Question

In my application, I am reading RSS feeds and saving them to a Core Data db using the URL of each specific article as the key. Passing these URLs around the system can be problematic because they can be lengthy, and I’d like a way to generate a unique identifier to store in the db and just pass that around.

I’d also like it to be reconstructable using the same string so that if I get a duplicate URL, I can generate the identifier from it and simply check Core Data for the identifier.

Is there an easy way to do this?

Answer 1

When most people are talking about hashes, they are generally thinking about one-way hashes like SHA1, SHA2, or MD5. While these are imminently useful, they will not allow you to take a hash and reverse it into its original form. They will, however, allow you to do things like compare a user entered password with one they’ve entered before without ever having to store the actual password — only the hash.

What you seem to want is string compression or deflation. Luckily, gzip is supported out of the box using the ASIHTTPRequest class. Here’s some code for using gzip found in this discussion.

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);  

There is a very good article that discusses hashing using MD5 here:
http://cocoawithlove.com/2009/07/hashvalue-object-for-holding-md5-and.html

Using the CommonCrypto library, there are a number of hash algorithms already built in. You can use the MD5 hasing algorithm like this:

#import <CommonCrypto/CommonDigest.h>

char input[] = "Some data value.";
char result[16];
CC_MD5(input, strlen(input), result);

This will print out the hash in human-readable hex form:

printf("MD5 (\"%s\") = %02x%02x%02x%02x%02x%02x
                       %02x%02x%02x%02x%02x%02x
                       %02x%02x%02x%02x\n",
    input,
    result[0], result[1], result[2], result[3], 
    result[4], result[5], result[6], result[7],
    result[8], result[9], result[10], result[11],
    result[12], result[13], result[14], result[15]);

If you would like more information on forward-only hashing, I posted some info as well as production-ready code in this SO answer.