Caching URL - Creating appropriate NSString representation - cocoa

Question

I want to cache an image I download from a URL in the local filesystem.
What I don't know, is how can I create from the URL a NSString that is compatible with the characters the iOS file system supports. For instance, if I try to create a file with a : in the name, it will fail.
What steps should I follow to create this NSString? Is a simple hash the best way to go? If so, what hash routine is available in iOS that I can use?

Answer 1

I'd use hashing:
it'll make for much more readable file names
you avoid problems with file name length
md5 should be perfectly fine for your purposes. Unfortunately, the ios5-sdk contains only a C-String function for this:
CC_MD5(in, in_len, out);
The function is contained in <CommonCrypto/CommonDigest.h>, there are other hash functions there, too.
You can find infos on how to wrap this up in a function that takes/returns an NSString here.

Answer 2

The following code belongs to ZDS_Shared. resolveLocalURLForRemoteURL accepts a remote URL and returns a URL pointing to the file on the iOS filesystem. The filename will be an alphanumeric string that doesn't resemble the original URL, but that shouldn't be a problem.
https://github.com/ZarraStudios/ZDS_Shared/blob/master/ZSAssetManager.m#L185
- (NSURL*)resolveLocalURLForRemoteURL:(NSURL*)url
{
if (!url) return nil;
NSString *filename = [[url absoluteString] zs_digest];
NSString *filePath = [[self cachePath] stringByAppendingPathComponent:filename];
return [NSURL fileURLWithPath:filePath];
}
https://github.com/ZarraStudios/ZDS_Shared/blob/master/NSString%2BZSAdditions.m#L38
// NSString category
- (NSString*)zs_digest
{
const char *cstr = [self cStringUsingEncoding:NSASCIIStringEncoding];
return [[NSData dataWithBytes:cstr length:strlen(cstr)] zs_digest];
}
https://github.com/ZarraStudios/ZDS_Shared/blob/master/NSData%2BZSAdditions.m#L38
// NSData category
- (NSString*)zs_digest
{
uint8_t digest[CC_SHA1_DIGEST_LENGTH];
CC_SHA1([self bytes], [self length], digest);
NSMutableString* outputHolder = [[NSMutableString alloc] initWithCapacity:CC_SHA1_DIGEST_LENGTH * 2];
for (int i = 0; i < CC_SHA1_DIGEST_LENGTH; i++) {
[outputHolder appendFormat:#"%02x", digest[i]];
}
NSString *output = [outputHolder copy];
MCRelease(outputHolder);
return [output autorelease];
}