String from NSInputStream is not valid utf8. How to convert to utf8 more 'lossy'

Question

I have an App that reads data from a server. Now and then, the data appears to be not valid UTF-8. If I convert from the byte array to an UTF8-String, the string appears nil. There must be some invalid not-UTF8 character in the byte array. Is there a way to 'lossy' convert the byte array to UTF8 and filter out only the invalid characters?

Any ideas?

My code looks like this:

- (void)stream:(NSStream *)theStream handleEvent:(NSStreamEvent)streamEvent {

switch (streamEvent){
    case NSStreamEventHasBytesAvailable:
    {
        uint8_t buffer[1024];
        int len;
        NSMutableData * inputData = [NSMutableData data];
        while ([directoryStream hasBytesAvailable]){
            len = [directoryStream read:buffer maxLength:sizeof(buffer)];
            if (len> 0) {
                [inputData appendBytes:(const void *)buffer length:len];
            }
        }
        NSString *directoryString = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
    }
    NSLog(@"directoryString: %@", directoryString);

    ...

Is there a way to do this conversion in a more 'lossy' way?

As you see I first append the chunks of data to an NSData value and do the conversion to utf8 when everything is read. This prevents that the (multi-byte) utf8 characters are split up resulting in even more invalid (empty) utf8 strings.

Answer 1

I created an NSString category with a -[validUTF8String] method which, in case UTF8String returns NULL, strips invalid surrogate characters then calls UTF8String on that cleaned string:

@interface NSString (ValidUTF8String)

- (const char *)validUTF8String;
- (NSString *)stringByStrippingInvalidUnicode;  // warning: very inefficient! should only be called when we are sure that the string contains invalid Unicode, e.g. when -[UTF8String] is NULL

@end

@implementation NSString (ValidUTF8String)

- (const char *)validUTF8String;
{
    const char *result=[self UTF8String];
    if (!result)
    {
        result=[[self stringByStrippingInvalidUnicode] UTF8String];
        if (!result)
            result="";
    }
    return result;
}

#define isHighSurrogate(k)  ((k>=0xD800) && (k<=0xDBFF))
#define isLowSurrogate(k)   ((k>=0xDC00) && (k<=0xDFFF))

- (NSString *)stringByStrippingInvalidUnicode
{
    NSMutableString *fixed=[[self mutableCopy] autorelease];
    for (NSInteger idx=0; idx<[fixed length]; idx++)
    {
        unichar k=[fixed characterAtIndex:idx];
        if (isHighSurrogate(k))
        {
            BOOL nextIsLowSurrogate=NO;
            if (idx+1<[fixed length])
            {
                unichar nextK=[fixed characterAtIndex:idx+1];
                nextIsLowSurrogate=isLowSurrogate(nextK);
            }
            if (!nextIsLowSurrogate)
            {
                [fixed deleteCharactersInRange:NSMakeRange(idx, 1)];
                idx--;
            }
        }
        else if (isLowSurrogate(k))
        {
            BOOL previousWasHighSurrogate=NO;
            if (idx>0)
            {
                unichar previousK=[fixed characterAtIndex:idx-1];
                previousWasHighSurrogate=isHighSurrogate(previousK);
            }
            if (!previousWasHighSurrogate)
            {
                [fixed deleteCharactersInRange:NSMakeRange(idx, 1)];
                idx--;
            }
        }
    }
    return fixed;
}

@end

Answer 2

It works! By combining the code snippet from Larme and the comment about the size of UTF-8 characters I managed to create a 'lossy' NSData to UTF-8 NSString conversion method.

+ (NSString *) data2UTF8String:(NSData *) data {

    // First try to do the 'standard' UTF-8 conversion 
    NSString * bufferStr = [[NSString alloc] initWithData:data
                                                 encoding:NSUTF8StringEncoding];

    // if it fails, do the 'lossy' UTF8 conversion
    if (!bufferStr) {
        const Byte * buffer = [data bytes];

        NSMutableString * filteredString = [[NSMutableString alloc] init];

        int i = 0;
        while (i < [data length]) {

            int expectedLength = 1;

            if      ((buffer[i] & 0b10000000) == 0b00000000) expectedLength = 1;
            else if ((buffer[i] & 0b11100000) == 0b11000000) expectedLength = 2;
            else if ((buffer[i] & 0b11110000) == 0b11100000) expectedLength = 3;
            else if ((buffer[i] & 0b11111000) == 0b11110000) expectedLength = 4;
            else if ((buffer[i] & 0b11111100) == 0b11111000) expectedLength = 5;
            else if ((buffer[i] & 0b11111110) == 0b11111100) expectedLength = 6;

            int length = MIN(expectedLength, [data length] - i);
            NSData * character = [NSData dataWithBytes:&buffer[i] length:(sizeof(Byte) * length)];

            NSString * possibleString = [NSString stringWithUTF8String:[character bytes]];
            if (possibleString) {
                [filteredString appendString:possibleString];
            }
            i = i + expectedLength;
        }
        bufferStr = filteredString;
    }

    return bufferStr;
}

If you have any comments, please let me know. Thanks Larme!