Compress the passed chunk of data

compressBytes.h

@interface compressBytes : NSObject {
z_stream zStream;
}
- (NSData *)compressBytes:(Bytef *)bytes length:(NSUInteger)length error:(NSError **)err shouldFinish:(BOOL)shouldFinish;

compressBytes.m

@implementation compressBytes
- (NSData *)compressBytes:(Bytef *)bytes length:(NSUInteger)length error:(NSError **)err shouldFinish:(BOOL)shouldFinish
{
	if (length == 0) return nil;
	
	NSUInteger halfLength = length/2;
	
	NSMutableData *outputData = [NSMutableData dataWithLength:length/2]; 
	
	int status;
	
	zStream.next_in = bytes;
	zStream.avail_in = (unsigned int)length;
	zStream.avail_out = 0;

	NSUInteger bytesProcessedAlready = zStream.total_out;
	while (zStream.avail_out == 0) {
		
		if (zStream.total_out-bytesProcessedAlready >= [outputData length]) {
			[outputData increaseLengthBy:halfLength];
		}
		
		zStream.next_out = (Bytef*)[outputData mutableBytes] + zStream.total_out-bytesProcessedAlready;
		zStream.avail_out = (unsigned int)([outputData length] - (zStream.total_out-bytesProcessedAlready));
		status = deflate(&zStream, shouldFinish ? Z_FINISH : Z_NO_FLUSH);
		
		if (status == Z_STREAM_END) {
			break;
		} else if (status != Z_OK) {
			if (err) {
				*err = [[self class] deflateErrorWithCode:status];
			}
			return NO;
		}
	}

	//length of data
	[outputData setLength: zStream.total_out-bytesProcessedAlready];
	return outputData;
}
@end