swift2 - How do you enumerate OptionSetType in Swift?

Question

I have a custom OptionSetType struct in Swift. How can I enumerate all values of an instance?

This is my OptionSetType:

struct WeekdaySet: OptionSetType {
    let rawValue: UInt8

    init(rawValue: UInt8) {
        self.rawValue = rawValue
    }

    static let Sunday        = WeekdaySet(rawValue: 1 << 0)
    static let Monday        = WeekdaySet(rawValue: 1 << 1)
    static let Tuesday       = WeekdaySet(rawValue: 1 << 2)
    static let Wednesday     = WeekdaySet(rawValue: 1 << 3)
    static let Thursday      = WeekdaySet(rawValue: 1 << 4)
    static let Friday        = WeekdaySet(rawValue: 1 << 5)
    static let Saturday      = WeekdaySet(rawValue: 1 << 6)
}

I would like to something like this:

let weekdays: WeekdaySet = [.Monday, .Tuesday]
for weekday in weekdays {
    // Do something with weekday
}

Answer 1

As of Swift 4, there are no methods in the standard library to enumerate the elements of an OptionSetType (Swift 2) resp. OptionSet (Swift 3, 4).

Here is a possible implementation which simply checks each bit of the underlying raw value, and for each bit which is set, the corresponding element is returned. The "overflow multiplication" &* 2 is used as left-shift because << is only defined for the concrete integer types, but not for the IntegerType protocol.

Swift 2.2:

public extension OptionSetType where RawValue : IntegerType {

    func elements() -> AnySequence<Self> {
        var remainingBits = self.rawValue
        var bitMask: RawValue = 1
        return AnySequence {
            return AnyGenerator {
                while remainingBits != 0 {
                    defer { bitMask = bitMask &* 2 }
                    if remainingBits & bitMask != 0 {
                        remainingBits = remainingBits & ~bitMask
                        return Self(rawValue: bitMask)
                    }
                }
                return nil
            }
        }
    }
}

Example usage:

let weekdays: WeekdaySet = [.Monday, .Tuesday]
for weekday in weekdays.elements() {
    print(weekday)
}

// Output:
// WeekdaySet(rawValue: 2)
// WeekdaySet(rawValue: 4)

Swift 3:

public extension OptionSet where RawValue : Integer {

    func elements() -> AnySequence<Self> {
        var remainingBits = rawValue
        var bitMask: RawValue = 1
        return AnySequence {
            return AnyIterator {
                while remainingBits != 0 {
                    defer { bitMask = bitMask &* 2 }
                    if remainingBits & bitMask != 0 {
                        remainingBits = remainingBits & ~bitMask
                        return Self(rawValue: bitMask)
                    }
                }
                return nil
            }
        }
    }
}

Swift 4:

public extension OptionSet where RawValue: FixedWidthInteger {

    func elements() -> AnySequence<Self> {
        var remainingBits = rawValue
        var bitMask: RawValue = 1
        return AnySequence {
            return AnyIterator {
                while remainingBits != 0 {
                    defer { bitMask = bitMask &* 2 }
                    if remainingBits & bitMask != 0 {
                        remainingBits = remainingBits & ~bitMask
                        return Self(rawValue: bitMask)
                    }
                }
                return nil
            }
        }
    }
}