import HTTPTypes
import Hummingbird
import Localization

/// Resolves the visitor's preferred language and records it on the request context.
///
/// Placed ahead of the localized responders in the middleware chain, it reads the request's
/// `Accept-Language` header, negotiates the best supported match (falling back to the default
/// language), and stores it on the context's ``LocalizedRequestContext/language``.
///
/// The request is otherwise passed through untouched — the URL and routing are not affected — so
/// each page is served at its existing path and varies its content by header.
public struct LocalizationMiddleware<Context: LocalizedRequestContext> {

    // MARK: Properties

    /// Negotiates the request's language from its `Accept-Language` header.
    private let negotiate: NegotiateLanguage

    // MARK: Initializers

    /// Creates a localization middleware.
    public init() {
        self.negotiate = .init()
    }

}

// MARK: - RouterMiddleware

extension LocalizationMiddleware: RouterMiddleware {

    // MARK: Functions

    /// Negotiates the request's language and records it on the context before passing it down.
    /// - 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.
    /// - Throws: any error thrown downstream.
    public func handle(
        _ request: Request,
        context: Context,
        next: (Request, Context) async throws -> Response
    ) async throws -> Response {
        var context = context

        context.language = negotiate(
            forAcceptLanguage: request.headers[.acceptLanguage]
        )

        return try await next(request, context)
    }

}