import Foundation
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 `lang` query parameter, its path, and its
/// `Accept-Language` header, negotiates the best supported match (falling back to the default language), and stores it on the context's
/// ``LocalizedRequestContext/language``.
///
/// The query parameter is the deliberate override a language switcher links to; failing that, a leading path segment naming a supported language
/// pins it, so an unrouted path under a language's prefix — its not-found page — answers in that language. Values naming no supported language
/// are ignored, leaving the header. The request is passed through untouched — the path and routing are unaffected.
public struct LocalizationMiddleware<Context: LocalizedRequestContext> {

    // MARK: Properties

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

    // MARK: Initializers

    /// Creates a localization middleware that negotiates against the given bundle's String Catalog languages.
    /// - Parameter bundle: the bundle whose String Catalog names the supported languages.
    public init(
        bundle: Bundle
    ) {
        self.negotiate = .init(bundle: bundle)
    }

}

// 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

        // The deliberate query override first; failing that, the leading path segment, so a language's whole URL
        // prefix — routed or not — answers in its language.
        let requested = request.uri.queryParameters[.Parameter.language].map(String.init)
            ?? request.uri.path.split(separator: "/").first.map(String.init)

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

        return try await next(request, context)
    }

}

// MARK: - Constants

private extension Substring {
    /// A namespace for the query parameters the middleware reads.
    enum Parameter {
        /// The query parameter carrying an explicit language choice; the site's language switcher appends it to the current path.
        static let language: Substring = "lang"
    }
}