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 naming the
/// language it was rendered in.
///
/// `Vary: Accept-Language` is the caller's to declare, since only the caller knows how it picks the language: a responder that negotiates the
/// header sets it, so shared caches key on the language rather than serving one to everyone; a responder that pins the language by route does
/// not, since the header would announce a negotiation that never happens.
public struct LocalizedHTMLCollectionResponse: Sendable {

    // MARK: Properties

    /// The supported languages and default language, derived from the bundle'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:
    ///   - bundle: the bundle whose String Catalog names the languages the document is rendered for.
    ///   - status: the status applied to every response. Defaults to `.ok`.
    ///   - variesOnAcceptLanguage: whether the responses declare `Vary: Accept-Language`. Defaults to `false`; a responder that
    ///     negotiates the header passes `true`.
    ///   - document: builds the document to render for a given locale.
    public init<Document: HTMLDocument>(
        bundle: Bundle,
        status: HTTPResponse.Status = .ok,
        variesOnAcceptLanguage: Bool = false,
        document: (Locale) -> Document
    ) {
        self.list = .init(bundle: bundle)
        self.responses = list.all
            .reduce(into: [:]) { responses, language in
                var headers: HTTPFields = [.contentLanguage: language]

                if variesOnAcceptLanguage {
                    headers[.vary] = "Accept-Language"
                }

                responses[language] = CachedHTMLResponse(
                    status: status,
                    additionalHeaders: headers,
                    document: document(.init(
                        identifier: language
                    ))
                )
            }
    }

    // MARK: Methods

    /// Builds the response for the given language, falling back to the default language.
    /// - Parameters:
    ///   - language: the negotiated language identifier.
    ///   - request: the request the response answers, consulted for conditional revalidation.
    /// - 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.
    public func response(
        for language: String,
        request: Request
    ) -> Response {
        guard
            let response = responses[language] ?? responses[list.default]
        else {
            return .init(
                status: .internalServerError
            )
        }

        return response.response(
            for: request
        )
    }

}