Hash value of NSDictionary - objective-c

Question

I ran into an issue, where I got the same hash value for different dictionaries. Maybe I'm doing something obvious wrong, but I thought, objects with different content (a.k.a. not equal objects) should have different hash values.
NSDictionary *dictA = #{ #"foo" : #YES };
NSDictionary *dictB = #{ #"foo" : #NO };
BOOL equal = [dictA hash] == [dictB hash];
NSAssert(!equal, #"Assuming, that different dictionaries have different hash values.");
Any thoughts?

Answer 1

There is no guarantee that two different objects will have different hash values.
In the latest open-source version of CoreFoundation, the hash of a CFDictionary (which is equivalent to an NSDictionary) is defined as:
static CFHashCode __CFDictionaryHash(CFTypeRef cf) {
return __CFBasicHashHash((CFBasicHashRef)cf);
}
and __CFBasicHashHash is defined as:
__private_extern__ CFHashCode __CFBasicHashHash(CFTypeRef cf) {
CFBasicHashRef ht = (CFBasicHashRef)cf;
return CFBasicHashGetCount(ht);
}
which is simply the number of entries stored in the collection. In the other words, both [dictA hash] and [dictB hash]'s hash value are 1, the number of entries in the dictionaries.
While it is a very bad hash algorithm, CF didn't do anything wrong here. If you need to have a more accurate hash value, you can provide one yourself in an Obj-C category.

Answer 2

With a dictionary with only integers, strings etc. I would use dict.description.hash as a quick code.

Answer 3

A solution based on igor-kulagin's answer which is not order dependent:
#implementation NSDictionary (Extensions)
- (NSUInteger) hash
{
NSUInteger prime = 31;
NSUInteger result = 1;
for (NSObject *key in [[self allKeys] sortedArrayUsingSelector:#selector(compare:)]) {
result = prime * result + [key hash];
result = prime * result + [self[key] hash];
}
return result;
}
#end
However, there is still a possibility of collision if the dictionary contains other dictionaries as values.

Answer 4

The function 'hash' is not a real hash function. It gives different values for strings (all predictable) but for collections (arrays and dictionaries) it just returns the count. If you want a unique hash you can calculate it yourself using primes, or the functions srandom() and random() or explore a real hash function like SHA1 available in CommonCrypto/CommonDigest.h

Answer 5

I created NSDictionary category and overridden hash method based on this answer: Best practices for overriding isEqual: and hash
#implementation NSDictionary (Extensions)
- (NSUInteger) hash {
NSUInteger prime = 31;
NSUInteger result = 1;
NSArray *sortedKeys = [self.allKeys sortedArrayUsingSelector: #selector(compare:)];
for (NSObject *key in sortedKeys) {
result = prime * result + key.hash;
id value = self[key];
if ([value conformsToProtocol: #protocol(NSObject)] == YES) {
result = prime * result + [value hash];
}
}
return result;
}
#end
And Swift implementation.
extension Dictionary where Key: Comparable, Value: Hashable {
public var hashValue: Int {
let prime = 31
var result = 1
let sortedKeys = self.keys.sorted()
for (key) in sortedKeys {
let value = self[key]!
result = Int.addWithOverflow(Int.multiplyWithOverflow(prime, result).0, key.hashValue).0
result = Int.addWithOverflow(Int.multiplyWithOverflow(prime, result).0, value.hashValue).0
}
return result
}
}
Perfectly this also requires to implement Equatable protocol for Dictionary so you can also add Hashable protocol conformance.