patch 9.0.1515: reverse() does not work for a String

Problem:    reverse() does not work for a String.
Solution:   Implement reverse() for a String. (Yegappan Lakshmanan,
            closes #12179)

runtime/doc/builtin.txt

@@ -484,7 +484,8 @@ rename({from}, {to})		Number	rename (move) file from {from} to {to}
 repeat({expr}, {count})		List/Blob/String
 					repeat {expr} {count} times
 resolve({filename})		String	get filename a shortcut points to
-reverse({list})			List	reverse {list} in-place
+reverse({obj})			List/Blob/String
+					reverse {obj}
 round({expr})			Float	round off {expr}
 rubyeval({expr})		any	evaluate |Ruby| expression
 screenattr({row}, {col})	Number	attribute at screen position
@@ -7404,11 +7405,13 @@ resolve({filename})					*resolve()* *E655*
 			GetName()->resolve()
 
 reverse({object})					*reverse()*
-		Reverse the order of items in {object} in-place.
-		{object} can be a |List| or a |Blob|.
-		Returns {object}.
-		Returns zero if {object} is not a List or a Blob.
-		If you want an object to remain unmodified make a copy first: >
+		Reverse the order of items in {object}.  {object} can be a
+		|List|, a |Blob| or a |String|.  For a List and a Blob the
+		items are reversed in-place and {object} is returned.
+		For a String a new String is returned.
+		Returns zero if {object} is not a List, Blob or a String.
+		If you want a List or Blob to remain unmodified make a copy
+		first: >
 			:let revlist = reverse(copy(mylist))
 <		Can also be used as a |method|: >
 			mylist->reverse()

runtime/doc/usr_41.txt

@@ -759,6 +759,7 @@ String manipulation:					*string-functions*
 	strdisplaywidth()	size of string when displayed, deals with tabs
 	setcellwidths()		set character cell width overrides
 	getcellwidths()		get character cell width overrides
+	reverse()		reverse the order of characters in a string
 	substitute()		substitute a pattern match with a string
 	submatch()		get a specific match in ":s" and substitute()
 	strpart()		get part of a string using byte index
@@ -797,7 +798,7 @@ List manipulation:					*list-functions*
 	reduce()		reduce a List to a value
 	slice()			take a slice of a List
 	sort()			sort a List
-	reverse()		reverse the order of a List or Blob
+	reverse()		reverse the order of items in a List
 	uniq()			remove copies of repeated adjacent items
 	split()			split a String into a List
 	join()			join List items into a String
@@ -864,6 +865,7 @@ Floating point computation:				*float-functions*
 Blob manipulation:					*blob-functions*
 	blob2list()		get a list of numbers from a blob
 	list2blob()		get a blob from a list of numbers
+	reverse()		reverse the order of numbers in a blob
 
 Other computation:					*bitwise-function*
 	and()			bitwise AND

src/list.c

@@ -2999,6 +2999,8 @@ f_reverse(typval_T *argvars, typval_T *rettv)
 
     if (argvars[0].v_type == VAR_BLOB)
 	blob_reverse(argvars[0].vval.v_blob, rettv);
+    else if (argvars[0].v_type == VAR_STRING)
+	string_reverse(argvars[0].vval.v_string, rettv);
     else if (argvars[0].v_type != VAR_LIST)
 	semsg(_(e_argument_of_str_must_be_list_or_blob), "reverse()");
     else

src/proto/strings.pro

@@ -23,6 +23,7 @@ int has_non_ascii(char_u *s);
 char_u *concat_str(char_u *str1, char_u *str2);
 char_u *string_quote(char_u *str, int function);
 long string_count(char_u *haystack, char_u *needle, int ic);
+void string_reverse(char_u *str, typval_T *rettv);
 void string_filter_map(char_u *str, filtermap_T filtermap, typval_T *expr, typval_T *rettv);
 void string_reduce(typval_T *argvars, typval_T *expr, typval_T *rettv);
 void f_byteidx(typval_T *argvars, typval_T *rettv);

src/strings.c

@@ -854,6 +854,47 @@ string_count(char_u *haystack, char_u *needle, int ic)
     return n;
 }
 
+/*
+ * Reverse the string in 'str' and set the result in 'rettv'.
+ */
+    void
+string_reverse(char_u *str, typval_T *rettv)
+{
+    rettv->v_type = VAR_STRING;
+    rettv->vval.v_string = NULL;
+    if (str == NULL)
+	return;
+
+    char_u	*rstr = vim_strsave(str);
+    rettv->vval.v_string = rstr;
+    if (rstr == NULL || *str == NUL)
+	return;
+
+    size_t	len = STRLEN(rstr);
+    if (has_mbyte)
+    {
+	char_u *src = str;
+	char_u *dest = rstr + len;
+
+	while (src < str + len)
+	{
+	    int clen = mb_ptr2len(src);
+	    dest -= clen;
+	    mch_memmove(dest, src, (size_t)clen);
+	    src += clen;
+	}
+    }
+    else
+    {
+	for (size_t i = 0; i < len / 2; i++)
+	{
+	    char tmp = rstr[len - i - 1];
+	    rstr[len - i - 1] = rstr[i];
+	    rstr[i] = tmp;
+	}
+    }
+}
+
 /*
  * Make a typval_T of the first character of "input" and store it in "output".
  * Return OK or FAIL.

src/testdir/test_functions.vim

@@ -3469,4 +3469,21 @@ func Test_delfunc_while_listing()
   call StopVimInTerminal(buf)
 endfunc
 
+" Test for the reverse() function with a string
+func Test_string_reverse()
+  call assert_equal('', reverse(test_null_string()))
+  for [s1, s2] in [['', ''], ['a', 'a'], ['ab', 'ba'], ['abc', 'cba'],
+        \ ['abcd', 'dcba'], ['«-«-»-»', '»-»-«-«'],
+        \ ['🇦', '🇦'], ['🇦🇧', '🇧🇦'], ['🇦🇧🇨', '🇨🇧🇦'],
+        \ ['🇦«🇧-🇨»🇩', '🇩»🇨-🇧«🇦']]
+    call assert_equal(s2, reverse(s1))
+  endfor
+
+  " test in latin1 encoding
+  let save_enc = &encoding
+  set encoding=latin1
+  call assert_equal('dcba', reverse('abcd'))
+  let &encoding = save_enc
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/testdir/test_listdict.vim

@@ -981,7 +981,7 @@ func Test_reverse_sort_uniq()
   END
   call v9.CheckLegacyAndVim9Success(lines)
 
-  call assert_fails('call reverse("")', 'E899:')
+  call assert_fails('call reverse({})', 'E899:')
   call assert_fails('call uniq([1, 2], {x, y -> []})', 'E745:')
   call assert_fails("call sort([1, 2], function('min'), 1)", "E1206:")
   call assert_fails("call sort([1, 2], function('invalid_func'))", "E700:")

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1515,
 /**/
     1514,
 /**/