/**
 *  GZip压缩数据
 *
 *  @param aUnData 未压缩数据
 *
 *  @return 已压缩数据
 */
+ (NSData *)toGZipCompressData:(NSData *)aUnData {
    if (![aUnData isKindOfClass:[NSData class]]) {
        return nil;
    }
    
    @try {
        if (aUnData.length == 0 || [self isGzippedData:aUnData]) {
            return aUnData;
        }
        
        z_stream stream;
        stream.zalloc = Z_NULL;
        stream.zfree = Z_NULL;
        stream.opaque = Z_NULL;
        stream.avail_in = (uint) aUnData.length;
        stream.next_in = (Bytef *) (void *) aUnData.bytes;
        stream.total_out = 0;
        stream.avail_out = 0;
        
        static const NSUInteger ChunkSize = 16384;
        
        NSMutableData *output = nil;
        if (deflateInit2(&stream, Z_DEFAULT_COMPRESSION, Z_DEFLATED, 31, 8, Z_DEFAULT_STRATEGY) == Z_OK) {
            output = [NSMutableData dataWithLength:ChunkSize];
            while (stream.avail_out == 0) {
                if (stream.total_out >= output.length) {
                    output.length += ChunkSize;
                }
                stream.next_out = (uint8_t *) output.mutableBytes + stream.total_out;
                stream.avail_out = (uInt)(output.length - stream.total_out);
                deflate(&stream, Z_FINISH);
            }
            deflateEnd(&stream);
            output.length = stream.total_out;
        }
        
        return output;
    }
    @catch (NSException *exception) {
        NSLog(@"%@", exception);
        return nil;
    }
}

+ (BOOL)isGzippedData:(NSData *)aData {
    const UInt8 *bytes = (const UInt8 *) aData.bytes;
    return (aData.length >= 2 && bytes[0] == 0x1f && bytes[1] == 0x8b);
}