import Hummingbird

/// Serves a custom error page for requests that match neither a route nor a static file.
///
/// Placed ahead of `FileMiddleware` in the middleware chain, it catches the `.notFound` error
/// that bubbles up when no file exists for the requested path and responds with the rendered
/// ``ErrorPage`` and a `404 Not Found` status. The page is served in the language stored on the
/// context by ``LocalizationMiddleware``, falling back to the default language.
public struct NotFoundMiddleware<Context: LocalizedRequestContext> {

    // MARK: Properties

    /// The error page, rendered once per supported language and reused for every not-found response.
    private let responses: LocalizedHTMLCollectionResponse

    // MARK: Initializers

    /// Creates a not-found middleware.
    public init() {
        self.responses = .init(
            status: .notFound
        ) {
            ErrorPage(locale: $0)
        }
    }

}

// MARK: - RouterMiddleware

extension NotFoundMiddleware: RouterMiddleware {

    // MARK: Functions

    /// Passes the request down the chain, rendering the error page if it results in a not-found
    /// response.
    ///
    /// Any error other than `.notFound` is rethrown unchanged.
    /// - Parameters:
    ///   - request: the incoming request.
    ///   - context: the context the request is resolved against.
    ///   - next: the next responder in the middleware chain.
    /// - Returns: the downstream response, or the rendered ``ErrorPage`` with a `404 Not Found` status.
    /// - Throws: any non-not-found error thrown downstream.
    public func handle(
        _ request: Request,
        context: Context,
        next: (Request, Context) async throws -> Response
    ) async throws -> Response {
        do {
            return try await next(request, context)
        }
        catch let error {
            guard
                let responseError = error as? any HTTPResponseError,
                responseError.status == .notFound
            else {
                throw error
            }

            return responses.response(
                for: context.language
            )
        }
    }

}