0.7.12.13:

[sbcl.git] / src / code / irrat.lisp

diff --git a/src/code/irrat.lisp b/src/code/irrat.lisp
index ec84564..bab7661 100644 (file)
--- a/src/code/irrat.lisp
+++ b/src/code/irrat.lisp
@@ -269,18 +269,54 @@
             (* base power)
             (exp (* power (log base)))))))))
 
+;;; FIXME: Maybe rename this so that it's clearer that it only works
+;;; on integers?
+(defun log2 (x)
+  (declare (type integer x))
+  ;; CMUCL comment:
+  ;;
+  ;;   Write x = 2^n*f where 1/2 < f <= 1.  Then log2(x) = n +
+  ;;   log2(f).  So we grab the top few bits of x and scale that
+  ;;   appropriately, take the log of it and add it to n.
+  ;;
+  ;; Motivated by an attempt to get LOG to work better on bignums.
+  (let ((n (integer-length x)))
+    (if (< n sb!vm:double-float-digits)
+       (log (coerce x 'double-float) 2.0d0)
+       (let ((f (ldb (byte sb!vm:double-float-digits
+                           (- n sb!vm:double-float-digits))
+                     x)))
+         (+ n (log (scale-float (coerce f 'double-float)
+                                (- sb!vm:double-float-digits))
+                   2.0d0))))))
+
 (defun log (number &optional (base nil base-p))
   #!+sb-doc
   "Return the logarithm of NUMBER in the base BASE, which defaults to e."
   (if base-p
-      (if (zerop base)
-         base                          ; ANSI spec
-         (/ (log number) (log base)))
+      (cond
+       ((zerop base) base) ; ANSI spec
+       ((and (typep number '(integer (0) *))
+             (typep base '(integer (0) *)))
+        (coerce (/ (log2 number) (log2 base)) 'single-float))
+       (t (/ (log number) (log base))))
       (number-dispatch ((number number))
-       (((foreach fixnum bignum ratio))
+       (((foreach fixnum bignum))
+        (if (minusp number)
+            (complex (log (- number)) (coerce pi 'single-float))
+            (coerce (/ (log2 number) (log (exp 1.0d0) 2.0d0)) 'single-float)))
+       ((ratio)
         (if (minusp number)
             (complex (log (- number)) (coerce pi 'single-float))
-            (coerce (%log (coerce number 'double-float)) 'single-float)))
+            (let ((numerator (numerator number))
+                  (denominator (denominator number)))
+              (if (= (integer-length numerator)
+                     (integer-length denominator))
+                  (coerce (%log1p (coerce (- number 1) 'double-float))
+                          'single-float)
+                  (coerce (/ (- (log2 numerator) (log2 denominator))
+                             (log (exp 1.0d0) 2.0d0))
+                          'single-float)))))
        (((foreach single-float double-float))
         ;; Is (log -0) -infinity (libm.a) or -infinity + i*pi (Kahan)?
         ;; Since this doesn't seem to be an implementation issue
@@ -436,7 +472,7 @@
                           (float-sign y pi))
                       (float-sign y (/ pi 2)))
                   (%atan2 y x))))
-       (number-dispatch ((y number) (x number))
+       (number-dispatch ((y real) (x real))
          ((double-float
            (foreach double-float single-float fixnum bignum ratio))
           (atan2 y (coerce x 'double-float)))