Hash value of NSDictionary

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

Answer 1

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.