Debugging Performance Issues

/* a terrible data structure which has a subset of the operations that Swift's
 * array does:
 *  - retrieving elements by index
 *     --> user's reasonable performance expectation: O(1)   (like Swift's Array)
 *     --> implementation's actual performance:       O(n)
 *  - adding elements
 *     --> user's reasonable performance expectation: amortised O(1)   (like Swift's Array)
 *     --> implementation's actual performance:       O(n)
 *
 * ie. the problem I'm trying to demo here is that this is an implementation
 * where the user would expect (amortised) constant time access but in reality
 * is linear time.
 */
struct TerribleArray<T: Comparable> {
    /* this is a terrible idea: storing the index inside of the array (so we can
     * waste some performance later ;)
     */
    private var storage: Array<(Int, T)> = Array()

    /* oh my */
    private func maximumIndex() -> Int {
        return (self.storage.map { $0.0 }.max()) ?? -1
    }

    /* expectation: amortised O(1) but implementation is O(n) */
    public mutating func append(_ value: T) {
        let maxIdx = self.maximumIndex()
        self.storage.append((maxIdx + 1, value))
        assert(self.storage.count == maxIdx + 2)
    }

    /* expectation: O(1) but implementation is O(n) */
    public subscript(index: Int) -> T? {
        get {
            return self.storage.filter({ $0.0 == index }).first?.1
        }
    }
}

protocol FavouriteNumbers {
    func addFavouriteNumber(_ number: Int)
    func isFavouriteNumber(_ number: Int) -> Bool
}

public class MyFavouriteNumbers: FavouriteNumbers {
    private var storage: TerribleArray<Int>
    public init() {
        self.storage = TerribleArray<Int>()
    }

    /* - user's expectation: O(n)
     * - reality O(n^2) because of TerribleArray */
    public func isFavouriteNumber(_ number: Int) -> Bool {
        var idx = 0
        var found = false
        while true {
            if let storageNum = self.storage[idx] {
                if number == storageNum {
                    found = true
                    break
                }
            } else {
                break
            }
            idx += 1
        }
        return found
    }

    /* - user's expectation: amortised O(1)
     * - reality O(n) because of TerribleArray */
    public func addFavouriteNumber(_ number: Int) {
        self.storage.append(number)
        precondition(self.isFavouriteNumber(number))
    }
}

let x: FavouriteNumbers = MyFavouriteNumbers()

for f in 0..<2_000 {
    x.addFavouriteNumber(f)
}