Re: compression of NSData... how to?

FROM : Andrew Merenbach
DATE : Tue Aug 29 16:46:42 2006

I've not used NSAssert() much, but might there not be a memory leak 
if it raises an NSInternalInconsistencyException (which, according 
the docs, it does)?  Your [compData release] call occurs only after 
the block in which NSAssert() is called.

Perhaps an exception block with @finally would prove useful here?

Besides that (which is possibly moot, anyway, for as I say, I have 
not used NSAssert() much), this is a really useful example to have!

Cheers,
   Andrew

On Aug 29, 2006, at 3:01 AM, Fredrik Olsson wrote:

> Benjámin Salánki skrev:

>> Hey there,
>>
>> I'm looking for a way to compress my NSData before writing it to 
>> file and then of course decompress it after reading it from file.
>>
>> I looked at zlib, but to tell you the truth I couldn't get it to 
>> work.
>>
>> Is there a sample code snippet that I can take a look at? Can 
>> anyone suggest a way to do this?
>>

> You can have mine, I have added compression and decompression as a 
> category to NSData, do not forget to link with zlib by adding -lz 
> to other linker flags. Change the AUTORELEASE and RELEASE macros to 
> normal release and autorelease messages if you need to, I jusy use 
> these macros not to forget to set pointers to nil for unused objects.
>
> - (NSData *) compressedData
> {
>    NSData *result = nil;
>    unsigned int srcLength = [self length];
>    if (srcLength > 0) {
>        uLong buffLength = srcLength * 1.001 + 12;
>        NSMutableData *compData = [[NSMutableData alloc] 
> initWithCapacity:buffLength];
>        [compData increaseLengthBy:buffLength];
>        int error = compress([compData mutableBytes], &buffLength, 
> [self bytes], srcLength);
>        switch( error ) {
>            case Z_OK:
>                [compData setLength: buffLength];
>                COPY(result, compData);
>                break;
>            default:
>                NSAssert(NO, @"Error compressing: Memory Error!");
>                break;
>        }
>        RELEASE(compData);
>    }
>    AUTORELEASE(result);
>    return result;
> }
>
> - (NSData *) decompressedData
> {
>    if ([self length] == 0) return self;
>      unsigned full_length = [self length];
>    unsigned half_length = [self length] / 2;
>      NSMutableData *decompressed = [NSMutableData dataWithLength: 
> full_length + half_length];
>    BOOL done = NO;
>    int status;
>      z_stream strm;
>    strm.next_in = (Bytef *)[self bytes];
>    strm.avail_in = [self length];
>    strm.total_out = 0;
>    strm.zalloc = Z_NULL;
>    strm.zfree = Z_NULL;
>    strm.opaque = Z_NULL;
>      if (inflateInit(&strm) != Z_OK) {
>        return nil;
>    }
>    while (!done) {
>        // Make sure we have enough room and reset the lengths.
>        if (strm.total_out >= [decompressed length]) {
>            [decompressed increaseLengthBy: half_length];
>        }
>        strm.next_out = [decompressed mutableBytes] + strm.total_out;
>        strm.avail_out = [decompressed length] - strm.total_out;
>              // Inflate another chunk.
>        status = inflate (&strm, Z_SYNC_FLUSH);
>        if (status == Z_STREAM_END) {
>            done = YES;
>        } else if (status != Z_OK) {
>            break;
>        }
>    }
>    if (inflateEnd (&strm) != Z_OK) {
>        return nil;
>    }
>      // Set real length.
>    if (done) {
>        [decompressed setLength: strm.total_out];
>        return [NSData dataWithData: decompressed];
>    } else {
>        return nil;
>    }
> }
>
> // Fredrik Olsson

>> Thanks,
>> Ben
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