可扩展协议

Question

我正在尝试转换以下Swift 2.3代码：

//Example usage:
//(0 ..< 778).binarySearch { $0 < 145 } // 145
extension CollectionType where Index: RandomAccessIndexType {

    /// Finds such index N that predicate is true for all elements up to
    /// but not including the index N, and is false for all elements
    /// starting with index N.
    /// Behavior is undefined if there is no such N.
    func binarySearch(predicate: Generator.Element -> Bool)
        -> Index {
        var low = startIndex
        var high = endIndex
        while low != high {
            let mid = low.advancedBy(low.distanceTo(high) / 2)
            if predicate(self[mid]) {
                low = mid.advancedBy(1)
            } else {
                high = mid
            }
        }
        return low
    }
}

进入Swift 3的步骤如下：

//Example usage:
//(0 ..< 778).binarySearch { $0 < 145 } // 145
extension Collection where Index: Strideable {

    /// Finds such index N that predicate is true for all elements up to
    /// but not including the index N, and is false for all elements
    /// starting with index N.
    /// Behavior is undefined if there is no such N.
    func binarySearch(predicate: (Generator.Element) -> Bool)
        -> Index {
        var low = startIndex
        var high = endIndex
        while low != high {
            let mid = low.advanced(by: low.distance(to: high) / 2)
            if predicate(self[mid]) {
                low = mid.advanced(to: 1)
            } else {
                high = mid
            }
        }
        return low
    }
}

误差

二进制运算符'/‘不能应用于'self.Index.Stride’和'Int‘类型的操作数

向let mid = low.advanced(by: low.distance(to: high) / 2)抛出

有什么能帮上忙的吗？

Answer 1

在Swift 3中，“集合移动他们的索引”，比较一种新的馆藏与指数模型关于Swift的演变。特别是，您不对索引调用advancedBy()，而是调用集合上的index()方法来提前索引。

因此，您的方法将在Swift 3中实现为

extension RandomAccessCollection {

    func binarySearch(predicate: (Iterator.Element) -> Bool) -> Index {
        var low = startIndex
        var high = endIndex
        while low != high {
            let mid = index(low, offsetBy: distance(from: low, to: high)/2)
            if predicate(self[mid]) {
                low = index(after: mid)
            } else {
                high = mid
            }
        }
        return low
    }
}

同样的方法也会编译(和工作)作为更一般的Collection类型的扩展，但是- as Vadim Yelagin正确地指出，如果索引/偏移量计算不能在固定时间内完成，则效率很低。