import HTTPTypes
import Hummingbird

/// Stamps a set of security-related HTTP headers onto every response.
///
/// Placed at (or near) the top of the middleware chain, it adds the configured headers to whatever response bubbles back up — the rendered pages, the
/// error page produced by ``NotFoundMiddleware``, and every static file served by `FileMiddleware` — so the browser applies the strict, hardened
/// interpretation of the content instead of its lenient legacy defaults.
///
/// The headers are precomputed once from the ``Configuration`` at initialization and reused for every request, so the per-request cost is a handful of
/// header copies.
public struct SecurityHeadersMiddleware<Context: RequestContext> {

    // MARK: Properties

    /// The precomputed headers applied to every response.
    private let fields: HTTPFields

    // MARK: Initializers

    /// Creates a security-headers middleware.
    /// - Parameter configuration: the headers applied to every response. Defaults to a hardened baseline suitable for a static site, with
    ///   `Strict-Transport-Security` left off (see ``Configuration``).
    public init(
        configuration: Configuration = .init()
    ) {
        self.fields = configuration.fields
    }

}

// MARK: - RouterMiddleware

extension SecurityHeadersMiddleware: RouterMiddleware {

    // MARK: Functions

    /// Passes the request down the chain and stamps the configured security headers onto the response on the way back up.
    ///
    /// Errors that can render themselves (`HTTPResponseError`, like the `HTTPError`s thrown by the controllers) are converted to their response
    /// here rather than left to the router: the router converts them above the middleware chain, where the response would escape these headers.
    /// Existing values for the same header names are replaced so downstream middleware cannot leave a weaker policy in place.
    /// - 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 with the security headers applied.
    /// - Throws: any downstream error that does not render as an HTTP response.
    public func handle(
        _ request: Request,
        context: Context,
        next: (Request, Context) async throws -> Response
    ) async throws -> Response {
        var response: Response

        do {
            response = try await next(
                request,
                context
            )
        } catch let error as any HTTPResponseError {
            response = try error.response(
                from: request,
                context: context
            )
        }

        for field in fields {
            response.headers[field.name] = field.value
        }

        return response
    }

}

// MARK: - Helpers

private extension SecurityHeadersMiddleware.Configuration {

    // MARK: Computed

    /// The configuration expressed as the headers to apply, omitting any whose value is `nil`.
    var fields: HTTPFields {
        var fields = HTTPFields()

        fields[.contentSecurityPolicy] = contentSecurityPolicy
        fields[.xContentTypeOptions] = contentTypeOptions
        fields[.frameOptions] = frameOptions
        fields[.referrerPolicy] = referrerPolicy
        fields[.permissionsPolicy] = permissionsPolicy
        fields[.strictTransportSecurity] = strictTransportSecurity

        return fields
    }

}

// MARK: - Configuration

extension SecurityHeadersMiddleware {
    /// The set of security headers a ``SecurityHeadersMiddleware`` applies.
    ///
    /// Each property maps to a single response header. A `nil` value omits that header entirely, which is how `Strict-Transport-Security` stays
    /// disabled by default: it is only safe to send over HTTPS and is "sticky" in browsers, so it must stay off in plain-HTTP development and be switched on
    /// (via configuration) only in TLS-terminated production.
    public struct Configuration: Sendable {
        
        // MARK: Properties
        
        /// The `Content-Security-Policy` value (controls which sources the browser will load).
        public let contentSecurityPolicy: String?
        /// The `X-Content-Type-Options` value (disables MIME sniffing when set to `nosniff`).
        public let contentTypeOptions: String?
        /// The `X-Frame-Options` value (controls whether the page may be framed).
        public let frameOptions: String?
        /// The `Referrer-Policy` value (controls how much referrer information is shared).
        public let referrerPolicy: String?
        /// The `Permissions-Policy` value (gates access to powerful browser features).
        public let permissionsPolicy: String?
        /// The `Strict-Transport-Security` value, or `nil` to omit the header (the default).
        public let strictTransportSecurity: String?
        
        // MARK: Initializers
        
        /// Creates a security-headers configuration.
        ///
        /// Every parameter defaults to the hardened baseline defined in `String.Security`, except `strictTransportSecurity`, which defaults
        /// to `nil` (omitted). Pass `nil` for any header to drop it from the response.
        /// - Parameters:
        ///   - contentSecurityPolicy: the `Content-Security-Policy` value.
        ///   - contentTypeOptions: the `X-Content-Type-Options` value.
        ///   - frameOptions: the `X-Frame-Options` value.
        ///   - referrerPolicy: the `Referrer-Policy` value.
        ///   - permissionsPolicy: the `Permissions-Policy` value.
        ///   - strictTransportSecurity: the `Strict-Transport-Security` value, or `nil` to omit it.
        public init(
            contentSecurityPolicy: String? = String.Security.contentSecurityPolicy,
            contentTypeOptions: String? = String.Security.contentTypeOptions,
            frameOptions: String? = String.Security.frameOptions,
            referrerPolicy: String? = String.Security.referrerPolicy,
            permissionsPolicy: String? = String.Security.permissionsPolicy,
            strictTransportSecurity: String? = nil
        ) {
            self.contentSecurityPolicy = contentSecurityPolicy
            self.contentTypeOptions = contentTypeOptions
            self.frameOptions = frameOptions
            self.referrerPolicy = referrerPolicy
            self.permissionsPolicy = permissionsPolicy
            self.strictTransportSecurity = strictTransportSecurity
        }
        
    }
}