Calc: GCD(0,x)=GCD(x,0)=|x|, not x (bug#41279)

Reported by David Ongaro.

* lisp/calc/calc-comb.el (calcFunc-gcd): Fix GCD simplification.
* test/lisp/calc/calc-tests.el (calc-gcd, calc-sum-gcd): New tests.

2 files changed, 30 insertions, 2 deletions

diff --git a/lisp/calc/calc-comb.el b/lisp/calc/calc-comb.el
index d4562a0cc86..c5d4d0837e7 100644
--- a/lisp/calc/calc-comb.el
+++ b/lisp/calc/calc-comb.el
@@ -241,8 +241,8 @@
          (calcFunc-gcd (math-neg a) b))
         ((Math-looks-negp b)
          (calcFunc-gcd a (math-neg b)))
-        ((Math-zerop a) b)
-        ((Math-zerop b) a)
+        ((Math-zerop a) (math-abs b))
+        ((Math-zerop b) (math-abs a))
         ((and (Math-ratp a)
               (Math-ratp b))
          (math-make-frac (math-gcd (if (eq (car-safe a) 'frac) (nth 1 a) a)
diff --git a/test/lisp/calc/calc-tests.el b/test/lisp/calc/calc-tests.el
index 9e36d91ac3e..c8cb97a8bca 100644
--- a/test/lisp/calc/calc-tests.el
+++ b/test/lisp/calc/calc-tests.el
@@ -368,6 +368,34 @@ An existing calc stack is reused, otherwise a new one is created."
                                      (vec 0 0 (var a var-a) 0)))
                  '(neg (var a var-a)))))
 
+(ert-deftest calc-gcd ()
+  (should (equal (calcFunc-gcd 3 4) 1))
+  (should (equal (calcFunc-gcd 12 15) 3))
+  (should (equal (calcFunc-gcd -12 15) 3))
+  (should (equal (calcFunc-gcd 12 -15) 3))
+  (should (equal (calcFunc-gcd -12 -15) 3))
+  (should (equal (calcFunc-gcd 0 5) 5))
+  (should (equal (calcFunc-gcd 5 0) 5))
+  (should (equal (calcFunc-gcd 0 -5) 5))
+  (should (equal (calcFunc-gcd -5 0) 5))
+  (should (equal (calcFunc-gcd 0 0) 0))
+  (should (equal (calcFunc-gcd 0 '(var x var-x))
+                 '(calcFunc-abs (var x var-x))))
+  (should (equal (calcFunc-gcd '(var x var-x) 0)
+                 '(calcFunc-abs (var x var-x)))))
+
+(ert-deftest calc-sum-gcd ()
+  ;; sum(gcd(0,n),n,-1,-1)
+  (should (equal (math-simplify '(calcFunc-sum (calcFunc-gcd 0 (var n var-n))
+                                               (var n var-n) -1 -1))
+                 1))
+  ;; sum(sum(gcd(n,k),k,-1,1),n,-1,1)
+  (should (equal (math-simplify
+                  '(calcFunc-sum
+                    (calcFunc-sum (calcFunc-gcd (var n var-n) (var k var-k))
+                                  (var k var-k) -1 1)
+                    (var n var-n) -1 1))
+                 8)))
 
 (provide 'calc-tests)
 ;;; calc-tests.el ends here