Why can't we set consecutive rawValue to Option Sets?

Question

Today I tried playing bit with OptionSet in Playground, and I notice this pattern

 struct Activities: OptionSet {
      let rawValue: Int
      static let eating = Activities(rawValue: 1)
      static let programming = Activities(rawValue: 2)
      static let breathing = Activities(rawValue: 3)
      static let saveGotham = Activities(rawValue: 4) 
}

 let act: Activities = [.eating, .programming, .saveGotham]

 act.contains(.breathing). //true /* this is unexpected */
 act.contains(.saveGotham) //true

Although, the array doesn't contain the value '.breathing' it still return true. I modified the same struct with different rawValue

 struct Activities: OptionSet {
      let rawValue: Int
      static let eating = Activities(rawValue: 1)
      static let programming = Activities(rawValue: 8)
      static let breathing = Activities(rawValue: 16)
      static let saveGotham = Activities(rawValue: 32) 
}

 let act: Activities = [.eating, .programming, .saveGotham]

 act.contains(.breathing). //false
 act.contains(.saveGotham) //true

and got the desired output. it would be awesome if someone shed the light on the problem and explain how the 'OptionSet' actually works.

Thank you.

Answer 1

The OptionSet protocol is meant to

... represent bitset types, where individual bits represent members of a set.

In your case,

let act: Activities = [.eating, .programming, .saveGotham]
print(act.rawValue) // 7

is stored as an integer containing the BITWISE OR of the raw values (1 | 2 | 4 = 7), and

  act.contains(.breathing). //true /* this is unexpected */

tests if the BITWISE AND 7 & 3 is non-zero (which is the case).

Therefore you should not use consecutive raw values, but powers of two, i.e. each of the mutually exclusive values is represented by one bit position:

struct Activities: OptionSet {
    let rawValue: Int
    static let eating = Activities(rawValue: 1)
    static let programming = Activities(rawValue: 2)
    static let breathing = Activities(rawValue: 4)
    static let saveGotham = Activities(rawValue: 8)
}

or equivalently:

struct Activities: OptionSet {
    let rawValue: Int
    static let eating = Activities(rawValue: 1 << 0)
    static let programming = Activities(rawValue: 1 << 1)
    static let breathing = Activities(rawValue: 1 << 2)
    static let saveGotham = Activities(rawValue: 1 << 3)
}

Now everything works as expected:

let act: Activities = [.eating, .programming, .saveGotham]
print(act.rawValue) // 11
print(act.contains(.breathing)) // false
print(act.contains(.saveGotham)) // true

Answer 2

From the documentation page for OptionSet,

You use the OptionSet protocol to represent bitset types, where individual bits represent members of a set. Adopting this protocol in your custom types lets you perform set-related operations such as membership tests, unions, and intersections on those types. What’s more, when implemented using specific criteria, adoption of this protocol requires no extra work on your part.

When creating an option set, include a rawValue property in your type declaration. The rawValue property must be of a type that conforms to the FixedWidthInteger protocol, such as Int or UInt8. Next, create unique options as static properties of your custom type using unique powers of two (1, 2, 4, 8, 16, and so forth) for each individual property’s raw value so that each property can be represented by a single bit of the type’s raw value.

The rawValue should be bit set type. So, what is going on with your code above,

let act: Activities = [.eating, .programming, .saveGotham]

Since eating is 1 equivalent to 00000001, programming is 2 which is 00000010, saveGotham is 4 which is 00000100. So, your act variable now sets all three bits at the end, which could be represented like this 00000111.

00000111 now also includes breathing.

From the note above, you should rather make the raw values unique bitwise.

And you can take an example from docs,

struct ShippingOptions: OptionSet {
    let rawValue: Int

    static let nextDay    = ShippingOptions(rawValue: 1 << 0)
    static let secondDay  = ShippingOptions(rawValue: 1 << 1)
    static let priority   = ShippingOptions(rawValue: 1 << 2)
    static let standard   = ShippingOptions(rawValue: 1 << 3)

    static let express: ShippingOptions = [.nextDay, .secondDay]
    static let all: ShippingOptions = [.express, .priority, .standard]
}