Objective-C -hash method clash

main.m

// This code demonstrates -hash collision on NSString. Two strings have the same
// hash when the first, center, and last 32 chars are identical. The other chars
// don't matter at all.
//
// I had to find this out the hard way when loading images from cache while
// using the url string hash as the cache key. Unfortunately the urls were all
// gravatar urls, thus first 32 chars were identical. Further, all urls also had
// a fallback image url appended as query which covered the 32
// center and end chars. The varying part was between the start and center parts
// and thus the hash was identical for all urls.

#import <Foundation/Foundation.h>

// Random string made out of ASCII chars.
NSString *randomString(NSUInteger length) {
	NSMutableString *s = [NSMutableString stringWithCapacity:length];
	for (NSUInteger i = 0U; i < length; i++)
	    [s appendFormat:@"%C", (unichar)(arc4random() % 128)];

	return s;
}

int main(int argc, char *argv[]) {
	@autoreleasepool {
		NSString *start  = randomString(32);
		NSString *center = randomString(32);
		NSString *end    = randomString(32);
		for (NSUInteger i = 1U; i <= 256; i++) {
			NSString *a = [NSString stringWithFormat:@"%@%@%@%@%@", start, randomString(i), center, randomString(i), end];
			NSString *b = [NSString stringWithFormat:@"%@%@%@%@%@", start, randomString(i), center, randomString(i), end];
			NSLog(@"%3lu: %d", i, [a hash] == [b hash]);
		}
	}
}