possible bugfix for CHAR_LEADING_CODE() for multibyte raw bytes

possible bugfix for CHAR_LEADING_CODE() for multibyte raw bytes

[Danny McClanahan]

Hello emacs-devel,

I have been getting familiar with the super simple and pragmatic Emacs multibyte encoding scheme and the work done in regex-emacs.c to manipulate it. I am now pretty confident that a byte-based automaton should be able to match against Emacs multibyte buffers (which iiuc are what most user-visible buffers are encoded in). This is super useful because it means:
(1) Emacs can make use of other external libraries which perform byte string matching without enforcing UTF-8 encoding (not just the one I'm working on).
(2) The matching loop can iterate directly over bytes in many cases instead of counting multibyte codepoints (I believe regex-emacs.c counts multibyte codepoints in more cases than necessary, but not sure of this yet).

While implementing the multibyte encoding scheme, I found what seemed to be an error in `CHAR_LEADING_CODE()` for raw bytes (multibyte codepoints encoded using two bytes which encode a single raw byte), where the multibyte code `c` isn't shifted downward the way it is in `char_string()` (see diff for fix at bottom).

This seems never to be triggered because regex-emacs.c never calls `CHAR_LEADING_CODE()` when generating the fastmap for raw bytes (e.g. lines 3095-3098):
```c
	  /* Cover the case of matching a raw char in a
	     multibyte regexp against unibyte.	*/
	  if (CHAR_BYTE8_HEAD_P (p[1]))
	    data->fastmap[CHAR_TO_BYTE8 (STRING_CHAR (p + 1))] = 1;
```

Not sure if I've got this right, so please correct me if not!

(Note: the below patch requires multiple trailing newlines in order to apply successfully, so I've enclosed it within "-----". I'm not sure how to avoid this: I just used `git diff master..`.)

-------------------------
diff --git a/src/character.h b/src/character.h
index 6d0f035c2bb..d7a6a4f525c 100644
--- a/src/character.h
+++ b/src/character.h
@@ -216,7 +216,7 @@ CHAR_LEADING_CODE (int c)
 	  : c <= MAX_3_BYTE_CHAR ? 0xE0 | (c >> 12)
 	  : c <= MAX_4_BYTE_CHAR ? 0xF0 | (c >> 18)
 	  : c <= MAX_5_BYTE_CHAR ? 0xF8
-	  : 0xC0 | ((c >> 6) & 0x01));
+	  : 0xC0 | ((CHAR_TO_BYTE8(c) >> 6) & 0x01));
 }
 
 
--------------------------

Re: possible bugfix for CHAR_LEADING_CODE() for multibyte raw bytes

[Eli Zaretskii]

> Date: Sun, 28 Apr 2024 10:14:53 +0000
> From: Danny McClanahan <dmcc2@hypnicjerk.ai>
> 
> While implementing the multibyte encoding scheme, I found what seemed to be an error in `CHAR_LEADING_CODE()` for raw bytes (multibyte codepoints encoded using two bytes which encode a single raw byte), where the multibyte code `c` isn't shifted downward the way it is in `char_string()` (see diff for fix at bottom).

Does the change you propose actually affect the result?  The
"((c >> 6) & 0x01)" part takes just the LSB of (c >> 6), and
that doesn't seem to be affected by running c through
CHAR_TO_BYTE8, does it?

The leading byte of a multibyte sequence for raw bytes is either 0xC0
or 0xC1.  That's what that part is trying to compute.  Am I missing
something?