autogen.swift

import Dispatch

/*
 Note: Thanks, Ian Keen and Zev Eisenberg and Sven Weidauer
 
 Zev Eisenberg: "Do you still have to specify 1 << _n_ manually for `OptionSet` conformance? There’s no magic?"
 
 This solution creates values that don't matter. They're simply  unique, since option sets should not be accessed by raw value outside the implementation. (Versus direct `UInt`, which supports bit manipulation operations)
 
*/

/// Stores `OptionSet` counts
///
/// - Note: I dislike unparented globals like this
private var _optionSetDict: [AnyHashable: Int] = [:]

/// Prevents concurrent reads/writes of `_optionSetDict`
///
/// - Note: I dislike unparented globals like this
private var _optionSetAccessQueue = DispatchQueue(label: "sadun.OptionSetGeneration", attributes: [.concurrent])


/// Extends `OptionSet` to allow automatic construction
/// of option entries
///
/// This approach:
///
/// * avoids specific option numbers
/// * hides implementation details
/// * allows better type editing to add/remove options
///
/// - Caution: the raw option value must not be meaningful
///   or accessed directly, as they may change during each
///   compilation/execution.
public extension OptionSet where RawValue == Int {
    
    /// Returns an auto-generated option for an option set
    /// using monotonically increasing bit offsets.
    ///
    /// - Caution: the raw option value must not be meaningful
    ///   or accessed directly, as they may change during each
    ///   compilation/execution.
    public static func generateOption() -> Self {
        // Thanks Sven Weidauer http://twitter.com/5sw_de
        let key = ObjectIdentifier(Self.self)
        return _optionSetAccessQueue.sync(flags: [.barrier]) {
            // This should be locked so there's a guarantee that
            // counts are unique for each generated option
            let count = _optionSetDict[key, default: 0]
            _optionSetDict[key] = count + 1
            return .init(rawValue: 1 << count)
        }
    }
}

// For example
public struct Traits: OptionSet {
    public typealias RawValue = Int
    public var rawValue = 0
    public init(rawValue: Traits.RawValue) {
        self.rawValue = rawValue
    }
    
    public static let bolded = generateOption()
    public static let italicized = generateOption()
    public static let monospaced = generateOption()
    public static let underlined = generateOption()
    public static let outlined = generateOption()
}

// For example
var mySet: Traits = []
print(mySet.rawValue) // 0
mySet.formUnion([.bolded, .monospaced])
print(mySet.rawValue) // 3
mySet.formUnion([.italicized, .monospaced])
print(mySet.rawValue) // 7