swift 符合 Stridable 后进入无限循环

Question

我有一个这样的枚举：

enum Rank: CaseIterable {
case Ace, King, Queen, ...
}

并希望使其符合Stridable。我试图通过像这样获取卡片索引来实现 func distance(to other: _)：

func distance(to other: Rank) -> Int {
            let indexOfSelf = Rank.allCases.firstIndex(of: self)!
            let indexOfOther = Rank.allCases.firstIndex(of: other)!
            return indexOfOther - indexOfSelf
}

只要我不符合 Stridable（例如， ace.distanceTo(Queen) 结果为 2。但是，一旦我符合 Stridable，代码就会进入无限循环，我得到错误 EXC_BAD_ACCESS (code=2, address=x)。

这是应该发生的吗？如果是这样，为什么会这样？

感谢大家的帮助！

我的实现advanced(by n: Int)：

func advanced(by n: Int) -> Rank {
            let index = Rank.allCases.firstIndex(of: self)!
            let resultIndex = index + n
            if resultIndex > Rank.allCases.count {
                return .Two
            }
            return Rank.allCases[resultIndex]
}

会导致错误的东西：

调用：ace.distanceTo(Queen) 会进入无限循环

Answer 1

来自 Strideable 协议的文档：

Important

The Strideable protocol provides default implementations for the equal-to (==) and less-than (<) operators that depend on the Stride type’s implementations. If a type conforming to Strideable is its own Stride type, it must provide concrete implementations of the two operators to avoid infinite recursion.

在你的例子中，Rank 不是它自己的 Stride 类型，而是 distance(to:) 方法调用 firstIndex(of:)，它在元素上调用 ==。现在 == 有一个针对 Strideable 类型的特殊实现——正如我们从 Stride.swift 中的实现中看到的那样——调用 distance(to:)。这会导致“无限”递归，并最终导致堆栈溢出。您还可以从堆栈回溯中看到：

(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=2, address=0x7ffeef3fffb8)
  * frame #0: 0x0000000100001b4b test`Rank.distance(other=ace, self=king) at main.swift:11
    frame #1: 0x0000000100001fd8 test`protocol witness for Strideable.distance(to:) in conformance Rank at <compiler-generated>:0
    frame #2: 0x00007fff7ac4927b libswiftCore.dylib`static (extension in Swift):Swift.Strideable.== infix(A, A) -> Swift.Bool + 219
    frame #3: 0x0000000100001f56 test`protocol witness for static Equatable.== infix(_:_:) in conformance Rank at <compiler-generated>:0
    frame #4: 0x00007fff7ab25ea9 libswiftCore.dylib`(extension in Swift):Swift.Collection< where A.Element: Swift.Equatable>.firstIndex(of: A.Element) -> Swift.Optional<A.Index> + 905
    frame #5: 0x0000000100001c31 test`Rank.distance(other=ace, self=ace) at main.swift:12:49
    ...
    frame #42256: 0x0000000100001fd8 test`protocol witness for Strideable.distance(to:) in conformance Rank at <compiler-generated>:0
    frame #42257: 0x00007fff7ac4927b libswiftCore.dylib`static (extension in Swift):Swift.Strideable.== infix(A, A) -> Swift.Bool + 219
    frame #42258: 0x0000000100001f56 test`protocol witness for static Equatable.== infix(_:_:) in conformance Rank at <compiler-generated>:0
    frame #42259: 0x00007fff7ab25ea9 libswiftCore.dylib`(extension in Swift):Swift.Collection< where A.Element: Swift.Equatable>.firstIndex(of: A.Element) -> Swift.Optional<A.Index> + 905
    frame #42260: 0x0000000100001c31 test`Rank.distance(other=ace, self=ace) at main.swift:12:49
    frame #42261: 0x0000000100001fd8 test`protocol witness for Strideable.distance(to:) in conformance Rank at <compiler-generated>:0
    frame #42262: 0x00007fff7ac4927b libswiftCore.dylib`static (extension in Swift):Swift.Strideable.== infix(A, A) -> Swift.Bool + 219
    frame #42263: 0x0000000100001f56 test`protocol witness for static Equatable.== infix(_:_:) in conformance Rank at <compiler-generated>:0
    frame #42264: 0x00007fff7ab25ea9 libswiftCore.dylib`(extension in Swift):Swift.Collection< where A.Element: Swift.Equatable>.firstIndex(of: A.Element) -> Swift.Optional<A.Index> + 905
    frame #42265: 0x0000000100001c31 test`Rank.distance(other=queen, self=ace) at main.swift:12:49
    frame #42266: 0x0000000100001830 test`main at main.swift:23:18
    frame #42267: 0x00007fff7b3843d5 libdyld.dylib`start + 1
(lldb)

正如 Joakim 所说，最简单的解决方案是根据枚举的原始值实现这些方法，这避免了（递归）使用 ==:

enum Rank: Int, Strideable {
    case ace, king, queen, jack
    
    func advanced(by n: Int) -> Rank {
        return Self(rawValue: self.rawValue + n)!
    }
    
    func distance(to other: Rank) -> Int {
        return other.rawValue - self.rawValue
    }
}

这也比一遍又一遍地查找 allCases 集合中的值更有效。

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