calc-math.el (math-largest-emacs-expt): Handle the cases when expt

doesn't give range errors.

1 files changed, 27 insertions, 11 deletions

diff --git a/lisp/calc/calc-math.el b/lisp/calc/calc-math.el
index 0b4c82d1292..1aab53524f8 100644
--- a/lisp/calc/calc-math.el
+++ b/lisp/calc/calc-math.el
@@ -53,17 +53,33 @@
 ;;; is an Emacs float, for acceptable d.dddd....
 
 (defvar math-largest-emacs-expt
-  (let ((x 1))
-    (while (condition-case nil
-               (expt 10.0 x)
-             (error nil))
-      (setq x (* 2 x)))
-    (setq x (/ x 2))
-    (while (condition-case nil
-               (expt 10.0 x)
-             (error nil))
-      (setq x (1+ x)))
-    (- x 2))
+  (let ((x 1)
+        (pow 1e2))
+    ;; The following loop is for efficiency; it should stop when 
+    ;; 10^(2x) is too large.  This could be indicated by a range 
+    ;; error when computing 10^(2x), an infinite value for 10^(2x), 
+    ;; or (!) a zero value for 10^(2x).
+    (while (and
+            pow
+            (< pow 1.0e+INF)
+            (> pow 0.0))
+      (setq x (* 2 x))
+      (setq pow (condition-case nil
+                    (expt 10.0 (* 2 x))
+                  (error nil))))
+    ;; The following loop should stop when 10^(x+1) is too large.
+    (setq pow (condition-case nil
+                    (expt 10.0 (1+ x))
+                  (error nil)))
+    (while (and
+            pow
+            (< pow 1.0e+INF)
+            (> pow 0.0))
+      (setq x (1+ x))
+      (setq pow (condition-case nil
+                    (expt 10.0 (1+ x))
+                  (error nil))))
+    (1- x))
   "The largest exponent which Calc will convert to an Emacs float.")
 
 (defvar math-smallest-emacs-expt