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up vote 2 down vote favorite
1

I'm loading a website HTML using this call -

    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
    [request setValue:@"utf-8" forHTTPHeaderField:@"Accept-Encoding"];
    [request setValue:@"text/html" forHTTPHeaderField:@"Accept"];
    [NSURLConnection sendAsynchronousRequest:request
                                       queue:[NSOperationQueue currentQueue]
                           completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) { ... }

and then, to convert NSData into NSString, I need to know the encoding, so I call -

NSString *textEncoding = [response textEncodingName];

from the code block, but it returns nil on websites that won't specify "Content-Encoding" header field.

If I don't know the encoding, [[NSString alloc] initWithData:data encoding:responseEncoding] won't give me readable HTML.

How can I detect the right encoding for websites that don't send "Content-Encoding" header field?

share|improve this question
up vote 2 down vote accepted

It is possible to try different encodings and see which one results with readable text -

static int encodingPriority[] = {
    NSUTF8StringEncoding,
    NSASCIIStringEncoding,
    NSISOLatin1StringEncoding,
    NSISOLatin2StringEncoding,
    NSUnicodeStringEncoding,
    NSWindowsCP1251StringEncoding,
    NSWindowsCP1252StringEncoding,
    NSWindowsCP1253StringEncoding,
    NSWindowsCP1254StringEncoding,
    NSWindowsCP1250StringEncoding,
    NSNEXTSTEPStringEncoding,
    NSJapaneseEUCStringEncoding,
    NSNonLossyASCIIStringEncoding,
    NSShiftJISStringEncoding,          /* kCFStringEncodingDOSJapanese */
    NSISO2022JPStringEncoding,        /* ISO 2022 Japanese encoding for e-mail */
    NSMacOSRomanStringEncoding,
    NSUTF16BigEndianStringEncoding,
    NSUTF16LittleEndianStringEncoding,
    NSUTF32StringEncoding,
    NSUTF32BigEndianStringEncoding,
    NSUTF32LittleEndianStringEncoding
};

#define REQUIRED_HTML_STRING    @"<html"

- (NSString *)htmlStringForUnknownEncodingData:(NSData *)data detectedEncoding:(NSStringEncoding *)detectedEncoding
{
    NSStringEncoding encoding;
    NSString *html;

    for (int i = 0; i < sizeof(encodingPriority); i++) {
        encoding = encodingPriority[i];

        // try this encoding
        html = [[NSString alloc] initWithData:data encoding:encoding];

        // we need to find a text, because bad encoding will return an unreadable text
        if (html && [html rangeOfString:REQUIRED_HTML_STRING options:NSCaseInsensitiveSearch].location != NSNotFound) {
            *detectedEncoding = encoding;
            return html;
        }
    }
    return nil;
}

then, to detect which encoding the HTML in your NSData is using, call -

NSStringEncoding encoding;
html = [self htmlStringForUnknownEncodingData:data detectedEncoding:&encoding];

if (html)
    NSLog("Encoding detected!");
else
    NSLog("No encoding detected");
share|improve this answer
up vote 0 down vote

I tried the code from @Kof. I have noticed I got encoding from response is utf-8 though. If you set encoding to [[NSString alloc] initWithData:data encoding:@"utf-8"] directly, it will definitely return null. It is because encoding accept type NSStringEncoding which has type of NSENUM. If you try [NSString alloc] initWithData:data encoding:NSUTF8StringEncoding, it will return you the result.

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