Re: docs for insert-file-contents use 'bytes'

[Ted Zlatanov <tzz@lifelogs.com>]

On Wed, 01 Oct 2008 09:44:31 +0900 Kenichi Handa <handa@m17n.org> wrote: 

KH> In article <8663od68yb.fsf@lifelogs.com>, Ted Zlatanov <tzz@lifelogs.com> writes:
>> There could be a insert-file-decoded-contents that seeks to a byte
>> position and gets the next character at or after that position.  That's
>> not too hard to implement and it's fast.

KH> It's not that easy.  Some encoding requires to seek back an
KH> escape sequence to get the next character.  And, for UTF-16
KH> with BOM, we have to check the first 2-byte.

OK.  Does it ever require going more than N*2 (where N = max sequence
length for the encoding) bytes back?  Is N ever bigger than 10?  If not,
it may be complicated code but at least it will be fairly fast.

The semantics could be (given N as above):

1) jump to character number C: scan from beginning of file and count
characters up to C if the encoding has a variable length.  Otherwise the
offset is obvious.

2) jump to character around/at byte B: jump to B-N*2 and scan characters
forward until you find the one that straddles or begins at B.  Also
should have a way to report that character's actual starting byte
position.

3) jump to byte: operate as now, just a fseek

For my purposes (2) is most useful, but I can use (3) and bypass
encodings.  (1) is not good for me, since the application is to view
large files, but (1) is OK for small files.

On Tue, 30 Sep 2008 11:58:12 -0400 Stefan Monnier <monnier@IRO.UMontreal.CA> wrote: 

>> I want to use it to implement a paged view of large files.  We discussed
>> this in emacs-help and you suggested using insert-file-contents IIRC.

SM> This is a very good application indeed.

It's on my TODO list :)  I'm more concerned with fixing the docs,
though, as that was the original intent of my post.  Can they just warn
about the issue, as I suggested?

>> Because the text will be corrupted if you seek in the middle of a
>> multibyte sequence, and there's no way to know in advance if a position
>> is safe without at least some scanning.

SM> It's not exactly "corrupted" in the sense that, while it is not
SM> displayed correctly, it should be correctly saved back so no information
SM> is lost.  Basically, some of the bytes are decoded with the wrong
SM> coding-system, but this coding system is supposed to be safe.

SM> No doubt that it's not "good enough" in general.

Right, it's the interpretation of the data, not the data itself, that's
incorrect.  Variable-length encodings are annoying.

>> There could be a insert-file-decoded-contents that seeks to a byte
>> position and gets the next character at or after that position.  That's
>> not too hard to implement and it's fast.

SM> It wouldn't be good enough for your application because you might then
SM> lose the chars that straddle a boundary.

That's fine.  I would just seek back N bytes and resynchronize (see
above).  I could also make the mode only work with bytes, bypassing
encoding schemes.  This would be a good thing, actually, for cases where
the user might page or search through many megabytes of data quickly.

Ted