Fix string-distance for two empty strings

* fns.c (Fstring_distance): Avoid using uninitialized memory.
* test/src/fns-tests.el (test-string-distance): Add test cases.

2 files changed, 10 insertions, 2 deletions

diff --git a/src/fns.c b/src/fns.c
index 4e74589ef26..a72e41aee5b 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -322,7 +322,7 @@ Letter-case is significant, but text properties are ignored. */)
 
   USE_SAFE_ALLOCA;
   ptrdiff_t *column = SAFE_ALLOCA ((len1 + 1) * sizeof (ptrdiff_t));
-  for (y = 1; y <= len1; y++)
+  for (y = 0; y <= len1; y++)
     column[y] = y;
 
   if (use_byte_compare)
diff --git a/test/src/fns-tests.el b/test/src/fns-tests.el
index 9f6593a177c..bd5a4358e65 100644
--- a/test/src/fns-tests.el
+++ b/test/src/fns-tests.el
@@ -786,7 +786,15 @@
   ;; string containing hanzi character, compare by character
   (should (equal 2 (string-distance "ab" "ab我她")))
   (should (equal 1 (string-distance "ab" "a我b")))
-  (should (equal 1 (string-distance "我" "她"))))
+  (should (equal 1 (string-distance "我" "她")))
+
+  ;; correct behaviour with empty strings
+  (should (equal 0 (string-distance "" "")))
+  (should (equal 0 (string-distance "" "" t)))
+  (should (equal 1 (string-distance "x" "")))
+  (should (equal 1 (string-distance "x" "" t)))
+  (should (equal 1 (string-distance "" "x")))
+  (should (equal 1 (string-distance "" "x" t))))
 
 (ert-deftest test-bignum-eql ()
   "Test that `eql' works for bignums."