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Re: readchars, seek back, and readchars again
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From:
William Michels via perl6-users
Date:
April 25, 2020 05:42
Subject:
Re: readchars, seek back, and readchars again
Message ID:
CAA99HCxS2-5XRnrHJuvS5ku88gEqaE-Aok7vo0sp2jS+OSn_gg@mail.gmail.com
Hi Joe,
I was able to run the code you posted and reproduced the exact same
result (Rakudo version 2020.02.1.0000.1 built on MoarVM version
2020.02.1 implementing Raku 6.d). I tried playing with file encodings a bit
(e.g. UTF8-C8), but I didn't see any improvement.
Yary has an issue posted regarding 'display-width' of UTF-16 encoded strings:
https://github.com/rakudo/rakudo/issues/3461
I know it might be far-fetched, but what if your UTF-8 issue and
Yary's UTF-16 issue were related? It would be nice to kill two birds
with one stone.
Best Regards, Bill.
On Fri, Apr 24, 2020 at 1:20 PM Joseph Brenner <doomvox@gmail.com> wrote:
>
> Another version of my test code, checking .tell throughout:
>
> use v6;
> use Test;
>
> my $tmpdir = IO::Spec::Unix.tmpdir;
> my $file = "$tmpdir/scratch_file.txt";
> my $unichar_str = "\x[1200]\x[2D80]\x[4DFC]\x[AAAA]\x[2CA4]\x[2C8E]"; # ሀⶀ䷼ꪪⲤⲎ
> my $ascii_str = "ABCDEFGHI";
>
> test_read_and_read_again($unichar_str, $file, 3);
> test_read_and_read_again($ascii_str, $file, 0);
>
> # write given string to file, then read the third character twice and check
> sub test_read_and_read_again($str, $file, $nudge = 0) {
> spurt $file, $str;
> my $fh = $file.IO.open;
> printf "%d: just opened\n", $fh.tell;
> $fh.readchars(2); # skip a few
> printf "%d: after skipping 2\n", $fh.tell;
> my $chr_1 = $fh.readchars(1);
> printf "%d: after reading 3rd: %s\n", $fh.tell, $chr_1;
> my $width = $chr_1.encode('UTF-8').bytes; # for our purposes, always 1 or 3
> my $step_back = $width + $nudge;
> $fh.seek: -$step_back, SeekFromCurrent;
> printf "%d: after seeking back %d\n", $fh.tell, $step_back;
> my $chr_2 = $fh.readchars(1);
> printf "%d: after re-reading 3rd: %s\n", $fh.tell, $chr_2;
> is( $chr_1, $chr_2,
> "read, seek back, and read again gets same char with nudge of $nudge" );
> }
>
>
> The output looks like so:
>
> /home/doom/End/Cave/Perl6/bin/trial-seeking_inner_truth.pl6
> 0: just opened
> 9: after skipping 2
> 12: after reading 3rd: ䷼
> 6: after seeking back 6
> 12: after re-reading 3rd: ䷼
> ok 1 - read, seek back, and read again gets same char with nudge of 3
> 0: just opened
> 2: after skipping 2
> 3: after reading 3rd: C
> 2: after seeking back 1
> 3: after re-reading 3rd: C
> ok 2 - read, seek back, and read again gets same char with nudge of 0
>
> It's really hard to see what I should do if I really wanted to
> intermix readchars and seeks like this... I'd need to check the range
> of the codepoint to see how far I need to seek to get where I expect
> to be.
>
>
>
> On 4/24/20, Joseph Brenner <doomvox@gmail.com> wrote:
> > Thanks, yes I understand unicode and utf-8 reasonably well.
> >
> >> So Rakudo has to read the next codepoint to make sure that it isn't a
> >> combining codepoint.
> >
> >> It is probably faking up the reads to look right when reading ASCII, but
> >> failing to do that for wider codepoints.
> >
> > I think it'd be the other way around... the idea here would be it's
> > doing an extra readchar behind the scenes just in-case there's
> > combining chars involved-- so you're figuring there's some confusion
> > about the actual point in the file that's being read and the
> > abstraction that readchars is supplying?
> >
> >
> > On 4/24/20, Brad Gilbert <b2gills@gmail.com> wrote:
> >> In UTF8 characters can be 1 to 4 bytes long.
> >>
> >> UTF8 was designed so that 7-bit ASCII is a subset of it.
> >>
> >> Any 8bit byte that has its most significant bit set cannot be ASCII.
> >> So multi-byte codepoints have the most significant bit set for all of the
> >> bytes.
> >> The first byte can tell you the number of bytes that follow it.
> >>
> >> That is how a singe codepoint is stored.
> >>
> >> A character can be made of several codepoints.
> >>
> >> "\c[LATIN SMALL LETTER E]\c[COMBINING ACUTE ACCENT]"
> >> "é"
> >>
> >> So Rakudo has to read the next codepoint to make sure that it isn't a
> >> combining codepoint.
> >>
> >> It is probably faking up the reads to look right when reading ASCII, but
> >> failing to do that for wider codepoints.
> >>
> >> On Fri, Apr 24, 2020 at 1:34 PM Joseph Brenner <doomvox@gmail.com> wrote:
> >>
> >>> I thought that doing a readchars on a filehandle, seeking backwards
> >>> the width of the char in bytes and then doing another read
> >>> would always get the same character. That works for ascii-range
> >>> characters (1-byte in utf-8 encoding) but not multi-byte "wide"
> >>> characters (commonly 3-bytes in utf-8).
> >>>
> >>> The question then, is why do I need a $nudge of 3 for wide chars, but
> >>> not ascii-range ones?
> >>>
> >>> use v6;
> >>> use Test;
> >>>
> >>> my $tmpdir = IO::Spec::Unix.tmpdir;
> >>> my $file = "$tmpdir/scratch_file.txt";
> >>> my $unichar_str = "\x[1200]\x[2D80]\x[4DFC]\x[AAAA]\x[2CA4]\x[2C8E]"; #
> >>> ሀⶀ䷼ꪪⲤⲎ
> >>> my $ascii_str = "ABCDEFGHI";
> >>>
> >>> subtest {
> >>> my $nudge = 3;
> >>> test_read_and_read_again($unichar_str, $file, $nudge);
> >>> }, "Wide unicode chars: $unichar_str";
> >>>
> >>> subtest {
> >>> my $nudge = 0;
> >>> test_read_and_read_again($ascii_str, $file, $nudge);
> >>> }, "Ascii-range chars: $ascii_str";
> >>>
> >>> # write given string to file, then read the third character twice and
> >>> check
> >>> sub test_read_and_read_again($str, $file, $nudge = 0) {
> >>> spurt $file, $str;
> >>> my $fh = $file.IO.open;
> >>> $fh.readchars(2); # skip a few
> >>> my $chr_1 = $fh.readchars(1);
> >>> my $width = $chr_1.encode('UTF-8').bytes; # for our purposes,
> >>> always
> >>> 1 or 3
> >>> my $step_back = $width + $nudge;
> >>> $fh.seek: -$step_back, SeekFromCurrent;
> >>> my $chr_2 = $fh.readchars(1);
> >>> is( $chr_1, $chr_2,
> >>> "read, seek back, and read again gets same char with nudge of
> >>> $nudge" );
> >>> }
> >>>
> >>
> >
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