New function 'string-distance'

* src/fns.c (Fstring_distance): New primitive.
(syms_of_fns): Defsubr it.

* test/lisp/subr-tests.el (subr-tests--string-distance): New test.

* etc/NEWS: Mention 'string-distance'.

etc/NEWS

@@ -534,6 +534,9 @@ manual for more details.
 +++
 ** New function assoc-delete-all.
 
+** New function string-distance to calculate Levenshtein distance
+between two strings.
+
 ** 'print-quoted' now defaults to t, so if you want to see
 (quote x) instead of 'x you will have to bind it to nil where applicable.
 

src/fns.c

@@ -153,6 +153,67 @@ If STRING is multibyte, this may be greater than the length of STRING.  */)
   return make_number (SBYTES (string));
 }
 
+DEFUN ("string-distance", Fstring_distance, Sstring_distance, 2, 3, 0,
+       doc: /* Return Levenshtein distance between STRING1 and STRING2.
+If BYTECOMPARE is nil, compare character of strings.
+If BYTECOMPARE is t, compare byte of strings.
+Case is significant, but text properties are ignored. */)
+  (Lisp_Object string1, Lisp_Object string2, Lisp_Object bytecompare)
+
+{
+  CHECK_STRING (string1);
+  CHECK_STRING (string2);
+
+  bool use_byte_compare = !NILP (bytecompare)
+    || (!STRING_MULTIBYTE (string1) && !STRING_MULTIBYTE (string2));
+  ptrdiff_t len1 = use_byte_compare? SBYTES (string1) : SCHARS (string1);
+  ptrdiff_t len2 = use_byte_compare? SBYTES (string2) : SCHARS (string2);
+  ptrdiff_t x, y, lastdiag, olddiag;
+
+  USE_SAFE_ALLOCA;
+  ptrdiff_t *column = SAFE_ALLOCA ((len1 + 1) * sizeof (ptrdiff_t));
+  for (y = 1; y <= len1; y++)
+    column[y] = y;
+
+  if (use_byte_compare)
+    {
+      char *s1 = SSDATA (string1);
+      char *s2 = SSDATA (string2);
+
+      for (x = 1; x <= len2; x++)
+        {
+          column[0] = x;
+          for (y = 1, lastdiag = x - 1; y <= len1; y++)
+            {
+              olddiag = column[y];
+              column[y] = min (min (column[y] + 1, column[y-1] + 1), lastdiag + (s1[y-1] == s2[x-1]? 0 : 1));
+              lastdiag = olddiag;
+            }
+        }
+    }
+  else
+    {
+      int c1, c2;
+      ptrdiff_t i1, i1_byte, i2 = 0, i2_byte = 0;
+      for (x = 1; x <= len2; x++)
+        {
+          column[0] = x;
+          FETCH_STRING_CHAR_ADVANCE (c2, string2, i2, i2_byte);
+          i1 = i1_byte = 0;
+          for (y = 1, lastdiag = x - 1; y <= len1; y++)
+            {
+              olddiag = column[y];
+              FETCH_STRING_CHAR_ADVANCE (c1, string1, i1, i1_byte);
+              column[y] = min (min (column[y] + 1, column[y-1] + 1), lastdiag + (c1 == c2? 0 : 1));
+              lastdiag = olddiag;
+            }
+        }
+    }
+
+  SAFE_FREE ();
+  return make_number (column[len1]);
+}
+
 DEFUN ("string-equal", Fstring_equal, Sstring_equal, 2, 2, 0,
        doc: /* Return t if two strings have identical contents.
 Case is significant, but text properties are ignored.
@@ -5226,6 +5287,7 @@ this variable.  */);
   defsubr (&Slength);
   defsubr (&Ssafe_length);
   defsubr (&Sstring_bytes);
+  defsubr (&Sstring_distance);
   defsubr (&Sstring_equal);
   defsubr (&Scompare_strings);
   defsubr (&Sstring_lessp);

test/lisp/subr-tests.el

@@ -281,6 +281,24 @@ indirectly `mapbacktrace'."
   (should (equal (string-match-p "\\`[[:blank:]]\\'" "\u3000") 0))
   (should-not (string-match-p "\\`[[:blank:]]\\'" "\N{LINE SEPARATOR}")))
 
+(ert-deftest subr-tests--string-distance ()
+  "Test `string-distance' behavior."
+  ;; ASCII characters are always fine
+  (should (equal 1 (string-distance "heelo" "hello")))
+  (should (equal 2 (string-distance "aeelo" "hello")))
+  (should (equal 0 (string-distance "ab" "ab" t)))
+  (should (equal 1 (string-distance "ab" "abc" t)))
+
+  ;; string containing hanzi character, compare by byte
+  (should (equal 6 (string-distance "ab" "ab我她" t)))
+  (should (equal 3 (string-distance "ab" "a我b" t)))
+  (should (equal 3 (string-distance "我" "她" t)))
+
+  ;; 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"))))
+
 (ert-deftest subr-tests--dolist--wrong-number-of-args ()
   "Test that `dolist' doesn't accept wrong types or length of SPEC,
 cf. Bug#25477."