Before sending out some data from the iphone, I wanted to compress it in a well known format. I didn’t find a simple solution to this, so I wrote this function:

#import "zlib.h"

- (NSData *)gzip:(NSData *)data {
    z_stream strm;
    strm.zalloc    = Z_NULL;
    strm.zfree     = Z_NULL;
    strm.opaque    = Z_NULL;
    strm.total_out = 0;
    strm.next_in   = (Bytef *)[data bytes];
    strm.avail_in  = (uInt)[data length];
    
    if (Z_OK != deflateInit2(&strm,
                             Z_DEFAULT_COMPRESSION, Z_DEFLATED,
                             (15+16), 8,
                             Z_DEFAULT_STRATEGY))
        return NULL;
    
    const NSUInteger inc = 16384;
    NSUInteger len = [data length];
    if (len < inc)
        len = inc;
    
    int flag;
    
    NSMutableData *cmp = [NSMutableData dataWithLength:len];
    do {
        if (strm.total_out >= [cmp length])
            [cmp increaseLengthBy: inc];
        
        strm.next_out = [cmp mutableBytes] + strm.total_out;
        strm.avail_out = (uInt)([cmp length] - strm.total_out);
        
        flag = deflate(&strm, Z_FINISH);
        
    } while (flag == Z_OK && strm.avail_out == 0);
    
    uInt inlen = strm.avail_in;
    uLong outlen = strm.total_out;
    
    deflateEnd(&strm);
    
    if (inlen == 0)
        [cmp setLength: outlen];
    else
        return NULL;

    return [NSData dataWithData:cmp];
}

The iphone includes the zlib library, which includes numerous compress routines. Although the library can handle streams, this function works on NSData. The strm structure is configured to read all of the bytes in the NSData. The call to deflateInit2 configures the compression to be gzip compatiable. The loop resizes the output buffer as necessary. The terminating condition is a bit subtle and could be improved. The loop will exit if deflate has a problem or the output buffer has spare capacity. The subtle aspect is if both conditions are false: is the compress complete when the input buffer is completely read but and error occured? I carefully copy out elements of the strm structure before calling deflateEnd.

deflateInit2 has a several parameters: Z_DEFAULT_COMPRESSION selects a balance between speed and compactness, Z_DEFLATED is required, (15+16) is the windowBits, 8 is the default value for the amount of memory to use, and Z_DEFAULT_STRATEGY uses a default value for the compression strategy. windowBits is the log base 2 of the historical window size: 15 indicates a 32K window size. Negative values are permitted too where the negative sign flags that a raw stream (lacking a header or check value) should be generated. Adding 16 to the value flags that a gzip header and check value should be used.