I have an on-disk file of 100mb (can be up to 300mb). There are nulls and some other control characters that should not be in there. At first I read the string into memory and then re-read it Char by Char and then removed the offending Chars and put the clean stuff into a StringBuilder and then did a ToString on that.
That uses too much memory of course. I need to figure out how to strip out bad ASCII values on disk. Maybe (.NET 4) Memory Mapped File Stream is the right thing (I checked this out from Memory Mapped File to Read End of File? a while ago)?
All ideas appreciated. Thanks.
If you need to shrink the file to remove bad characters then simply read the file in a character or block at a time and write it out to a new file skipping bad characters.
This also gives you an undo!
If you can replace bad characters in place so that the length of the file doesn't change then map the file and scan over the memory replacing each bad character with eg space (ascii 32). This is simplest and probably faster - but either way you are going to be dominated by the raw disk i/o
Related
My non-Unicode Delphi 7 application allows users to open .txt files.
Sometimes UTF-8/UNICODE .txt files are tried to be opened causing a problem.
I need a function that detects if the user is opening a txt file with UTF-8 or Unicode encoding and Converts it to the system's default code page (ANSI) encoding automatically when possible so that it can be used by the app.
In cases when converting is not possible, the function should return an error.
The ReturnAsAnsiText(filename) function should open the txt file, make detection and conversion in steps like this;
If the byte stream has no bytes values over x7F, its ANSI, return as is
If the byte stream has bytes values over x7F, convert from UTF-8
If the stream has BOM; try Unicode conversion
If conversion to the system's current code page is not possible, return NULL to indicate an error.
It will be an OK limit for this function, that the user can open only those files that match their region/codepage (Control Panel Regional Region Settings for non-Unicode apps).
The conversion function ReturnAsAnsiText, as you designed, will have a number of issues:
The Delphi 7 application may not be able to open files where the filename using UTF-8 or UTF-16.
UTF-8 (and other Unicode) usage has increased significantly from 2019. Current web pages are between 98% and 100% UTF-8 depending on the language.
You design will incorrectly translate some text that a standards compliant would handle.
Creating the ReturnAsAnsiText is beyond the scope of an answer, but you should look at locating a library you can use instead of creating a new function. I haven't used Delphi 2005 (I believe that is 7), but I found this MIT licensed library that may get you there. It has a number of caveats:
It doesn't support all forms of BOM.
It doesn't support all encodings.
There is no universal "best-fit" behavior for single-byte character sets.
There are other issues that are tangentially described in this question. You wouldn't use an external command, but I used one here to demonstrate the point:
% iconv -f utf-8 -t ascii//TRANSLIT < hello.utf8
^h'elloe
iconv: (stdin):1:6: cannot convert
% iconv -f utf-8 -t ascii < hello.utf8
iconv: (stdin):1:0: cannot convert
Enabling TRANSLIT in standards based libraries supports converting characters like é to ASCII e. But still fails on characters like π, since there are no similar in form ASCII characters.
Your required answer would need massive UTF-8 and UTF-16 translation tables for every supported code page and BMP, and would still be unable to reliably detect the source encoding.
Notepad has trouble with this issue.
The solution as requested, would probably entail more effort than you put into the original program.
Possible solutions
Add a text editor into your program. If you write it, you will be able to read it.
The following solution pushes the translation to established tables provided by Windows.
Use the Win32 API native calls translate strings using functions like WideCharToMultiByte, but even this has its drawbacks(from the referenced page, the note is more relevant to the topic, but the caution is important for security):
Caution Using the WideCharToMultiByte function incorrectly can compromise the security of your application. Calling this function can easily cause a buffer overrun because the size of the input buffer indicated by lpWideCharStr equals the number of characters in the Unicode string, while the size of the output buffer indicated by lpMultiByteStr equals the number of bytes. To avoid a buffer overrun, your application must specify a buffer size appropriate for the data type the buffer receives.
Data converted from UTF-16 to non-Unicode encodings is subject to data loss, because a code page might not be able to represent every character used in the specific Unicode data. For more information, see Security Considerations: International Features.
Note The ANSI code pages can be different on different computers, or can be changed for a single computer, leading to data corruption. For the most consistent results, applications should use Unicode, such as UTF-8 or UTF-16, instead of a specific code page, unless legacy standards or data formats prevent the use of Unicode. If using Unicode is not possible, applications should tag the data stream with the appropriate encoding name when protocols allow it. HTML and XML files allow tagging, but text files do not.
This solution still has the guess the encoding problem, but if a BOM is present, this is one of the best translators possible.
Simply require the text file to be saved in the local code page.
Other thoughts:
ANSI, ASCII, and UTF-8 are all separate encodings above 127 and the control characters are handled differently.
In UTF-16 every other byte(zero first) of ASCII encoded text is 0. This is not covered in your "rules".
You simply have to search for the Turkish i to understand the complexities of Unicode translations and comparisons.
Leverage any expectations of the file contents to establish a coherent baseline comparison to make an educated guess.
For example, if it is a .csv file, find a comma in the various formats...
Bottom Line
There is no perfect general solution, only specific solutions tailored to your specific needs, which were extremely broad in the question.
This may be a stupid question, but compressing an empty file doesn't make any sense right?. The Huffman encoding algorithm on an empty file wouldn't work because it relies on the fact there have to be at least 2 nodes in the priority queue. If we run the algorithm on an empty file, the only node we would get is the one corresponding to EOF.
Ya, that's right, it doesn't make much sense to run the Huffman encoding on it. Depending on the details of the implementation, it may not crash.
But why would you try to compress an empty file?
You need to somehow encode at the start of the compressed data what symbols correspond to what Huffman codes. It is in that representation that the number of symbols would be indicated. If there is only one symbol, which has to be EOF per your description, then the Huffman coding is implied to be zero bits. If there is only one symbol, then you need zero bits to represent it.
I got a script that is no longer supported and I'm looking for a way to change the value of a variable in it... The script is encrypted (loadstring/bytecode/something like that) e.g.: loadstring('\27\76\117\97\81\0\1\4\4\4\8\0\')
I can find what I want to change (through notepad after I compile the script), but if I try to change the value, the script won't work, if I change and try to recompile it still won't work: "luac: Testing09.lua: unexpected end in precompiled chunk" ...
Any ideas? I did something like that with a program long a go using ollydbg but I can't use it with lua scripts... I'm kinda lost here, doing some Googling for quite a while couldn't find a way... Any ideas?
It is easy to change a string in a Lua bytecode. You just have to adjust the length of the string after you change it. The length comes before the string. It probably takes four or eight bytes just before the string, depending on whether you have a 32-bit or 64-bit platform. The length is stored in the endianness of the machine where the bytecode was generated. Note that strings include a trailing '\0' and this counts in the length.
Perhaps it is easier to just copy some bytes directly. Write this file
return "this is the new string you want"
Generate bytecode from it with luac and look at an dump of luac.out and locate the string and its length. Copy those bytes to the original file.
I don't know whether notepad handles binary data. if it doesn't, you'll need an hex editor to do this.
Another solution is to write a Lua program that reads the bytecode as a strings, generate bytecode for return "this is the new string you want", perform the change in the original bytecode using string operations and write it back to file.
You can also try my bytecode inspector library lbci, which allows you to change constants in functions. You'd load the bytecode (but not execute it), and use setconstant after locating the constant that has the string you want to change.
In all, there is some fun to be had here...
For a work project I am using headless Squeak on a (displayless, remote) Linuxserver and also using Squeak on a Windows developer-machine.
Code on the developer machine is managed using Monticello. I have to copy the mcz to the server using SFTP unfortunately (e.g. having a push-repository on the server is not possible for security reasons). The code is then merged by eg:
MczInstaller installFileNamed: 'name-b.18.mcz'.
Which generally works.
Unfortunately our code-base contains strings that contain Umlauts and other non-ascii characters. During the Monticello-reimport some of them get replaced with other characters and some get replaced with nothing.
I also tried e.g.
MczInstaller installStream: (FileStream readOnlyFileNamed: '...') binary
(note .mcz's are actually .zip's, so binary should be appropriate, i guess it is the default anyway)
Finding out how to make Monticello's transfer preserve the Squeak internal-encoding of non-ascii's is the main Goal of my question. Changing all the source code to only use ascii-strings is (at least in this codebase) much less desirable because manual labor is involved. If you are interested in why it is not a simple grep-replace in this case read this side note:
(Side note: (A simplified/special case) The codebase uses Seaside's #text: method to render strings that contain chars that have to be html-escaped. This works fine with our non-ascii's e.g. it converts ä into ä, if we were to grep-replace the literal ä's by ä explicitly, then we would have to use the #html: method instead (else double-escape), however that would then require that we replace all other characters that have to be html-escaped as well (e.g. &), but then again the source-code itself contains such characters. And there are other cases, like some #text:'s that take third-party strings, they may not be replaced by #html's...)
Squeak does use unicode (ISO 10646) internally for encoding characters in a String.
It might use extension like CP1252 for characters in range 16r80 to: 16r9F, but I'm not really sure anymore.
The characters codes are written as is on the stream source.st, and these codes are made of a single byte for a ByteString when all characters are <= 16rFF. In this case, the file should look like encoded in ISO-8859-L1 or CP1252.
If ever you have character codes > 16rFF, then a WideString is used in Squeak. Once again the codes are written as is on the stream source.st, but this time these are 32 bits codes (written in big-endian order). Technically, the encoding is thus UTF-32BE.
Now what does MczInstaller does? It uses the snapshot/source.st file, and uses setConverterForCode for reading this file, which is either UTF-8 or MacRoman... So non ASCII characters might get changed, and this is even worse in case of WideString which will be re-interpreted as ByteString.
MC itself doesn't use the snapshot/source.st member in the archive.
It rather uses the snapshot.bin (see code in MCMczReader, MCMczWriter).
This is a binary file whose format is governed by DataStream.
The snippet that you should use is rather:
MCMczReader loadVersionFile: 'YourPackage-b.18.mcz'
Monticello isn't really aware of character encoding. I don't know the present situation in squeak but the last time I've looked into it there was an assumed character encoding of latin1. But that would mean it should work flawlessly in your situation.
It should work somehow anyway if you are writing and reading from the same kind of image. If the proper character encoding fails usually the internal byte representation is written from memory to disk. While this prevents any cross dialect exchange of packages it should work if using the same image kind.
Anyway there are things that should or could work but they often go wrong. So most projects try to avoid using non 7bit characters in their code.
You don't need to convert non 7bit characters to HTML entities. You can use
Character value: 228
for producing an ä in your code without using non 7bit characters. On every character you like to add a conversion you can do
$ä asciiValue => 228
I know this is not the kind of answer some would want to get. But monticello is one of these things that still need to be adjusted for proper character encoding.
I'm downloading a CSV from Google Docs and in it characters like “ are saved as \xE2\x80\x9C and ” are saved as \xE2\x80\x9D.
My question is... what charset are those being saved in? How might I go about figuring that out?
It is in UTF-8.. You can tell by decoding it as UTF-8 and it shows the correct characters.
UTF-8 also has a unique and very distinctive pattern, just 3 bytes with highest bit set forming a valid UTF-8 sequence are enough to tell if something is UTF-8 with 99% confidence. Even with 2 bytes with highest bit set forming a valid UTF-8 sequence, you can already get to 90%.
In a case it wasn't UTF-8, and was some 8-bit code page instead, it would be impossible to tell just by looking at the bytes alone. Without any other information, you would basically have to brute force by decoding it in various 8-bit encodings and then seeing if it looks correct. The other possibility is using an algorithm that would go through the encodings automatically, and see if it the result makes sense in any language.
With more information like what operating system and locale the file was saved in, you could reduce the amount of possible encodings to try by a huge deal though.