String(data:encoding:) doesn't seem to work with "advanced" encodings


The String.init?(data:encoding:) initializer seems to not work properly (or at all) with, what I call, "advanced" encodings. By "advanced" encodings I mean encodings that can be (only?) accessed via String.Encoding.init(rawValue:) initializer.

Let's discuss two examples. The first example is the one character string ":relaxed:" (U+263A unicode scalar). According to this wikipedia article, this string can be encoded as one byte 0x01 using CP437 aka DOS Latin-US encoding. As I understand from the output of { String.localizedName(of: $0) } this encoding is present in Swift as String.Encoding(rawValue: 0x8000_0400) (assuming that "Latin-US (DOS)", as reported by localizedName(of:), is indeed the same thing as CP437). So by doing the following:

let cp437 = String.Encoding(rawValue: 0x80000400) 
print(String(data: Data([0x01]), encoding: cp437)!)

I expect to get my smiley face, but instead I get "\u{1}", which is the exact same output you would get if you instead tried to use .utf8 encoding.

The second example is the following: "А" (Cyrillic letter, U+0410 unicode scalar). Using the CP866 aka Cyrillic DOS encoding it is encoded as one byte 0x80. In Swift this encoding can be found as String.Encoding(rawValue: 0x8000_0413). Doing a similar exercise:

let cp866 = String.Encoding(rawValue: 0x8000_0413)
print(String(data: Data([0x80]), encoding: cp866)!)

I get a mysterious "ђ" output, instead of my simple letter А.

So, I guess, my question is: am I missing something or is this a bug?

String.Encoding is a bridged representation of the various NS<*>StringEncoding values from Foundation. It does not support any encoding that isn't already represented as a static value. Its init(rawValue:) is only exposed due to it's public conformance to RawRepresentable. Most likely that initializer defaults to a particular encoding when you pass it a value it doesn't support. There is a bunch of CFStringEncodings that aren't exposed through String.Encoding, including .dosLatinUS, but I'll leave that conversion to you.

Well, that means that String.availableStringEncodings is at best misleading, doesn't it?

Regardless, I am aware of CFStringEncodings. I've been using them in the past for this exact purpose, but since, as I understand, using CoreFoundation is discouraged on non-darwin (and maybe non-linux) platforms, I am trying other ways to access "advanced" encodings.

Anyhow, I tried to use CFStringEncodings again right now:

let CfCp437 = UInt32(truncatingIfNeeded: CFStringEncodings.dosLatinUS.rawValue)
print(CFStringIsEncodingAvailable(CfCp437)) // prints "true"
let cfstring = CFStringCreateWithCString(nil, [0x01, 0x00], CfCp437)
print(cfstring) // prints "Optional()"

let convertedToNsCfCp437 = CFStringConvertEncodingToNSStringEncoding(CfCp437)
print(convertedToNsCfCp437) // prints "2147484672" which is exactly 0x8000_0400
print(NSString(data: Data([0x01]), encoding: convertedToNsCfCp437)) // prints "Optional()"

Note, that CoreFoundation's CP437 encoding converts seemingly into the exact same encoding that I've been trying to use in my original post. But more importantly, the CFStringEncodings doesn't seem to work either.

You're right, it does appear to be an available value, and the instance created from the raw value does seem to be the correct one. In my testing applying the encoding does have different results.

let data = Data("☺️".utf8)
let encoding = String.Encoding(rawValue: 0x80000400)
print(String(data: data, encoding: encoding))
print(String(data: data, encoding: .utf8))
print(NSString(data: data, encoding: encoding.rawValue))
print(NSString(data: data, encoding: String.Encoding.utf8.rawValue))

results in


Whether any of that is correct I don't know.

Well, technically, this is correct, but I was trying to perform the reverse operation: using the encoding decode back into a string the byte representation of the smiley face in this encoding. Basically, convert 0x01 into

I've also noticed, that the encoding works correctly for some bytes, but not the others. For example,

print(String(data: Data([0xe2, 0x98, 0xba, 0x7f, 0x15]), encoding: cp437)!)

prints "Γÿ║ " instead of expected "Γÿ║⌂§" (I had to use quotation marks when typing these to show whitespaces in the first result). Perhaps, the issue is with those bytes that are normal characters in CP437, but are control characters in UTF-8/ASCII (in this example, the last two characters are DEL and NAK correspondingly in UTF-8/ASCII). So it looks like it mixes two encodings, but, honestly, I have no idea what is happening and why.

I do not know if this table is the correct source, but it maps 0x01 to U+0001 and seems to be consistent with your other results:

0x01	0x0001	#START OF HEADING
0x7f	0x007f	#DELETE

Hmm, the link from Unicode seems to be a more authoritative source than Wikipedia by itself, which makes Wikipedia wrong.

However, Wikipedia cites specifications from IBM, the authors of this encoding:

These files corroborate the table from Wikipedia, and contradict the file from Unicode.

So, I don't know who is right in this case.

why are you doing this to begin with? is it to be compatible with documents created on pre-unicode systems or to save a few bytes or what?

The former. In particular, ZIP file format's default encoding for string fields is CP437, and while it has facilities to support UTF-8, by nature of being a format used for archiving purposes, when parsing ZIP archives one should always expect to encounter a file created a long time ago, before UTF-8 support was clarified in the spec.

Anyhow, by looking further on the Internet, it seems CP437 is special in a sense, that some of its ranges can be interpreted as either control characters or normal character "depending on context". This last part is a bit vague, but I guess in light of this one can say that String(data:encoding:) is allowed to interpret them however it wants (i.e. not as normal characters).

With regards to the second example from my original post, apparently, it is actually CP855, not CP866 encoding hiding behind the name of "Cyrillic (DOS)" and it works as expected:

let cp855 = String.Encoding(rawValue: 0x8000_0413)
print(String.localizedName(of: cp855)) // prints "Cyrillic (DOS)"
print(String(data: Data([0xDD, 0xE1, 0xB7, 0xEB, 0xA8, 0xE5]), encoding: cp855)!) // prints "Привет"
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