A native Swift implementation of a Mutable Ordered Set:
public struct OrderedSet<Element: Hashable> {
public init() { }
private var elements: [Element] = []
private var set: Set<Element> = []
}
Conforming to the MutableCollection Protocol
To add conformance to the MutableCollection protocol to your own custom collection, upgrade your type’s subscript to support both read and write access. A value stored into a subscript of a MutableCollection instance must subsequently be accessible at that same position. That is, for a mutable collection instance a, index i, and value x, the two sets of assignments in the following code sample must be equivalent:
extension OrderedSet: MutableCollection {
public subscript(index: Index) -> Element {
get { elements[index] }
// set {
// guard set.update(with: newValue) == nil else {
// insert(remove(at: elements.firstIndex(of: newValue)!), at: index)
// return
// }
// elements[index] = newValue
// }
set {
guard let newMember = set.update(with: newValue) else { return }
elements[index] = newMember
}
}
}
Conforming to the RandomAccessCollection Protocol
The RandomAccessCollection protocol adds further constraints on the associated Indices and SubSequence types, but otherwise imposes no additional requirements over the BidirectionalCollection protocol. However, in order to meet the complexity guarantees of a random-access collection, either the index for your custom type must conform to the Strideable protocol or you must implement the index(_:offsetBy:) and distance(from:to:) methods with O(1) efficiency.
extension OrderedSet: RandomAccessCollection {
public typealias Index = Int
public typealias Indices = Range<Int>
public typealias SubSequence = Slice<OrderedSet<Element>>
public typealias Iterator = IndexingIterator<Self>
// Generic subscript to support `PartialRangeThrough`, `PartialRangeUpTo`, `PartialRangeFrom` and `FullRange`
public subscript<R: RangeExpression>(range: R) -> SubSequence where Index == R.Bound { .init(elements[range]) }
public var endIndex: Index { elements.endIndex }
public var startIndex: Index { elements.startIndex }
public func formIndex(after i: inout Index) { elements.formIndex(after: &i) }
public var isEmpty: Bool { elements.isEmpty }
@discardableResult
public mutating func append(_ newElement: Element) -> Bool { insert(newElement).inserted }
}
Conforming to the Hashable Protocol
To use your own custom type in a set or as the key type of a dictionary, add Hashable conformance to your type. The Hashable protocol inherits from the Equatable protocol, so you must also satisfy that protocol’s requirements. The compiler automatically synthesizes your custom type’s Hashable and requirements when you declare Hashable conformance in the type’s original declaration and your type meets these criteria: For a struct, all its stored properties must conform to Hashable. For an enum, all its associated values must conform to Hashable. (An enum without associated values has Hashable conformance even without the declaration.)
extension OrderedSet: Hashable {
public static func ==(lhs: Self, rhs: Self) -> Bool { lhs.elements.elementsEqual(rhs.elements) }
}
Conforming to the SetAlgebra Protocol
When implementing a custom type that conforms to the SetAlgebra protocol, you must implement the required initializers and methods. For the inherited methods to work properly, conforming types must meet the following axioms. Assume that:
? S is a custom type that conforms to the SetAlgebra protocol, x and y are instances of S, and e is of type S.Element—the type that the set holds.
? S() == [ ]
? x.intersection(x) == x
? x.intersection([ ]) == [ ]
? x.union(x) == x
? x.union([ ]) == x x.contains(e) implies
? x.union(y).contains(e)
? x.union(y).contains(e) implies x.contains(e) || y.contains(e)
? x.contains(e) && y.contains(e) if and only if x.intersection(y).contains(e)
? x.isSubset(of: y) implies x.union(y) == y
? x.isSuperset(of: y) implies x.union(y) == x
? x.isSubset(of: y) if and only if y.isSuperset(of: x)
? x.isStrictSuperset(of: y) if and only if x.isSuperset(of: y) && x != y
? x.isStrictSubset(of: y) if and only if x.isSubset(of: y) && x != y
extension OrderedSet: SetAlgebra {
public mutating func insert(_ newMember: Element) -> (inserted: Bool, memberAfterInsert: Element) {
let insertion = set.insert(newMember)
if insertion.inserted {
elements.append(newMember)
}
return insertion
}
public mutating func remove(_ member: Element) -> Element? {
if let index = elements.firstIndex(of: member) {
elements.remove(at: index)
return set.remove(member)
}
return nil
}
public mutating func update(with newMember: Element) -> Element? {
if let index = elements.firstIndex(of: newMember) {
elements[index] = newMember
return set.update(with: newMember)
} else {
elements.append(newMember)
set.insert(newMember)
return nil
}
}
public func union(_ other: Self) -> Self {
var orderedSet = self
orderedSet.formUnion(other)
return orderedSet
}
public func intersection(_ other: Self) -> Self {
var orderedSet = self
orderedSet.formIntersection(other)
return orderedSet
}
public func symmetricDifference(_ other: Self) -> Self {
var orderedSet = self
orderedSet.formSymmetricDifference(other)
return orderedSet
}
public mutating func formUnion(_ other: Self) {
other.forEach { append($0) }
}
public mutating func formIntersection(_ other: Self) {
self = .init(filter { other.contains($0) })
}
public mutating func formSymmetricDifference(_ other: Self) {
self = .init(filter { !other.set.contains($0) } + other.filter { !set.contains($0) })
}
}
Conforming to ExpressibleByArrayLiteral
Add the capability to be initialized with an array literal to your own custom types by declaring an init(arrayLiteral:) initializer. The following example shows the array literal initializer for a hypothetical OrderedSet type, which has setlike semantics but maintains the order of its elements.
extension OrderedSet: ExpressibleByArrayLiteral {
public init(arrayLiteral: Element...) {
self.init()
for element in arrayLiteral {
self.append(element)
}
}
}
extension OrderedSet: CustomStringConvertible {
public var description: String { .init(describing: elements) }
}
Conforming to the AdditiveArithmetic Protocol
To add AdditiveArithmetic protocol conformance to your own custom type, implement the required operators, and provide a static zero property using a type that can represent the magnitude of any value of your custom type.
extension OrderedSet: AdditiveArithmetic {
public static var zero: Self { .init() }
public static func + (lhs: Self, rhs: Self) -> Self { lhs.union(rhs) }
public static func - (lhs: Self, rhs: Self) -> Self { lhs.subtracting(rhs) }
public static func += (lhs: inout Self, rhs: Self) { lhs.formUnion(rhs) }
public static func -= (lhs: inout Self, rhs: Self) { lhs.subtract(rhs) }
}
Conforming to the RangeReplaceableCollection Protocol
To add RangeReplaceableCollection conformance to your custom collection, add an empty initializer and the replaceSubrange(:with:) method to your custom type. RangeReplaceableCollection provides default implementations of all its other methods using this initializer and method. For example, the removeSubrange(:) method is implemented by calling replaceSubrange(_:with:) with an empty collection for the newElements parameter. You can override any of the protocol’s required methods to provide your own custom implementation.
extension OrderedSet: RangeReplaceableCollection {
public init<S>(_ elements: S) where S: Sequence, S.Element == Element {
elements.forEach { set.insert($0).inserted ? self.elements.append($0) : () }
}
mutating public func replaceSubrange<C: Collection, R: RangeExpression>(_ subrange: R, with newElements: C) where Element == C.Element, C.Element: Hashable, Index == R.Bound {
elements[subrange].forEach { set.remove($0) }
elements.removeSubrange(subrange)
newElements.forEach { set.insert($0).inserted ? elements.append($0) : () }
}
}
OrderedSet Playground Sample
Quick Playground Test (OrderedSet should have all methods that are available to Swift native Array and Set structures)
var ordereSet1: OrderedSet = [1,2,3,4,5,6,1,2,3] // [1, 2, 3, 4, 5, 6]
var ordereSet2: OrderedSet = [4,5,6,7,8,9,7,8,9] // [4, 5, 6, 7, 8, 9]
ordereSet1 == ordereSet2 // false
ordereSet1.union(ordereSet2) // [1, 2, 3, 4, 5, 6, 7, 8, 9]
ordereSet1.intersection(ordereSet2) // [4, 5, 6]
ordereSet1.symmetricDifference(ordereSet2) // [1, 2, 3, 7, 8, 9]
ordereSet1.subtract(ordereSet2) // [1, 2, 3]
ordereSet2.popLast() // 9
