AssociativeArray.swift

import Foundation

/// A "collection" of paired items
public struct AssociativeArray<PairedItem: Hashable>: CustomStringConvertible, ExpressibleByDictionaryLiteral {
    
    /// Constructable outside module
    public init() {}

    public typealias DictForm = Dictionary<PairedItem, PairedItem>
    
    /// The true 2-way association backing dictionary
    private var backingDict: DictForm = [:]
    
    /// The user-facing version which updates synchronously
    /// so there is no overhead for user facing forms.
    private var userFacingDict: DictForm = [:]
    
    /// Supports thread safety
    private var accessQueue = DispatchQueue(label: "org.sadun.2WayArray", attributes: [.concurrent])
    
    /// Allow paired access
    public subscript(item1: PairedItem) -> PairedItem? {
        get {
            return accessQueue.sync { return backingDict[item1] }
        }
        
        set(newItem2) {
            return accessQueue.sync(flags: [.barrier]) {
                
                // No op for same value
                let item2 = backingDict[item1]
                guard item2 != newItem2 else { return }
                
                switch (item2, newItem2) {
                case (let item2?, let newItem2?):
                    
                    // Sanitize newItem2 if paired
                    let newItem1 = backingDict[newItem2]
                    backingDict.removeValue(forKey: newItem2)
                    userFacingDict.removeValue(forKey: newItem2)
                    if newItem1 != nil {
                        let newItem1: PairedItem! = newItem1
                        backingDict.removeValue(forKey: newItem1)
                        userFacingDict.removeValue(forKey: newItem1)
                    }
                    
                    // Sanitize item2
                    backingDict.removeValue(forKey: item2)
                    userFacingDict.removeValue(forKey: item2)
                    
                    // Introduce new pair
                    backingDict[newItem2] = item1
                    backingDict[item1] = newItem2
                    userFacingDict[item1] = newItem2
                    
                case (let item2?, nil):
                    backingDict.removeValue(forKey: item1)
                    backingDict.removeValue(forKey: item2)
                    userFacingDict.removeValue(forKey: item1)
                    userFacingDict.removeValue(forKey: item2)
                    
                case (nil, let newItem2?):
                    backingDict[newItem2] = item1
                    backingDict[item1] = newItem2
                    userFacingDict[item1] = newItem2
                    
                default:
                    assertionFailure("Should never get here")
                }
            }
        }        
    }
    
    // Support dictionary literal conformance
    public typealias Key = PairedItem
    public typealias Value = PairedItem
    
    /// Allows dictionary literal initialization
    public init(dictionaryLiteral elements: (PairedItem, PairedItem)...) {
        self.init()
        for (item1, item2) in elements {
            self[item1] = item2
        }
    }

    /// Returns reduced version with duplicates removed
    public var dictionaryRepresentation: [PairedItem: PairedItem] {
        return userFacingDict
    }
    
    /// Provides human-facing description
    public var description: String {
        return "\(dictionaryRepresentation)"
    }
}

/// For public consumption in most common use case
public typealias PairedStrings = AssociativeArray<String>

main.swift

import Foundation

var test1 = AssociativeArray<String>()
test1["a"] = "b"
print(test1)
assert(test1["b"] == "a")
assert(test1["a"] == "b")

var test2: PairedStrings = [:]
print(test2)

var test3: PairedStrings = [
    "a": "b",
    "c": "d",
    "e": "f",
    "d": "a" // removes "a":"b" and "c":"d"
]
assert(test3.dictionaryRepresentation == ["f": "e", "a": "d"])
print(test3)

test3["b"] = "c"
assert(test3.dictionaryRepresentation == ["b": "c", "f": "e", "a": "d"])
print(test3)