如何使用zlib gzip NSData？

#import <zlib.h>

// 压缩NSData为gzip格式
- (NSData *)gzipCompressData:(NSData *)data {
    if ([data length] == 0) {
        return nil;
    }
    
    z_stream zlibStreamStruct;
    zlibStreamStruct.zalloc = Z_NULL;
    zlibStreamStruct.zfree = Z_NULL;
    zlibStreamStruct.opaque = Z_NULL;
    zlibStreamStruct.total_out = 0;
    zlibStreamStruct.next_in = (Bytef *)[data bytes];
    zlibStreamStruct.avail_in = (uInt)[data length];
    
    if (deflateInit2(&zlibStreamStruct, Z_DEFAULT_COMPRESSION, Z_DEFLATED, (15+16), 8, Z_DEFAULT_STRATEGY) != Z_OK) {
        return nil;
    }
    
    NSMutableData *compressedData = [NSMutableData dataWithLength:16384];
    do {
        if (zlibStreamStruct.total_out >= [compressedData length]) {
            [compressedData increaseLengthBy:16384];
        }
        
        zlibStreamStruct.next_out = [compressedData mutableBytes] + zlibStreamStruct.total_out;
        zlibStreamStruct.avail_out = (uInt)([compressedData length] - zlibStreamStruct.total_out);
        
        deflate(&zlibStreamStruct, Z_FINISH);
        
    } while (zlibStreamStruct.avail_out == 0);
    
    deflateEnd(&zlibStreamStruct);
    [compressedData setLength:zlibStreamStruct.total_out];
    
    return compressedData;
}

// 解压缩gzip格式的NSData
- (NSData *)gzipDecompressData:(NSData *)compressedData {
    if ([compressedData length] == 0) {
        return nil;
    }
    
    z_stream zlibStreamStruct;
    zlibStreamStruct.zalloc = Z_NULL;
    zlibStreamStruct.zfree = Z_NULL;
    zlibStreamStruct.opaque = Z_NULL;
    zlibStreamStruct.total_out = 0;
    zlibStreamStruct.next_in = (Bytef *)[compressedData bytes];
    zlibStreamStruct.avail_in = (uInt)[compressedData length];
    
    if (inflateInit2(&zlibStreamStruct, (15+32)) != Z_OK) {
        return nil;
    }
    
    NSMutableData *decompressedData = [NSMutableData dataWithLength:(NSUInteger)([compressedData length] * 1.5)];
    do {
        if (zlibStreamStruct.total_out >= [decompressedData length]) {
            [decompressedData increaseLengthBy:(NSUInteger)([compressedData length] * 0.5)];
        }
        
        zlibStreamStruct.next_out = [decompressedData mutableBytes] + zlibStreamStruct.total_out;
        zlibStreamStruct.avail_out = (uInt)([decompressedData length] - zlibStreamStruct.total_out);
        
        inflate(&zlibStreamStruct, Z_SYNC_FLUSH);
        
    } while (zlibStreamStruct.avail_out == 0);
    
    inflateEnd(&zlibStreamStruct);
    [decompressedData setLength:zlibStreamStruct.total_out];
    
    return decompressedData;
}