import Foundation
import Hummingbird
import NIOCore

/// Serves a custom error page for requests that match neither a route nor a static file.
///
/// Placed ahead of `FileMiddleware` in the middleware chain, it catches the `.notFound` error
/// that bubbles up when no file exists for the requested path and responds with the preloaded
/// error page and a `404 Not Found` status.
public struct NotFoundMiddleware<Context: RequestContext> {
    
    // MARK: Properties
    
    /// The body of the error page served on a not-found response.
    private let page: ByteBuffer
    
    // MARK: Initializers
    
    /// Creates a middleware that serves the error page (`404.html`) from the static files folder.
    ///
    /// The page is read once, at construction. A minimal fallback body is used when the file is
    /// missing.
    /// - Parameter staticFilesPath: the folder, relative to the working directory, the static files are served from.
    public init(
        _ staticFilesPath: String
    ) {
        let path = StaticFile.errorHTML.path(relativeTo: staticFilesPath)
        
        if let data = try? Data(contentsOf: URL(fileURLWithPath: path)) {
            self.init(page: .init(bytes: data))
        } else {
            self.init(page: .init(string: "404 Not Found"))
        }
    }
    
    /// Creates a middleware that serves the given error page on a not-found response.
    /// - Parameter page: the body of the error page.
    init(
        page: ByteBuffer
    ) {
        self.page = page
    }
    
}

// MARK: - RouterMiddleware

extension NotFoundMiddleware: RouterMiddleware {

    // MARK: Functions

    public func handle(
        _ request: Request,
        context: Context,
        next: (Request, Context) async throws -> Response
    ) async throws -> Response {
        do {
            return try await next(request, context)
        } catch let error {
            // Only intercept "not found"; let every other error propagate.
            guard
                let responseError = error as? any HTTPResponseError,
                responseError.status == .notFound
            else {
                throw error
            }

            var headers = HTTPFields()

            headers[.contentType] = StaticFile.errorHTML.contentType

            return Response(
                status: .notFound,
                headers: headers,
                body: .init(byteBuffer: page)
            )
        }
    }

}