import Foundation

protocol Identifiable {
    associatedtype Identifier: Hashable
    var identifier: Identifier { get }
}

protocol Updatable {
    associatedtype Source: Identifiable

    // implement this to update this type from a source representation
    func update(from source: Source)

    // implement this to perform any cleanup before removing this element
    func willBeDeleted()
}

protocol CollectionDiffable {
    associatedtype Element: Updatable
    var elementMap: [Element.Source.Identifier: Element] { get }

    func update(source: [Element.Source])

    // implement this to create a new Element from a Source
    func add(source: Element.Source)

    // implement this to remove an Element from the collection
    func remove(identifier: Element.Source.Identifier)
}

extension CollectionDiffable {
    func update(sources: [Element.Source]) {
        let identifiers = sources.map { $0.identifier }
        elementMap.keys
            .filter { !identifiers.contains($0) }
            .forEach {
                if let itemToDelete = elementMap[$0] {
                    itemToDelete.willBeDeleted()
                }
                self.remove(identifier: $0)
        }

        sources.forEach { source in
            let identifier = source.identifier
            if let existingUpdatable = elementMap[identifier] {
                existingUpdatable.update(from: source)
            } else {
                self.add(source: source)
            }
        }
    }
}