import Elementary
import Foundation
import HTTPTypes
import Hummingbird
import Localization

/// A per-language collection of pre-rendered HTML responses.
///
/// At initialization it renders the document once for each language the bundle's ``LanguageList``
/// reports and caches the bytes, mirroring ``CachedHTMLResponse``'s render-once model but keyed by
/// language. Each cached response carries a `Content-Language` header and `Vary: Accept-Language`, so
/// shared caches key on the negotiated language instead of serving one language to everyone.
struct LocalizedHTMLCollectionResponse: Sendable {

    // MARK: Properties

    /// The supported languages and default language, derived from the module's String Catalog.
    private let list: LanguageList

    /// The pre-rendered responses, keyed by language identifier.
    private let responses: [String: CachedHTMLResponse]

    // MARK: Initializers

    /// Renders the document once per supported language.
    /// - Parameters:
    ///   - status: the status applied to every response. Defaults to `.ok`.
    ///   - document: builds the document to render for a given locale.
    init<Document: HTMLDocument>(
        status: HTTPResponse.Status = .ok,
        document: (Locale) -> Document
    ) {
        self.list = .init(bundle: .module)
        self.responses = list.all
            .reduce(into: [:]) { responses, language in
                responses[language] = CachedHTMLResponse(
                    status: status,
                    additionalHeaders: [
                        .contentLanguage: language,
                        .vary: "Accept-Language",
                    ],
                    document: document(.init(identifier: language))
                )
            }
    }

    // MARK: Methods

    /// Builds the response for the given language, falling back to the default language.
    /// - Parameter language: the negotiated language identifier.
    /// - Returns: the cached response for the language, the default language's response when the
    ///   language is unavailable, or a `500 Internal Server Error` if neither is cached.
    func response(
        for language: String
    ) -> Response {
        guard
            let response = responses[language] ?? responses[list.default]
        else {
            return .init(status: .internalServerError)
        }

        return response.response()
    }

}