更严格的泛型的扩展功能

Question

我有一个基于 Comparable 的通用二叉搜索树：

public class BSTree<T: Comparable> {
    public func insert(_ val: T, _ n: Int) {
        // ...
    }

    @discardableResult
    public func delete(_ val: T, _ n: Int) -> Int {
        // ...
    }
}

如果 T 是算术类型，我想添加提供值总和的功能。我尝试了以下方法：

public class BSTree<T: Comparable> {
    private var sumStorage: T?

    public func insert(_ val: T, _ n: Int) {
        if let arithVal = val as? AdditiveArithmetic {
            for _ in 0 ..< n { sumStorage += arithVal }
        }
        // ...
    }

    @discardableResult
    public func delete(_ val: T, _ n: Int) -> Int {
        // ...
        numDeleted = ...
        if let arithVal = val as? AdditiveArithmetic {
            for _ in 0 ..< numDeleted { sumStorage -= arithVal }
       }        
    }
}

extension BSTree where T: AdditiveArithmetic {
    public var sum: T {
        sumStorage as? T ?? T.zero
    }
}

当然，当我尝试将val 转换为AdditiveArithmetic 时，我得到“协议'AdditiveArithmetic' 只能用作通用约束，因为它具有Self 或关联的类型要求”。加上sumStorage 不是AdditiveArithmetic，所以我不能添加它，我不能让它成为扩展的存储属性，因为......你不能。

我最后想到的是使用继承：

class SummedBSTree<T>: BSTree<T> where T: AdditiveArithmetic & Comparable {

    public var sum = T.zero

    override public func insert(_ val: T, _ n: Int) {
        super.insert(val, n)
        for _ in 0 ..< n { sum += val }
    }

    @discardableResult
    override public func delete(_ val: T, _ n: Int) -> Int {
        let numDeleted = super.delete(val, n)
        for _ in 0 ..< numDeleted { sum -= val }
        return numDeleted
    }

}

这行得通，但它似乎是使用大锤的情况，珠宝商的螺丝刀应该能够做到这一点。令人沮丧的是，在 Objective-C（和其他强类型语言）中很容易做到的事情在 Swift 中却如此困难。有人可以想出一种无需子类化即可添加求和功能的方法吗？

Answer 1

import UIKit
//https://stackoverflow.com/questions/61784548/swift-extended-capability-for-a-more-restrictive-generic

protocol ON1Speedable {
    
    associatedtype Item: Comparable

    var sumStorage: Item? { get set }
    
}

public class BSTree<T: Comparable> {
        
    var sumStorage: T?
    
    init(sumStorage: T? = nil) {
        self.sumStorage = sumStorage
    }
    
}

extension ON1Speedable where Item: AdditiveArithmetic, Item: Strideable, Item.Stride: SignedInteger {
    
    mutating func insert(_ val: Item, _ n: Int) {
        sumStorage = sumStorage ?? Item.zero
        for _ in 0..<n { sumStorage! += val }
    }
    
    @discardableResult
    mutating func delete(_ val: Item, _ n: Int) -> Item? {
        sumStorage = sumStorage ?? Item.zero
        for _ in Item.zero..<val { sumStorage! -= val }
        
        return sumStorage
    }
    
}

extension BSTree: ON1Speedable { }

var g2 = BSTree<Int>(sumStorage: 0)

g2.sumStorage
g2.insert(5, 5)
g2.sumStorage // 25
g2.delete(5, 5) // 0

var g3 = BSTree<String>()

g3.sumStorage // nil
//g3.insert("foo", 5) // Error: Referencing instance method 'insert' on 'ON1Speedable' requires that 'String.Stride' conform to 'SignedInteger'
g3.sumStorage // nil
//g3.delete("bar", 5) // Error: Referencing instance method 'delete' on 'ON1Speedable' requires that 'String.Stride' conform to 'SignedInteger'