如何将NSString溢出到多个八位字节中

Question

I want to split a NSString into NSStrings that have no more than 75 octets (in UTF8 representation) each.

If my NSString would be completely ascii it would be a no-brainer.

But since the resulting string could have any length between 18 and 75 characters I have no real idea how to do it.

what's the way to do this?

Convert the string into octets, take the first 75, convert it back to NSString and hope that NSString tells me that I've ripped an utf8-character into two parts?

Answer 1

the answer of warren showed me that I was on the right track. This is what I came up with:

- (NSString *)foldString:(NSString *)string withOctetCount:(NSInteger)octetCount {
    NSMutableString *output = [NSMutableString string];
    NSData *stringData = [string dataUsingEncoding:NSUTF8StringEncoding allowLossyConversion:YES];
    NSInteger convertedBytes = 0;
    while (convertedBytes < [stringData length]) {
        NSString *subString = nil;
        NSInteger usedLength = octetCount;
        if (convertedBytes > 0) {
            // all lines after the first one get a space as prefix. so use one octet less in those lines
            usedLength--;
        }
        if (convertedBytes + usedLength > [stringData length]) {
            usedLength = [stringData length] - convertedBytes;
        }
        while (!subString) {
            NSData *data = [stringData subdataWithRange:NSMakeRange(convertedBytes, usedLength)];
            subString = [[[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding] autorelease];
            if (!subString) {
                usedLength--;
                if (usedLength == 0) {
                    // TODO: remove abort
                    abort();
                    return nil;
                }
            }
        }
        //      NSLog(@"Used %d octets", usedLength);
        if (convertedBytes == 0) {
            // Dont prefix with space on first line
            [output appendString:subString];
        }
        else {
            [output appendFormat:@"\r\n %@", subString];
        }
        convertedBytes += usedLength;
    }
    return output;
}

Answer 2

NSString has

- (NSData *)dataUsingEncoding:(NSStringEncoding)encoding allowLossyConversion:(BOOL)flag

gives a unterminated C string but allows loss of chars

- (const char *)cStringUsingEncoding:(NSStringEncoding)encoding

will attempt to give you a C string but barf to NULL if it cant do it losslessly

which could be a way to detect that you are about to break up a pair.