in reply to Re^3: Processing an encoded file backwards
in thread Processing an encoded file backwards

> So it doesn't allow you to differentiate between a character that was broken by the read, and an actually malformed input file.

providing a call back helps identifying the malformed bytes 

  DB<131> dd $rr
"\x84\xC3\x96\xC3\x9C.\r\n\r\n"

  DB<132> $start=0

  DB<133> $rru = Encode::decode('utf8',$rr, sub{ my $broken = shift; $
+start++; "" });

  DB<134> dd $rru
"\xD6\xDC.\r\n\r\n"

  DB<135> p $start
1
  DB<136>

[download]

> I don't use the debugger often, so reading its output doesn't come naturally to me ;-)

as commented

furthermore debugger commands 

 DB<79> h p
p expr        Same as "print {DB::OUT} expr" in current package.

  DB<80> h x
x expr        Evals expression in list context, dumps the result.

[download]

update

one way to identify how many malformed bytes are at the start and to be sure the rest is well. 

  DB<159> $start=0

  DB<160> $rru = Encode::decode('utf8',$rr,sub{ $start++; return "" })
+;

  DB<161> $sub= substr $rr,$start

  DB<162> $rru2 = Encode::decode('utf8',$sub ,Encode::FB_CROAK);

  DB<163> p $rru2 eq $rru
1
  DB<164>

[download]

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery FootballPerl is like chess, only without the dice

Replies are listed 'Best First'.
Re^5: Processing an encoded file backwards
by haukex (Archbishop) on Jan 18, 2020 at 21:53 UTC
    The Flag FB_QUIET seems to be the answer
    providing a call back helps identifying the malformed bytes

    Unfortunately, that doesn't always seem to be the case:

    use warnings;
use strict;
use Encode qw/decode/;
use Data::Dump;

dd decode('UTF-16-BE', "\xD8\x3D\xDD\xFA", Encode::FB_QUIET|Encode::LE
+AVE_SRC);
dd decode('UTF-16-BE', "\x3D\xDD\xFA", Encode::FB_QUIET|Encode::LEAVE_
+SRC);
dd decode('UTF-16-BE', "\x3D\xDD\xFA", sub{ sprintf "<U+%04X>", shift 
+});

dd decode('UTF-16-LE', "\x3D\xD8\xFA\xDD", Encode::FB_QUIET|Encode::LE
+AVE_SRC);
dd decode('UTF-16-LE', "\xD8\xFA\xDD", Encode::FB_QUIET|Encode::LEAVE_
+SRC);
dd decode('UTF-16-LE', "\xD8\xFA\xDD", sub{ sprintf "<U+%04X>", shift 
+});

__END__

"\x{1F5FA}"
"\x{3DDD}"
"\x{3DDD}"
"\x{1F5FA}"
"\x{FAD8}"
"\x{FAD8}"

    [download]

    It could be argued that this is a bug / oversight in Encode, and of course if we know we're reading UTF-16 we should always read an even number of bytes. But still, because of this behavior, and because I don't yet know if there are encodings where chopped up byte sequences might end up in valid characters, I'm doubtful that a generalized "read any file backwards with Encode" is reliable. Personally I'd just make a version for UTF-8 and UTF-16, and any others as needed, or other encodings can be converted to the supported ones.

[reply]
[d/l]
      Haukex++

      NICE example for ambiguity of UTF-16 if you don't get the start right!!!

      For clarification: your point is that "\xFA" (first block) and "\xDD" (second block) should raise errors?

      use warnings;
use strict;
use Encode qw/decode/;
use Data::Dump qw/dd/;


dd decode('UTF-16-BE', "\x3D\xDD\xFA", Encode::FB_CROAK|Encode::LEAVE_
+SRC );
dd decode('UTF-16-BE', "\xFA",         Encode::FB_CROAK|Encode::LEAVE_
+SRC );

dd decode('UTF-16-LE', "\xD8\xFA\xDD", Encode::FB_CROAK|Encode::LEAVE_
+SRC );
dd decode('UTF-16-LE', "\xDD",         Encode::FB_CROAK|Encode::LEAVE_
+SRC);

__END__

"\x{3DDD}"
""
"\x{FAD8}"
""

      [download]

      I agree, looks like a bug we should report.

      A really strange one too ...

      > Personally I'd just make a version for UTF-8 and UTF-16, and any others as needed, or other encodings can be converted to the supported ones.

      I don't even know other wide encodings except unicode , so I prefer relying on Encode for utf8 and make sure utf16 are read modulo 4 bytes 2 bytes.

      update

      From the docs

          As of version 2.12, "Encode" supports coderef values for "CHECK"; 
+see
    below.

    NOTE: Not all encodings support this feature. Some encodings ignor
+e the
    *CHECK* argument. For example, Encode::Unicode ignores *CHECK* and
+ it
    always croaks on error.

      [download]

      ... but it doesn't croak

      Cheers Rolf
      (addicted to the Perl Programming Language :)
      Wikisyntax for the Monastery FootballPerl is like chess, only without the dice

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[d/l]
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        What Perl version? I'm getting

        UTF-16BE:Partial character at /home/choroba/1.pl line 8.

        [download]
        in 5.26.1.

        map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]
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[d/l]
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