Fix and speed up en/decoding of UTF-8 strings

* src/coding.c (get_char_bytes, encode_string_utf_8)
(decode_string_utf_8): Fix commentary.
(encode_string_utf_8): Return the original ASCII string only
if NOCOPY is non-zero.
(decode_string_utf_8): Accept 2 additional arguments STR and
STR_LEN, which allow to pass the input text as a C string.
(make_string_from_utf8): Delegate the job to decode_string_utf_8.
* src/coding.h: Update the prototype of decode_string_utf_8.
* src/json.c (json_encode): Call encode_string_utf_8.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/coding.h b/src/coding.h
index 8efddbf55c4..d552e7b4c8f 100644
--- a/src/coding.h
+++ b/src/coding.h
@@ -691,7 +691,8 @@ extern Lisp_Object code_convert_string_norecord (Lisp_Object, Lisp_Object,
                                                  bool);
 extern Lisp_Object encode_string_utf_8 (Lisp_Object, Lisp_Object, bool,
                                         Lisp_Object, Lisp_Object);
-extern Lisp_Object decode_string_utf_8 (Lisp_Object, Lisp_Object, bool,
+extern Lisp_Object decode_string_utf_8 (Lisp_Object, const char *, ptrdiff_t,
+                                        Lisp_Object, bool,
                                         Lisp_Object, Lisp_Object);
 extern Lisp_Object encode_file_name (Lisp_Object);
 extern Lisp_Object decode_file_name (Lisp_Object);