Compress/Decompress NSString in objective-c (iphone) using GZIP or deflate

Question

Hi,

I have a web-service running on Windows Azure which returns JSON that I consume in my iPhone app.

Unfortunately, Windows Azure doesn't seem to support the compression of dynamic responses yet (long story) so I decided to get around it by returning an uncompressed JSON package, which contains a compressed (using GZIP) string.

e.g

{"Error":null,"IsCompressed":true,"Success":true,"Value":"vWsAAB+LCAAAAAAAB..etc.."}

... where value is the compressed string of a complex object represented in JSON.

This was really easy to implement on the server, but for the life of me I can't figure out how to decompress a gzipped NSString into an uncompressed NSString, all the examples I can find for zlib etc are dealing with files etc.

Can anyone give me any clues on how to do this? (I'd also be happy for a solution that used deflate as I could change the server-side implementation to use deflate too).

Thanks!!

Steven

Edit 1: Aaah, I see that ASIHTTPRequest is using the following function in it's source code:

//uncompress gzipped data with zlib
+ (NSData *)uncompressZippedData:(NSData*)compressedData;

... and I'm aware that I can convert NSString to NSData, so I'll see if this leads me anywhere!

Edit 2: Unfortunately, the method described in Edit 1 didn't lead me anywhere.

Edit 3: Following the advice below regarding base64 encoding/decoding, I came up with the following code. The encodedGzippedString is as you can guess, a string "Hello, my name is Steven Elliott" which is gzipped and then converted to a base64 string. Unfortunately, the result that prints using NSLog is just blank.

NSString *encodedGzippedString = @"GgAAAB+LCAAAAAAABADtvQdgHEmWJSYvbcp7f0r1StfgdKEIgGATJNiQQBDswYjN5pLsHWlHIymrKoHKZVZlXWYWQMztnbz33nvvvffee++997o7nU4n99//P1xmZAFs9s5K2smeIYCqyB8/fnwfPyK+uE6X2SJPiyZ93eaX+TI9Lcuiatvx/wOwYc0HGgAAAA==";
NSData *decodedGzippedData = [NSData dataFromBase64String:encodedGzippedString];
NSData* unGzippedJsonData = [ASIHTTPRequest uncompressZippedData:decodedGzippedData];   
NSString* unGzippedJsonString = [[NSString alloc] initWithData:unGzippedJsonData encoding:NSASCIIStringEncoding];       
NSLog(@"Result: %@", unGzippedJsonString);  

Answer 1

Your "compressed" string is not raw GZIP'd data, it's in some encoding that allows those bytes to be stored in a string-- looks like base-64 or something like it. To get an NSData out of this, you'll need to decode it into the NSData.

If it's really base-64, check out this blog post an accompanying code: http://cocoawithlove.com/2009/06/base64-encoding-options-on-mac-and.html which will do what you want.

Once you have an NSData object, the ASIHTTPRequest method will probably do as you like.

Answer 2

This worked for me: from a string gzipeed, then base64 encoded to un-gzipped string (all utf8).

#import "base64.h"
#import "NSData+Compression.h"

...

+(NSString *)gunzipBase64StrToStr:(NSString *)stringValue {
    //now we decode from Base64
    Byte inputData[[stringValue lengthOfBytesUsingEncoding:NSUTF8StringEncoding]];//prepare a Byte[]
    [[stringValue dataUsingEncoding:NSUTF8StringEncoding] getBytes:inputData];//get the pointer of the data
    size_t inputDataSize = (size_t)[stringValue length];
    size_t outputDataSize = EstimateBas64DecodedDataSize(inputDataSize);//calculate the decoded data size
    Byte outputData[outputDataSize];//prepare a Byte[] for the decoded data
    Base64DecodeData(inputData, inputDataSize, outputData, &outputDataSize);//decode the data
    NSData *theData = [[NSData alloc] initWithBytes:outputData length:outputDataSize];//create a NSData object from the decoded data
                                                                                      //NSLog(@"DATA: %@ \n",[theData description]);

    //And now we gunzip:
    theData=[theData gzipInflate];//make bigger==gunzip
    return [[NSString alloc] initWithData:theData encoding:NSUTF8StringEncoding];
}

@end