swift - 我应该如何计算哈希值？

Question

我看到了几个 Hashable 变量 hashValue 的实现示例，例如:

extension Country: Hashable {
  var hashValue: Int {
    return name.hashValue ^ capital.hashValue ^ visited.hashValue
  }
}


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


var hashValue : Int {
    get {
        return String(self.scalarArray.map { UnicodeScalar($0) }).hashValue
    }
}

等等。在某些情况下，会使用OR、XOR 或BITWISE 运算符。在其他情况下，其他算法、映射或过滤函数等。

现在我有点困惑。计算良好的 hashValue 的经验法则是什么？

在最简单的有两个字符串变量的情况下，我应该用 OR 运算符将这两个变量组合起来吗？

Answer 1

从 Swift 4.2 开始，您可以使用 Hasher

Swift 4.2 implements hashing based on the SipHash family of pseudorandom functions, specifically SipHash-1-3 and SipHash-2-4, with 1 or 2 rounds of hashing per message block and 3 or 4 rounds of finalization, respectively.

Now if you want to customize how your type implements Hashable, you can override the hash(into:) method instead of hashValue. The hash(into:) method passes a Hasher object by reference, which you call combine(_:) on to add the essential state information of your type.

// Swift >= 4.2
struct Color: Hashable {
    let red: UInt8
    let green: UInt8
    let blue: UInt8

    // Synthesized by compiler
    func hash(into hasher: inout Hasher) {
        hasher.combine(self.red)
        hasher.combine(self.green)
        hasher.combine(self.blue)
    }

    // Default implementation from protocol extension
    var hashValue: Int {
        var hasher = Hasher()
        self.hash(into: &hasher)
        return hasher.finalize()
    }
}