Swift Hash Functions

Country.swift

import Foundation

struct Country {
    let name: String
    let capital: String
    var visited: Bool
}

extension Country: Equatable {
    static func == (lhs: Country, rhs: Country) -> Bool {
        return lhs.name == rhs.name &&
            lhs.capital == rhs.capital &&
            lhs.visited == rhs.visited
    }
}

extension Country: Comparable {

    static func < (lhs: Country, rhs: Country) -> Bool {
        return lhs.name < rhs.name ||
        (lhs.name == rhs.name && lhs.capital < rhs.capital) ||
        (lhs.name == rhs.name && lhs.capital == rhs.capital && rhs.visited)
    }
}

// http://www.cse.yorku.ca/~oz/hash.html
extension String {

    // hash(0) = 5381
    // hash(i) = hash(i - 1) * 33 ^ str[i];
    var djb2hash: Int {
        let unicodeScalars = self.unicodeScalars.map { $0.value }
        return unicodeScalars.reduce(5381) {
            ($0 << 5) &+ $0 &+ Int($1)
        }
    }

    // hash(0) = 0
    // hash(i) = hash(i - 1) * 65599 + str[i];
    var sdbmhash: Int {
        let unicodeScalars = self.unicodeScalars.map { $0.value }
        return unicodeScalars.reduce(0) {
            Int($1) &+ ($0 << 6) &+ ($0 << 16) - $0
        }
    }
}

// https://github.com/krzysztofzablocki/Sourcery

// MIT License
//
// Copyright (c) 2016 Krzysztof Zabłocki
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

fileprivate func combineHashes(_ hashes: [Int]) -> Int {
    return hashes.reduce(0, combineHashValues)
}

fileprivate func combineHashValues(_ initial: Int, _ other: Int) -> Int {
    #if arch(x86_64) || arch(arm64)
        let magic: UInt = 0x9e3779b97f4a7c15
    #elseif arch(i386) || arch(arm)
        let magic: UInt = 0x9e3779b9
    #endif
    var lhs = UInt(bitPattern: initial)
    let rhs = UInt(bitPattern: other)
    lhs ^= rhs &+ magic &+ (lhs << 6) &+ (lhs >> 2)
    return Int(bitPattern: lhs)
}

// Three different approachs to combining the hash values
// of the properties of a structure.

extension Country: Hashable {

    // XOR the three values. Simple and works well in this case
    // where we do not expect our name and capital to have
    // matching values.

    var simpleHashValue: Int {
        return name.hashValue ^ capital.hashValue ^ visited.hashValue

    }

    // Avoid common collisions when name and capital can have
    // matching values by applying a different hash method to
    // one of the values

    var djb2HashValue: Int {
        return name.djb2hash ^ capital.hashValue ^ visited.hashValue
    }

    // Use the boost hash combine function to combine an array
    // of hashValues.

    var boostHashValue: Int {
        return combineHashes([name.hashValue,capital.hashValue,visited.hashValue])
    }

    // Return the hash value using one of the above approaches
    var hashValue: Int {
        return boostHashValue
    }
}

String.sdbmhash crashed on the first run