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/// Enum that contains states for paginated data loading.
///
/// - initial: Initial state. Before something will happen.
/// - initialLoading: Initial loading state. When data was requested initially.
/// - loadingMore: Loading more state. When additional data was requested.
/// - results: Result state from a specific data source after a given state.
/// - error: Error state with a specific error after a given state.
/// - empty: Empty state. When data was initially requested and empty result was received.
/// - exhausted: Exhausted state. When no more results can be received.
public indirect enum PaginationDataLoadingState<DS: DataSource> {

    case initial
    case initialLoading(after: PaginationDataLoadingState)
    case loadingMore(after: PaginationDataLoadingState)
    case results(newItems: DS.ResultType, from: DS, after: PaginationDataLoadingState)
    case error(error: Error, after: PaginationDataLoadingState)
    case empty
    case exhausted
}

extension PaginationDataLoadingState: DataLoadingState {

    public typealias DataSourceType = DS

    public static var initialState: PaginationDataLoadingState<DS> {
        .initial
    }

    public static var emptyState: PaginationDataLoadingState<DS> {
        .empty
    }

    public static func initialLoadingState(after: PaginationDataLoadingState<DS>) -> PaginationDataLoadingState<DS> {
        .initialLoading(after: after)
    }

    public static func resultState(result: DS.ResultType,
                                   from: DataSourceType,
                                   after: PaginationDataLoadingState<DS>) -> PaginationDataLoadingState<DS> {

        .results(newItems: result, from: from, after: after)
    }

    public static func errorState(error: Error,
                                  after: PaginationDataLoadingState<DS>) -> PaginationDataLoadingState<DS> {

        .error(error: error, after: after)
    }

    public var isInitialState: Bool {
        switch self {
        case .initial:
            return true

        default:
            return false
        }
    }

    public var result: DS.ResultType? {
        switch self {
        case .results(let newItems, _, _):
            return newItems

        default:
            return nil
        }
    }

    public var error: Error? {
        switch self {
        case .error(let error, _):
            return error

        default:
            return nil
        }
    }
}