Update

Martin R writes:

As of Swift 4.1, the compiler can synthesize Equatable and Hashable
for types conformance automatically, if all members conform to
Equatable/Hashable (SE0185). And as of Swift 4.2, a high-quality hash
combiner is built-in into the Swift standard library (SE-0206).

Therefore there is no need anymore to define your own hashing
function, it suffices to declare the conformance:

struct ScalarString: Hashable, ... {

    private var scalarArray: [UInt32] = []

    // ... }

Thus, the answer below needs to be rewritten (yet again). Until that happens refer to Martin R’s answer from the link above.

---

Old Answer:

This answer has been completely rewritten after submitting my original answer to code review.

How to implement to Hashable protocol

The Hashable protocol allows you to use your custom class or struct as a dictionary key. In order to implement this protocol you need to

1. Implement the Equatable protocol (Hashable inherits from Equatable)
2. Return a computed hashValue

These points follow from the axiom given in the documentation:

x == y implies x.hashValue == y.hashValue

where x and y are values of some Type.

Implement the Equatable protocol

In order to implement the Equatable protocol, you define how your type uses the == (equivalence) operator. In your example, equivalence can be determined like this:

func ==(left: ScalarString, right: ScalarString) -> Bool {
    return left.scalarArray == right.scalarArray
}

The == function is global so it goes outside of your class or struct.

Return a computed hashValue

Your custom class or struct must also have a computed hashValue variable. A good hash algorithm will provide a wide range of hash values. However, it should be noted that you do not need to guarantee that the hash values are all unique. When two different values have identical hash values, this is called a hash collision. It requires some extra work when there is a collision (which is why a good distribution is desirable), but some collisions are to be expected. As I understand it, the == function does that extra work. (Update: It looks like == may do all the work.)

There are a number of ways to calculate the hash value. For example, you could do something as simple as returning the number of elements in the array.

var hashValue: Int {
    return self.scalarArray.count
} 

This would give a hash collision every time two arrays had the same number of elements but different values. NSArray apparently uses this approach.

DJB Hash Function

A common hash function that works with strings is the DJB hash function. This is the one I will be using, but check out some others here.

A Swift implementation provided by @MartinR follows:

var hashValue: Int {
    return self.scalarArray.reduce(5381) {
        ($0 << 5) &+ $0 &+ Int($1)
    }
}

This is an improved version of my original implementation, but let me also include the older expanded form, which may be more readable for people not familiar with reduce. This is equivalent, I believe:

var hashValue: Int {

    // DJB Hash Function
    var hash = 5381

    for(var i = 0; i < self.scalarArray.count; i++)
    {
        hash = ((hash << 5) &+ hash) &+ Int(self.scalarArray[i])
    }

    return hash
} 

The &+ operator allows Int to overflow and start over again for long strings.

Big Picture

We have looked at the pieces, but let me now show the whole example code as it relates to the Hashable protocol. ScalarString is the custom type from the question. This will be different for different people, of course.

// Include the Hashable keyword after the class/struct name
struct ScalarString: Hashable {

    private var scalarArray: [UInt32] = []

    // required var for the Hashable protocol
    var hashValue: Int {
        // DJB hash function
        return self.scalarArray.reduce(5381) {
            ($0 << 5) &+ $0 &+ Int($1)
        }
    }
}

// required function for the Equatable protocol, which Hashable inheirits from
func ==(left: ScalarString, right: ScalarString) -> Bool {
    return left.scalarArray == right.scalarArray
}

Other helpful reading

• Which hashing algorithm is best for uniqueness and speed?
• Overflow Operators
• Why are 5381 and 33 so important in the djb2 algorithm?
• How are hash collisions handled?

Credits

A big thanks to Martin R over in Code Review. My rewrite is largely based on his answer. If you found this helpful, then please give him an upvote.

Update

Swift is open source now so it is possible to see how hashValue is implemented for String from the source code. It appears to be more complex than the answer I have given here, and I have not taken the time to analyze it fully. Feel free to do so yourself.