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