Writing a good Hashable implementation in Swift

Question

In Objective-C (and other languages) a relatively good default implementation of - (NSUInteger)hash might be:

- (NSUInteger)hash {
   return 31u * [self.property1 hash] + [self.property2 hash];
}

Assuming both property1 and property2 return good values for hash.

This doesn't work in Swift's equivalent var hashValue: Int method defined on its Hashable protocol.

The equivalent Swift code is likely to overflow and this a runtime error in Swift.

var hashValue: Int {
    return 31 * property1.hashValue + property2.hashValue // overflow-tastic
}

So my question is, what is the best technique for generating hash values (implementing Hashable) in Swift? Should I just use XOR? Though my understanding is that XOR is not ideal for creating uniform hash distributions. Perhaps something more exotic?

You could simply use the overflow operators (&+ and &*) to allow integer overflows in your hash calculation. developer.apple.com/library/prerelease/ios/documentation/swift/… — Fabian Kreiser
– Fabian Kreiser, Commented Jun 16, 2014 at 8:51
You might want to edit your question. hashValue is not a func. Instead, it's a computed property. — pohl
– pohl, Commented Jun 25, 2014 at 1:01
Does this answer your question? How to Implement hash(into:) from hashValue in Swift? — E. Rivera
– E. Rivera, Commented Oct 1, 2020 at 19:45

orj · Accepted Answer · 2014-10-13 07:41:58Z

27

As suggested by Fabian Kreiser one can use the overflow operators to make the hashValue method as follows:

var hashValue: Int {
    return (31 &* property1.hashValue) &+ property2.hashValue 
}

The value still overflows, but at least it doesn't crash

edited Oct 13, 2014 at 7:41

answered Jun 16, 2014 at 9:03

orj

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3 Comments

pohl Over a year ago

You might want to edit your answer. hashValue is not a func. Instead, it's a computed property.

kfmfe04 Over a year ago

does it make a difference which one I choose to be property1 or property2? suppose property1 goes from 0 to 2 while property2 goes from 0 to some big n...

user1951992 Over a year ago

This approach slowed my compilation time down by a factor of 10000. Xcode Version 8.0 (8A218a).

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