 (in-package :xlisp)
   2 (export 'rationalize)
   3 (defun rationalize (val)        ; hopefully readable conversion
   4        (unless (typep val 'flonum)
   5                (if (typep val 'rational)
   6                    (return-from rationalize val)
   7                    (error "~s is invalid type" val)))
   8        (let ((fraction (abs (rem val 1.0))))
   9             (if (zerop fraction)
  10                 (round val)
  11                 (let ((limit (expt 10 (- (+ 7 (truncate (log fraction 10)))
  12                                           (max 0 (truncate (log (abs val) 10))))))
  13                       divisor)
  14                      (cond ((>= limit 10000)    ; allow primes 3 3 7 11 13
  15                             (setq limit (* 9009 (/ limit 10000))))
  16                            ((>= limit 1000)     ; allow primes 3 3 7 11
  17                             (setq limit (* 693 (/ limit 1000))))
  18                            ((>= limit 100)      ; allow primes 3 3 7
  19                             (setq limit (* 63 (/ limit 100)))))
  20                      (setq divisor (round (/ limit fraction)))
  21                       (if (floatp divisor)
  22                           (round val)   ; Doesn't fit
  23                           (/ (round (* val divisor)) divisor))))))