Update of /project/oct/cvsroot/oct In directory clnet:/tmp/cvs-serv8223

Modified Files: qd-methods.lisp qd-package.lisp Log Message: Add RATIONALIZE methods. Algorithm graciously provided by Bruno Haible.

qd-package.lisp: o Shadow RATIONALIZE

qd-methods.lisp: o Add RATIONALIZE methods for CL:REAL's and QD-REAL's..

--- /project/oct/cvsroot/oct/qd-methods.lisp 2007/09/19 17:30:04 1.59 +++ /project/oct/cvsroot/oct/qd-methods.lisp 2007/09/20 21:04:05 1.60 @@ -835,6 +835,102 @@ (cl:rational x2) (cl:rational x3))))

+(defmethod rationalize ((x cl:real)) + (cl:rationalize x)) + +;;; The algorithm here is the method described in CLISP. Bruno Haible has +;;; graciously given permission to use this algorithm. He says, "You can use +;;; it, if you present the following explanation of the algorithm." +;;; +;;; Algorithm (recursively presented): +;;; If x is a rational number, return x. +;;; If x = 0.0, return 0. +;;; If x < 0.0, return (- (rationalize (- x))). +;;; If x > 0.0: +;;; Call (integer-decode-float x). It returns a m,e,s=1 (mantissa, +;;; exponent, sign). +;;; If m = 0 or e >= 0: return x = m*2^e. +;;; Search a rational number between a = (m-1/2)*2^e and b = (m+1/2)*2^e +;;; with smallest possible numerator and denominator. +;;; Note 1: If m is a power of 2, we ought to take a = (m-1/4)*2^e. +;;; But in this case the result will be x itself anyway, regardless of +;;; the choice of a. Therefore we can simply ignore this case. +;;; Note 2: At first, we need to consider the closed interval [a,b]. +;;; but since a and b have the denominator 2^(|e|+1) whereas x itself +;;; has a denominator <= 2^|e|, we can restrict the seach to the open +;;; interval (a,b). +;;; So, for given a and b (0 < a < b) we are searching a rational number +;;; y with a <= y <= b. +;;; Recursive algorithm fraction_between(a,b): +;;; c := (ceiling a) +;;; if c < b +;;; then return c ; because a <= c < b, c integer +;;; else +;;; ; a is not integer (otherwise we would have had c = a < b) +;;; k := c-1 ; k = floor(a), k < a < b <= k+1 +;;; return y = k + 1/fraction_between(1/(b-k), 1/(a-k)) +;;; ; note 1 <= 1/(b-k) < 1/(a-k) +;;; +;;; You can see that we are actually computing a continued fraction expansion. +;;; +;;; Algorithm (iterative): +;;; If x is rational, return x. +;;; Call (integer-decode-float x). It returns a m,e,s (mantissa, +;;; exponent, sign). +;;; If m = 0 or e >= 0, return m*2^e*s. (This includes the case x = 0.0.) +;;; Create rational numbers a := (2*m-1)*2^(e-1) and b := (2*m+1)*2^(e-1) +;;; (positive and already in lowest terms because the denominator is a +;;; power of two and the numerator is odd). +;;; Start a continued fraction expansion +;;; p[-1] := 0, p[0] := 1, q[-1] := 1, q[0] := 0, i := 0. +;;; Loop +;;; c := (ceiling a) +;;; if c >= b +;;; then k := c-1, partial_quotient(k), (a,b) := (1/(b-k),1/(a-k)), +;;; goto Loop +;;; finally partial_quotient(c). +;;; Here partial_quotient(c) denotes the iteration +;;; i := i+1, p[i] := c*p[i-1]+p[i-2], q[i] := c*q[i-1]+q[i-2]. +;;; At the end, return s * (p[i]/q[i]). +;;; This rational number is already in lowest terms because +;;; p[i]*q[i-1]-p[i-1]*q[i] = (-1)^i. +;;; +(defmethod rationalize ((x qd-real)) + ;; This is a fairly straigtforward implementation of the iterative + ;; algorithm above. + (multiple-value-bind (frac expo sign) + (integer-decode-float x) + (cond ((or (zerop frac) (>= expo 0)) + (if (minusp sign) + (- (ash frac expo)) + (ash frac expo))) + (t + ;; expo < 0 and (2*m-1) and (2*m+1) are coprime to 2^(1-e), + ;; so build the fraction up immediately, without having to do + ;; a gcd. + (let ((a (/ (- (* 2 frac) 1) (ash 1 (- 1 expo)))) + (b (/ (+ (* 2 frac) 1) (ash 1 (- 1 expo)))) + (p0 0) + (q0 1) + (p1 1) + (q1 0)) + (do ((c (ceiling a) (ceiling a))) + ((< c b) + (let ((top (+ (* c p1) p0)) + (bot (+ (* c q1) q0))) + (/ (if (minusp sign) + (- top) + top) + bot))) + (let* ((k (- c 1)) + (p2 (+ (* k p1) p0)) + (q2 (+ (* k q1) q0))) + (psetf a (/ (- b k)) + b (/ (- a k))) + (setf p0 p1 + q0 q1 + p1 p2 + q1 q2))))))))  (define-compiler-macro + (&whole form &rest args) (declare (ignore form)) --- /project/oct/cvsroot/oct/qd-package.lisp 2007/09/18 12:46:36 1.40 +++ /project/oct/cvsroot/oct/qd-package.lisp 2007/09/20 21:04:05 1.41 @@ -168,6 +168,7 @@ #:decf #:float-digits #:rational + #:rationalize ) ;; Export types (:export #:qd-real @@ -237,6 +238,7 @@ #:decf #:float-digits #:rational + #:rationalize ) ;; Constants (:export #:+pi+