patch 8.2.4695: JSON encoding could be faster

Problem:    JSON encoding could be faster.
Solution:   Optimize encoding JSON strings. (closes #10086)

src/json.c

@@ -114,23 +114,45 @@ json_encode_lsp_msg(typval_T *val)
 }
 #endif
 
+/*
+ * Lookup table to quickly know if the given ASCII character must be escaped.
+ */
+static const char ascii_needs_escape[128] = {
+    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, // 0x0.
+    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, // 0x1.
+    0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0x2.
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0x3.
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0x4.
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, // 0x5.
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0x6.
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0x7.
+};
+
+/*
+ * Encode the utf-8 encoded string "str" into "gap".
+ */
     static void
 write_string(garray_T *gap, char_u *str)
 {
     char_u	*res = str;
     char_u	numbuf[NUMBUFLEN];
-
+    char_u	*from;
-    if (res == NULL)
-	ga_concat(gap, (char_u *)"\"\"");
-    else
-    {
 #if defined(USE_ICONV)
     vimconv_T   conv;
     char_u	*converted = NULL;
+#endif
+    int		c;
 
+    if (res == NULL)
+    {
+	ga_concat(gap, (char_u *)"\"\"");
+	return;
+    }
+
+#if defined(USE_ICONV)
     if (!enc_utf8)
     {
-	    // Convert the text from 'encoding' to utf-8, the JSON string is
+	// Convert the text from 'encoding' to utf-8, because a JSON string is
 	// always utf-8.
 	conv.vc_type = CONV_NONE;
 	convert_setup(&conv, p_enc, (char_u*)"utf-8");
@@ -140,11 +162,24 @@ write_string(garray_T *gap, char_u *str)
     }
 #endif
     ga_append(gap, '"');
-	while (*res != NUL)
+    // `from` is the beginning of a sequence of bytes we can directly copy from
+    // the input string, avoiding the overhead associated to decoding/encoding
+    // them.
+    from = res;
+    while ((c = *res) != NUL)
     {
-	    int c;
 	// always use utf-8 encoding, ignore 'encoding'
-	    c = utf_ptr2char(res);
+	if (c < 0x80)
+	{
+	    if (!ascii_needs_escape[c])
+	    {
+		res += 1;
+		continue;
+	    }
+
+	    if (res != from)
+		ga_concat_len(gap, from, res - from);
+	    from = res + 1;
 
 	    switch (c)
 	    {
@@ -164,26 +199,44 @@ write_string(garray_T *gap, char_u *str)
 		    ga_append(gap, c);
 		    break;
 		default:
-		    if (c >= 0x20)
+		    vim_snprintf((char *)numbuf, NUMBUFLEN, "\\u%04lx",
-		    {
+								      (long)c);
-			numbuf[utf_char2bytes(c, numbuf)] = NUL;
 		    ga_concat(gap, numbuf);
 	    }
+
+	    res += 1;
+	}
 	else
 	{
-			vim_snprintf((char *)numbuf, NUMBUFLEN,
+	    int l = utf_ptr2len(res);
-							 "\\u%04lx", (long)c);
+
+	    if (l > 1)
+	    {
+		res += l;
+		continue;
+	    }
+
+	    // Invalid utf-8 sequence, replace it with the Unicode replacement
+	    // character U+FFFD.
+	    if (res != from)
+		ga_concat_len(gap, from, res - from);
+	    from = res + 1;
+
+	    numbuf[utf_char2bytes(0xFFFD, numbuf)] = NUL;
 	    ga_concat(gap, numbuf);
+
+	    res += l;
 	}
     }
-	    res += utf_ptr2len(res);
+
-	}
+    if (res != from)
+	ga_concat_len(gap, from, res - from);
+
     ga_append(gap, '"');
 #if defined(USE_ICONV)
     vim_free(converted);
 #endif
 }
-}
 
 /*
  * Return TRUE if "key" can be used without quotes.

src/testdir/test_json.vim

@@ -107,6 +107,9 @@ func Test_json_encode()
   call assert_equal('"café"', json_encode("caf\xe9"))
   let &encoding = save_encoding
 
+  " Invalid utf-8 sequences are replaced with U+FFFD (replacement character)
+  call assert_equal('"foo' . "\ufffd" . '"', json_encode("foo\xAB"))
+
   call assert_fails('echo json_encode(function("tr"))', 'E1161: Cannot json encode a func')
   call assert_fails('echo json_encode([function("tr")])', 'E1161: Cannot json encode a func')
 

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4695,
 /**/
     4694,
 /**/