I don't know what the original code is, so I assume that the original code is IBM850 or ISO8859-1.My process below
IBM850 -> UTF8
if this is OK, I consider the original code is IBM850, if NOK,do next step:
ISO8859-1 -> UTF8
if this is OK, I consider the original code is UTF8.
But there is a problem，
if the original code is ISO8859-1, it will be recognised to IBM850.
if the original code is IBM850, it will be recognised to ISO8859-1.
It seems that there are common ground between IBM850 and ISO8859-1.
Who can help me, thanks.
Yes, only the most trivial kind of autodetection is possible by testing whether conversion fails or succeeds. It's not going to work for input encodings where (almost) any input is valid.
You should know something more about your likely output, to test whether if it makes more sense after translating from IBM850 or from ISO8859-1. That's what enca and libenca do. You can probably start with some simple expectations to check:
Does your source happen to be within the ASCII subset of both encodings? Then you're happy with any conversion (but you have no way to know the original encoding at all).
Does your code use box drawing characters? If it does not, it would be easy to reject some candidates for IBM850.
Does your code use control characters from ISO8859-1? If it does not, it would be easy to reject some candidates for ISO8859-1 if codepoints 0x80-0x9F are used.
Do the fragments of your code which are non-ASCII always represent a text in a natural language? Then you can use frequency tables for characters and their pairs, selecting the source encoding which makes the result closer to your natural language(s) on these criteria. (If both variants are almost equally acceptable, it's probably better to give an error message and leave the final decision to humans).
Related
I have a legacy app in Perl processing XML encoded in UTF-8 most likely and which needs to store some data of that XML in some database, which uses windows-1252 for historical reasons. Yes, this setup can't support all possible characters of the Unicode standard, but in practice I don't need to anyway and can try to be reasonable compatible.
The specific problem currently is a file containing LATIN SMALL LETTER U, COMBINING DIAERESIS (U+0075 U+0308), which makes Perl break the existing encoding of the Unicode string to windows-1252 with the following exception:
"\x{0308}" does not map to cp1252
I was able to work around that problem using Unicode::Normalize::NFKC, which creates the character U+00FC (ü), which perfectly fine maps to windows-1252. That lead to some other problem of course, e.g. in case of the character VULGAR FRACTION ONE HALF (½, U+00BD), because NFKC creates DIGIT ONE, FRACTION SLASH, DIGIT TWO (1/2, U+0031 U+2044 U+0032) for that and Perl dies again:
"\x{2044}" does not map to cp1252
According to normalization rules, this is perfectly fine for NFKC. I used that because I thought it would give me the most compatible result, but that was wrong. Using NFC instead fixed both problems, as both characters provide a normalization compatible with windows-1252 in that case.
This approach gets additionally problematic for characters for which a normalization compatible with windows-1252 is available in general, only different from NFC. One example is LATIN SMALL LIGATURE FI (fi, U+FB01). According to it's normalization rules, it's representation after NFC is incompatible with windows-1252, while using NFKC this time results in two characters compatible with windows-1252: fi (U+0066 U+0069).
My current approach is to simply try encoding as windows-1252 as is, if that fails I'm using NFC and try again, if that fails I'm using NFKC and try again and if that fails I'm giving up for now. This works in the cases I'm currently dealing with, but obviously fails if all three characters of my examples above are present in a string at the same time. There's always one character then which results in windows-1252-incompatible output, regardless the order of NFC and NFKC. The only question is which character breaks when.
BUT the important point is that each character by itself could be normalized to something being compatible with windows-1252. It only seems that there's no one-shot-solution.
So, is there some API I'm missing, which already converts in the most backwards compatible way?
If not, what's the approach I would need to implement myself to support all the above characters within one string?
Sounds like I would need to process each string Unicode-character by Unicode-character, normalize individually with what is most compatible with windows-1252 and than concatenate the results again. Is there some incremental Unicode-character parser available which deals with combining characters and stuff already? Does a simple Unicode-character based regular expression handles this already?
Unicode::Normalize provides additional functions to work on partial strings and such, but I must admit that I currently don't fully understand their purpose. The examples focus on concatenation as well, but from my understanding I first need some parsing to be able to normalize individual characters differently.
I don't think you're missing an API because a best-effort approach is rather involved. I'd try something like the following:
Normalize using NFC. This combines decomposed sequences like LATIN SMALL LETTER U, COMBINING DIAERESIS.
Extract all codepoints which aren't combining marks using the regex /\PM/g. This throws away all combining marks remaining after NFC conversion which can't be converted to Windows-1252 anyway. Then for each code point:
If the codepoint can be converted to Windows-1252, do so.
Otherwise try to normalize the codepoint with NFKC. If the NFKC mapping differs from the input, apply all steps recursively on the resulting string. This handles things like ligatures.
As a bonus: If the codepoint is invariant under NFKC, convert to NFD and try to convert the first codepoint of the result to Windows-1252. This converts characters like Ĝ to G.
Otherwise ignore the character.
There are of course other approaches that convert unsupported characters to ones that look similar but they require to create mappings manually.
Since it seems that you can convert individual characters as needed (to cp-1252 encoding), one way is to process character by character, as proposed, once a word fails the procedure.
The \X in Perl's regex matches a logical Unicode character, an extended grapheme cluster, either as a single codepoint or a sequence. So if you indeed can convert all individual (logical) characters into the desired encoding, then with
while ($word =~ /(\X)/g) { ... }
you can access the logical characters and apply your working procedure to each.
In case you can't handle all logical characters that may come up, piece together an equivalent of \X using specific character properties, for finer granularity with combining marks or such (like /((.)\p{Mn}?)/, or \p{Nonspacing_Mark}). The full, grand, list is in perluniprops.
I've been studying RFC1951 and 'puff.c', and have a question about the issue of "incomplete length".
As near I can tell, defining a "dynamic" Huffman code table that allows for more codes than specified by HLIT+257 will produce an error, at least by puff.c. For example, an error is produced by 'puff.c' if, as a simple debugging test, I were to use a Huffman table of all 9-bit codes to define only 257 lit/lens. Is this outcome purposeful or a bug? And can I assume that any "inflator" based on the 'zlib' library will produce the same error?
I can't find any specification in RFC 1951 that should REQUIRE the use of a sufficiently tight Huffman code. Certainly, I can see that using an "under-subscribed" Huffman table might be inefficient, in terms of compression, but I'm not sure why such a table should be prohibited from use.
My interest isn't simply hypothetical. I really want to use an under-subscribed, literal-only, Huffman code (but NOT the example cited above) to compress some application specific images into PNG files. But I want to make sure it will work with any PNG image viewer.
The RFC specifies that the codes are Huffman codes, which by definition are complete codes. (Complete means that all bit patterns are used.)
zlib will reject incomplete or oversubscribed codes, except in the special case noted in the RFC:
If only one distance code is used, it is encoded using one bit, not
zero bits; in this case there is a single code length of one, with one
unused code.
There the incomplete code 0 for the single symbol, with code 1 unused, is permitted.
(That, by the way, is unnecessary. If there is only one distance symbol, then you don't need any bits to specify it. You know that that distance symbol must be used with any length. If that symbol needs extra bits, then those extra bits immediately follow the length. But, oh well -- for that case Phil Katz put an extraneous zero bit in every match, and now we're stuck with it.)
The fact that the RFC even had to note this special case is another clue that incomplete codes are not accepted otherwise.
There is sort of another exception in deflate, in that the fixed literal/length code is incomplete, with two unused codes at the end.
The bottom line is, no, you will not be able to use an incomplete code in a dynamic header (except the special case) and expect zlib or any compliant deflate decoder to be able to decode it.
As for why this strictness is useful, constraints on dynamic headers permit rapid detection of non-deflate streams or corrupted deflate streams. Similarly, a dynamic header with no end code is not permitted by zlib, so as to avoid the case of a bogus dynamic header permitting any following random bits to be decodable forever, never detecting an error. The unused fixed codes also help in this regard, since eventually they trigger an error in random input.
By the way, if you want to define a fixed, complete Huffman code for your case, it's super simple, and would reduce the size of almost all of your codes by one bit. Just encode eight bits for the symbols 0..253, using that symbol number directly as the code (reversing the bits of course), and nine bits for symbols 254..257, using the codes 508..511 (bits reversed).
In Haskell's Data.Text.Encoding, presuming one presents a pure ASCII ByteString, is decodeLatin1 very much faster than decodeUtf8? Intuitively It seems like there would be at least one more machine instruction given the nature of utf-8 (ie a test of the top bit). I know I could do my own profiling, but I presume this may have been done already so that is why I ask.
Here is the underlying C code that the text library uses internally for the decoder. Specifically the function _hs_text_decode_latin1:
http://hackage.haskell.org/package/text-1.0.0.1/src/cbits/cbits.c
is decodeLatin1 very much faster than decodeUtf8
The answer to this is simply that it shouldn't matter, you should choose to use latin1 decoder if you need to work with existing latin1 text data. Anything else is just microoptimization for almost all cases, the text library is already very heavily optimized.
I am writing an erlang module that has to deal a bit with strings, not too much, however, I do some tcp recv and then some parsing over the data.
While matching data and manipulating strings, I am using binary module all the time like binary:split(Data,<<":">>) and basically using <<"StringLiteral">> all the time.
Till now I have not encounter difficulties or missing methods from the alternative( using lists) and everything is coming out quite naturally except maybe for adding the <<>>, but I was wondering if this way of dealing with strings might have drawbacks I am not aware of.
Any hint?
As long as you and your team remember that your strings are binaries and not lists, there are no inherent problems with this approach. In fact, Couch DB took this approach as an optimization which apparently paid nice dividends.
You do need to be very aware of how your string is encoded in your binaries. When you do <<"StringLiteral">> in your code, you have to be aware that this is simply a binary serialization of the list of code-points. Your Erlang compiler reads your code as ISO-8859-1 characters, so as long as you only use Latin-1 characters and do this consistently, you should be fine, But this isn't very friendly to internationalization.
Most application software these day should prefer a unicode encoding. UTF-8 is compatible with your <<"StringLiteral">> for the first 128 codepoints, but not for the second 128, so be careful. You might be surprised what you see on your UTF-8 encoded web applications if you use <<"StrïngLïteral">> in your code.
There was an EEP proposal for binary support in the form of <<"StrïngLïteral"/utf8>>, but I don't think this is finalized.
Also be aware that your binary:split/2 function may have unexpected results in UTF-8 if there is a multi-byte character that contains the IS0-8859-1 byte that to are splitting on.
Some would argue that UTF-16 is a better encoding to use because it can be parsed more efficiently and can be more easily split by index, if you are assuming or verify that there are no 32-bit characters.
The unicode module should be use, but tread carefully when you use literals.
The only thing to be aware of is that a binary is a slice of bytes, whereas a list is a list of unicode codepoints. In other words, the latter is naturally unicode whereas the former requires you to do some sort of encoding, usually UTF-8.
To my knowledge, there is no drawbacks to your method.
Binaries are very efficient structures to store strings. If they are longer than 64B they are also stored outside process heap so they are not object of GC (still GC'ed by ref counting when last ref lost). Don't forget use iolists for concatenation them to avoid copying when performance matter.
A few days ago, I asked why its not possible to store binary data, such as a jpg file into a string variable.
Most of the answers I got said that string is used for textual information such as what I'm writing now.
What is considered textual data though? Bytes of a certain nature represent a jpg file and those bytes could be represented by character byte values...I think. So when we say strings are for textual information, is there some sort of range or list of characters that aren't stored?
Sorry if the question sounds silly. Just trying to 'get it'
I see three major problems with storing binary data in strings:
Most systems assume a certain encoding within string variables - e.g. if it's a UTF-8, UTF-16 or ASCII string. New line characters may also be translated depending on your system.
You should watch out for restrictions on the size of strings.
If you use C style strings, every null character in your data will terminate the string and any string operations performed will only work on the bytes up to the first null.
Perhaps the most important: it's confusing - other developers don't expect to find random binary data in string variables. And a lot of code which works on strings might also get really confused when encountering binary data :)
I would prefer to store binary data as binary, you would only think of converting it to text when there's no other choice since when you convert it to a textual representation it does waste some bytes (not much, but it still counts), that's how they put attachments in email.
Base64 is a good textual representation of binary files.
I think you are referring to binary to text encoding issue. (translate a jpg into a string would require that sort of pre-processing)
Indeed, in that article, some characters are mentioned as not always supported, other can be confusing:
Some systems have a more limited character set they can handle; not only are they not 8-bit clean, some can't even handle every printable ASCII character.
Others have limits on the number of characters that may appear between line breaks.
Still others add headers or trailers to the text.
And a few poorly-regarded but still-used protocols use in-band signaling, causing confusion if specific patterns appear in the message. The best-known is the string "From " (including trailing space) at the beginning of a line used to separate mail messages in the mbox file format.
Whoever told you you can't put 'binary' data into a string was wrong. A string simply represents an array of bytes that you most likely plan on using for textual data... but there is nothing stopping you from putting any data in there you want.
I do have to be careful though, because I don't know what language you are using... and in some languages \0 ends the string.
In C#, you can put any data into a string... example:
byte[] myJpegByteArray = GetBytesFromSomeImage();
string myString = Encoding.ASCII.GetString(myJpegByteArray);
Before internationalization, it didn't make much difference. ASCII characters are all bytes, so strings, character arrays and byte arrays ended up having the same implementation.
These days, though, strings are a lot more complicated, in order to deal with thousands of foreign language characters and the linguistic rules that go with them.
Sure, if you look deep enough, everything is just bits and bytes, but there's a world of difference in how the computer interprets them. The rules for "text" make things look right when it's displayed to a human, but the computer is free to monkey with the internal representation. For example,
In Unicode, there are many encoding systems. Changing between them makes every byte different.
Some languages have multiple characters that are linguistically equivalent. These could switch back and forth when you least expect it.
There are different ways to end a line of text. Unintended translations between CRLF and LF will break a binary file.
Deep down everything is just bytes.
Things like strings and pictures are defined by rules about how to order bytes.
strings for example end in a byte with value 32 (or something else)
jpg's don't
Depends on the language. For example in Python string types (str) are really byte arrays, so they can indeed be used for binary data.
In C the NULL byte is used for string termination, so a sting cannot be used for arbitrary binary data, since binary data could contain null bytes.
In C# a string is an array of chars, and since a char is basically an alias for 16bit int, you can probably get away with storing arbitrary binary data in a string. You might get errors when you try to display the string (because some values might not actually correspond to a legal unicode character), and some operations like case conversions will probably fail in strange ways.
In short it might be possible in some langauges to store arbitrary binary data in strings, but they are not designed for this use, and you may run into all kinds of unforseen trouble. Most languages have a byte-array type for storing arbitrary binary data.
I agree with Jacobus' answer:
In the end all data structures are made up of bytes. (Well, if you go even deeper: of bits). With some abstraction, you could say that a string or a byte array are conventions for programmers, on how to access them.
In this regard, the string is an abstraction for data interpreted as a text. Text was invented for communication among humans, computers or programs do not communicate very well using text. SQL is textual, but is an interface for humans to tell a database what to do.
So in general, textual data, and therefore strings, are primarily for human to human, or human to machine interaction (say for the content of a message box). Using them for something else (e.g. reading or writing binary image data) is possible, but carries lots of risk bacause you are using the data type for something it was not designed to handle. This makes it much more error prone. You may be able to store binary data in strings, mbut just because you are able to shoot yourself in the foot, you should avoid doing so.
Summary: You can do it. But you better don't.
Your original question (c# - What is string really good for?) made very little sense. So the answers didn't make sense, either.
Your original question said "For some reason though, when I write this string out to a file, it doesn't open." Which doesn't really mean much.
Your original question was incomplete, and the answers were misleading and confusing. You CAN store anything in a String. Period. The "strings are for text" answers were there because you didn't provide enough information in your question to determine what's going wrong with your particular bit of C# code.
You didn't provide a code snippet or an error message. That's why it's hard to 'get it' -- you're not providing enough details for us to know what you don't get.