Hash value of an array containing structs

Question

I have a struct called Survey. It conforms to both Equatable and Hashable protocols.

import Foundation

public struct Survey {
    public let id: String
    public let createdAt: Date
    public let updatedAt: Date
    public let title: String
    public let type: String
}

extension Survey: Equatable { }

public func ==(lhs: Survey, rhs: Survey) -> Bool {
    return lhs.id == rhs.id && lhs.createdAt == rhs.createdAt && lhs.updatedAt == rhs.updatedAt && lhs.title == rhs.title && lhs.type == rhs.type
}

extension Survey: Hashable {
    public var hashValue: Int {
        return id.hashValue ^ createdAt.hashValue ^ updatedAt.hashValue ^ title.hashValue ^ type.hashValue
    }
}

I can get the hash value of individual Survey objects.

But how do I get the hash value of an array containing multiple Survey objects?

Firstly, you can simply do public struct Survey: Hashable and get rid of everything else (except the struct variables ofcourse) — staticVoidMan
– staticVoidMan, Commented Apr 27, 2018 at 11:19
Synthesizing Hashable conformance for arrays is implemented in Swift 4.2: github.com/apple/swift-evolution/blob/master/proposals/… — Martin R
– Martin R, Commented Apr 27, 2018 at 11:20
Secondly, what do you mean get the hash value of an array containing multiple Survey objects. Include the expected code that you would like to achieve for this. Something like [survey_1, survey_N].hashValue? — staticVoidMan
– staticVoidMan, Commented Apr 27, 2018 at 11:20

Tj3n · Accepted Answer · 2018-04-27 11:32:56Z

2

Maybe something like this?

extension Array: Hashable where Iterator.Element: Hashable {
    public var hashValue: Int {
        return self.reduce(1, { $0.hashValue ^ $1.hashValue })
    }
}

Custom hash value is just value you define anyway

*Edit: This would also work if you only want Hashable for Survey array

extension Array: Hashable where Element == Survey {
    public var hashValue: Int {
        return self.reduce(1, { $0.hashValue ^ $1.hashValue })
    }
}

edited Apr 27, 2018 at 11:32

answered Apr 27, 2018 at 11:25

Tj3n

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3 Comments

staticVoidMan Over a year ago

you can actually reduce that to extension Array where Element: Hashable

Tj3n Over a year ago

@staticVoidMan I think the OP want to do something with the Array that must conform Hashable (like using NSOrderedSet), so I think its good to keep it

staticVoidMan Over a year ago

Noted! My bad :) Extend Array to conform to Hashable explictly and use the where condition appropriately.

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