import Foundation
import HTTPTypes
import Hummingbird
import Localization

public extension Negotiate {

    // MARK: Methods

    /// The language to serve a request in.
    ///
    /// The `lang` query parameter wins as a deliberate override; failing that, a leading path segment naming a supported language pins it, so an
    /// unrouted path under a language's prefix answers in that language. Anything naming no supported language is ignored, leaving the
    /// `Accept-Language` header and its fallback to the default.
    ///
    /// Called where the answer is used rather than stamped onto every request on the way past: page routes that pin their language by URL never ask.
    /// - Parameter request: the request to negotiate for.
    /// - Returns: the identifier of the supported language to serve.
    func callAsFunction(
        for request: Request
    ) -> String {
        let requested = request.uri.queryParameters[.Parameter.language].map(String.init)
            ?? request.uri.path.split(separator: "/").first.map(String.init)

        return callAsFunction(
            requested: requested,
            acceptLanguage: request.headers[.acceptLanguage]
        )
    }

}

// MARK: - Constants

private extension Substring {
    /// A namespace for the query parameters the negotiation 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"
    }
}