[Git][cmucl/cmucl][master] Add special case for (expt 0 power)

[Raymond Toy]

Raymond Toy pushed to branch master at cmucl / cmucl


Commits:
f3b73541 by Raymond Toy at 2016-01-01T09:30:18Z
Add special case for (expt 0 power)

We know the result of (expt 0 power) so return it immediately without
first checking if the power exceeds the limit.

Also took the opportunity to add a better message to the
intexp-limit-error condition to make it more explicit what is being
computed and why it's failing.

Tests added too.

- - - - -


2 changed files:

- src/code/irrat.lisp
- tests/irrat.lisp


Changes:

=====================================
src/code/irrat.lisp
=====================================
--- a/src/code/irrat.lisp
+++ b/src/code/irrat.lisp
@@ -252,6 +252,13 @@
     (return-from intexp base))
   (when (eql base -1)
     (return-from intexp (if (oddp power) -1 1)))
+
+  ;; Handle 0 raised to a power.  Return 0 if the power is
+  ;; non-negative or signal a divide-by-zero if the power is negative.
+  (when (zerop base)
+    (if (minusp power)
+	(error 'division-by-zero)
+	(return-from intexp base)))
   
   (when (> (abs power) *intexp-maximum-exponent*)
     ;; Allow user the option to continue with calculation, possibly


=====================================
tests/irrat.lisp
=====================================
--- a/tests/irrat.lisp
+++ b/tests/irrat.lisp
@@ -152,3 +152,23 @@
 	for logx = (kernel::dd-%log2 x)
 	for log1/x = (kernel::dd-%log2 (/ x))
 	do (assert-true (<= (abs (+ logx log1/x)) (* 1 double-float-epsilon)))))
+
+(define-test expt-integer
+  (let ((power (1+ kernel::*intexp-maximum-exponent*)))
+    ;; Make sure we error out in the usual case with the power too
+    ;; large.
+    (assert-error 'kernel::intexp-limit-error
+		  (expt 2 power))
+    (assert-error 'kernel::intexp-limit-error
+		  (expt 2 (- power)))
+    ;; But raising 0 or 1 to a power shouldn't signal anything, except
+    ;; the obvious division-by-zero.
+    (assert-eql 1 (expt 1 power))
+    (cond ((evenp power)
+	   (assert-eql 1 (expt -1 power))
+	   (assert-eql -1 (expt -1 (1+ power))))
+	  (t
+	   (assert-eql -1 (expt -1 power))
+	   (assert-eql 1 (expt -1 (1+ power)))))
+    (assert-eql 0 (expt 0 power))
+    (assert-error 'division-by-zero (expt 0 (- power)))))



View it on GitLab: https://gitlab.common-lisp.net/cmucl/cmucl/commit/f3b73541a4918c50abdc17da4164113e40fa6036
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