Re: 22.1.50; calc-math.el patch: fix non-termination

[Markus Triska]

Jay Belanger <address@hidden> writes:

> There was talk about adjusting `expt' to give a range error for large
> x; if that doesn't happen then something like Markus's patch (I'll
> tweak it to take care of the 0 case, for example) will be needed.

The 0 case cannot arise for math-largest-emacs-expt, and it is handled
for math-smallest-emacs-expt. I have now tested on another OS and
suggest the following improved patch, which should work in all cases:

2007-09-27  Markus Triska  <address@hidden>

        * calc/calc-math.el (math-largest-emacs-expt)
        (math-smallest-emacs-expt): more robust computation

diff --git a/lisp/calc/calc-math.el b/lisp/calc/calc-math.el
index 3e4743d..1cfc582 100644
--- a/lisp/calc/calc-math.el
+++ b/lisp/calc/calc-math.el
@@ -53,14 +53,17 @@
 ;;; is an Emacs float, for acceptable d.dddd....
 
 (defvar math-largest-emacs-expt
-  (let ((x 1))
+  (let ((x 1)
+        err)
     (while (condition-case nil
-               (expt 10.0 x)
-             (error nil))
+               (< (expt 10.0 x) (expt 10.0 (* 2 x)))
+             (error (setq err t)
+                    nil))
       (setq x (* 2 x)))
-    (setq x (/ x 2))
+    (unless err
+      (setq x (/ x 2)))
     (while (condition-case nil
-               (expt 10.0 x)
+               (< (expt 10.0 x) (expt 10.0 (1+ x)))
              (error nil))
       (setq x (1+ x)))
     (- x 2))
@@ -69,12 +72,12 @@
 (defvar math-smallest-emacs-expt
   (let ((x -1))
     (while (condition-case nil
-               (expt 10.0 x)
+               (> (expt 10.0 x) 0.0)
              (error nil))
       (setq x (* 2 x)))
     (setq x (/ x 2))
     (while (condition-case nil
-               (expt 10.0 x)
+               (> (expt 10.0 x) 0.0)
              (error nil))
       (setq x (1- x)))
     (+ x 2))