Implementing a hash combiner in Swift

Question

I\'m extending a struct conform to Hashable. I\'ll use the DJB2 hash combiner to accomplish this.

To make it easy to write hash function f

Answer 1

Use the method hash(into:) from the Apple Developer Documentation:

https://developer.apple.com/documentation/swift/hashable

struct GridPoint {
    var x: Int
    var y: Int
}

extension GridPoint: Hashable {

    static func == (lhs: GridPoint, rhs: GridPoint) -> Bool {
        return lhs.x == rhs.x && lhs.y == rhs.y
    }

    func hash(into hasher: inout Hasher) {
        hasher.combine(x)
        hasher.combine(y)
    }

}

Answer 2

You cannot define a parameter of type P if P is a protocol which has Self or associated type requirements. In this case it is the Equatable protocol from which Hashable inherits, which has a Self requirement:

public static func ==(lhs: Self, rhs: Self) -> Bool

What you can do is to define a generic method instead:

extension Hashable {
    func combineHash<T: Hashable>(with hashableOther: T) -> Int {
        let ownHash = self.hashValue
        let otherHash = hashableOther.hashValue
        return (ownHash << 5) &+ ownHash &+ otherHash
    }
}