seraching backward in files

[Patrick Gundlach]

Dear ruby hackers,

I'd wonder if there is a simple method in searching backward in
files. I want to read a pdf file, so I have to do the following:

1) goto the end of file (pdffile.seek(0,IO::SEEK_END))
2) search backward for the string "%%EOF" within the last 1024 bytes
3) search backward for "startxref"
4) read the next bytes,.....


Do I have to write my own seach backward routine or is there some
functionality for it?

Best regards,

Patrick

 

[Simon Strandgaard]

I'd wonder if there is a simple method in searching backward in
files. I want to read a pdf file, so I have to do the following:

1) goto the end of file (pdffile.seek(0,IO::SEEK_END))
2) search backward for the string "%%EOF" within the last 1024 bytes
3) search backward for "startxref"
4) read the next bytes,.....


Do I have to write my own seach backward routine or is there some
functionality for it?

Ruby's native regexp engine cannot go backwards... thats why I
am working on my own regexp-engine. However I don't have a
FileInputStream class yet (but I can easily add one).

The text is spelled backwards, because we want to search backwards.

# pseudo code
iterator = file.create_iterator_end.reverse
re = NewRegexp.new('(?xm) .{0,1024} FOE%% .{0,4000} ferxtrats')
matchdata = re.match(iterator)

The engine is written entirely in Ruby, so speed isn't impressive.
Its fairly compatible with Ruby1.8's builtin GNU regexp, and its
carefully tested (more than 2000 tests).
http://raa.ruby-lang.org/list.rhtml?name=regexp

 

[Carlos]

Dear ruby hackers,

I'd wonder if there is a simple method in searching backward in
files. I want to read a pdf file, so I have to do the following:

1) goto the end of file (pdffile.seek(0,IO::SEEK_END))
2) search backward for the string "%%EOF" within the last 1024 bytes
3) search backward for "startxref"
4) read the next bytes,.....


Do I have to write my own seach backward routine or is there some
functionality for it?

You can load the last 1024 characters in a string and use String#rindex for
(2). Then, use String#rindex again to find (3). If you don't find, load the
second-to-last 1024 characters, use String#rindex again, etc...

Good luck.

--

 

[Ara.T.Howard]

Dear ruby hackers,

I'd wonder if there is a simple method in searching backward in
files. I want to read a pdf file, so I have to do the following:

1) goto the end of file (pdffile.seek(0,IO::SEEK_END))
2) search backward for the string "%%EOF" within the last 1024 bytes
3) search backward for "startxref"
4) read the next bytes,.....


Do I have to write my own seach backward routine or is there some
functionality for it?

Best regards,

Patrick

if you are on linux you might want to simply

tac = IO.popen "tac #{ path }"

# process file in 'normal' fashion - only backwards...




man 1 tac

cheers.

-a
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[Patrick Gundlach]

Hi,

You can load the last 1024 characters in a string and use String#rindex for
(2). Then, use String#rindex again to find (3). If you don't find, load the
second-to-last 1024 characters, use String#rindex again, etc...

Yes, this is cool. It works like charm. Thanks for pointing this out.


Patrick

 

[Patrick Gundlach]

Hi,

your solution would probably the most clean one, but would require an
additional library. I think I'll go for IO->String, String#rindex.


Thank you for your answer,


Patrick

 

[Patrick Gundlach]

if you are on linux you might want to simply

tac = IO.popen "tac #{ path }"

# process file in 'normal' fashion - only backwards...

Oh, right, I forgot about tac. But this could be slow on large files
(and non portable).

Thank you for your answer,

Patrick

 

[Simon Strandgaard]

your solution would probably the most clean one, but would require an
additional library. I think I'll go for IO->String, String#rindex.

Good luck.

BTW: I have just made an experimental File iterator.