NodeをCollectionに。 extension LinkedListObject.Node: Collection { public enum Index: Comparable { case node(LinkedListObject.Node), none public static func < (lhs: Index, rhs: Index) -> Bool { switch (lhs, rhs) { case (.node(let lhs), .node(let rhs)): return lhs < rhs case (.node(_), .none): return true default: return false } } init(_ node: LinkedListObject.Node?) { self = node.map(Index.node) ?? .none } var node: LinkedListObject.Node? { guard case .node(let node) = self else { return nil } return node } } public var startIndex: Index { return .node(self) } public var endIndex: Index { return .none } public subscript(position: Index) -> LinkedListObject.Node { guard let node = position.node else { preconditionFailure("Index out of range") } return node } public func index(after i: Index) -> Index { guard let node = i.node, let nextIndex = node.next.map(Index.node) else { return .none } return nextIndex } }
