CacheCreek/Source/AnyKey.swift

//
//  AnyKey.swift
//  CacheCreek
//
//  Created by Christopher Luu on 1/26/16.
//
//

import Foundation

/// This code was taken from:
/// http://stackoverflow.com/questions/24119624/how-to-create-dictionary-that-can-hold-anything-in-key-or-all-the-possible-type
///
/// `AnyKey` is a simple struct that conforms to `Hashable` to allow any other `Hashable` key to be used in the cache dictionary
struct AnyKey: Hashable {
    /// The underlying value
    fileprivate let underlying: Any
    /// The hashing function
    fileprivate let hashValueFunc: () -> Int
    /// The equality function
    fileprivate let equalityFunc: (Any) -> Bool

    init<T: Hashable>(_ key: T) {
        underlying = key
        // Capture the key's hashability and equatability using closures.
        // The Key shares the hash of the underlying value.
        hashValueFunc = { key.hashValue }
        
        // The Key is equal to a Key of the same underlying type,
        // whose underlying value is "==" to ours.
        equalityFunc = {
            if let other = $0 as? T {
                return key == other
            }
            return false
        }
    }

    /// `Hashable` protocol conformance
    var hashValue: Int { return hashValueFunc() }
}

func ==(x: AnyKey, y: AnyKey) -> Bool {
    return x.equalityFunc(y.underlying)
}