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

Question

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

After all this time, I finally found a solution to this problem!

None of the answers above helped me, as promising as they all looked. In the end, I was able to compress the string on the server with gzip using the chilkat framework for .net ... and then decompress it on the iphone using the chilkat framework for iOS (not yet released, but available if you email the guy directly).

The chilkat framework made this super easy to do so big thumbs up to the developer!

Answer 2

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 3

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