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    use std::collections::HashMap;

    use error::Result;
    use http::StatusCode;
    use httprequest::HttpRequest;
    use httpresponse::HttpResponse;
    use middleware::{Middleware, Response};

    type ErrorHandler<S> = Fn(&HttpRequest<S>, HttpResponse) -> Result<Response>;

    /// `Middleware` for allowing custom handlers for responses.
    ///
    /// You can use `ErrorHandlers::handler()` method  to register a custom error
    /// handler for specific status code. You can modify existing response or
    /// create completely new one.
    ///
    /// ## Example
    ///
    /// ```rust
    /// # extern crate actix_web;
    /// use actix_web::middleware::{ErrorHandlers, Response};
    /// use actix_web::{http, App, HttpRequest, HttpResponse, Result};
    ///
    /// fn render_500<S>(_: &HttpRequest<S>, resp: HttpResponse) -> Result<Response> {
    ///     let mut builder = resp.into_builder();
    ///     builder.header(http::header::CONTENT_TYPE, "application/json");
    ///     Ok(Response::Done(builder.into()))
    /// }
    ///
    /// fn main() {
    ///     let app = App::new()
    ///         .middleware(
    ///             ErrorHandlers::new()
    ///                 .handler(http::StatusCode::INTERNAL_SERVER_ERROR, render_500),
    ///         )
    ///         .resource("/test", |r| {
    ///             r.method(http::Method::GET).f(|_| HttpResponse::Ok());
    ///             r.method(http::Method::HEAD)
    ///                 .f(|_| HttpResponse::MethodNotAllowed());
    ///         })
    ///         .finish();
    /// }
    /// ```
    pub struct ErrorHandlers<S> {
        handlers: HashMap<StatusCode, Box<ErrorHandler<S>>>,
    }

    impl<S> Default for ErrorHandlers<S> {
        fn default() -> Self {
            ErrorHandlers {
                handlers: HashMap::new(),
            }
        }
    }

    impl<S> ErrorHandlers<S> {
        /// Construct new `ErrorHandlers` instance
        pub fn new() -> Self {
            ErrorHandlers::default()
        }

        /// Register error handler for specified status code
        pub fn handler<F>(mut self, status: StatusCode, handler: F) -> Self
        where
            F: Fn(&HttpRequest<S>, HttpResponse) -> Result<Response> + 'static,
        {
            self.handlers.insert(status, Box::new(handler));
            self
        }
    }

    impl<S: 'static> Middleware<S> for ErrorHandlers<S> {
        fn response(&self, req: &HttpRequest<S>, resp: HttpResponse) -> Result<Response> {
            if let Some(handler) = self.handlers.get(&resp.status()) {
                handler(req, resp)
            } else {
                Ok(Response::Done(resp))
            }
        }
    }

    #[cfg(test)]
    mod tests {
        use super::*;
        use error::{Error, ErrorInternalServerError};
        use http::header::CONTENT_TYPE;
        use http::StatusCode;
        use httpmessage::HttpMessage;
        use middleware::Started;
        use test::{self, TestRequest};

        fn render_500<S>(_: &HttpRequest<S>, resp: HttpResponse) -> Result<Response> {
            let mut builder = resp.into_builder();
            builder.header(CONTENT_TYPE, "0001");
            Ok(Response::Done(builder.into()))
        }

        #[test]
        fn test_handler() {
            let mw =
                ErrorHandlers::new().handler(StatusCode::INTERNAL_SERVER_ERROR, render_500);

            let mut req = TestRequest::default().finish();
            let resp = HttpResponse::InternalServerError().finish();
            let resp = match mw.response(&mut req, resp) {
                Ok(Response::Done(resp)) => resp,
                _ => panic!(),
            };
            assert_eq!(resp.headers().get(CONTENT_TYPE).unwrap(), "0001");

            let resp = HttpResponse::Ok().finish();
            let resp = match mw.response(&mut req, resp) {
                Ok(Response::Done(resp)) => resp,
                _ => panic!(),
            };
            assert!(!resp.headers().contains_key(CONTENT_TYPE));
        }

        struct MiddlewareOne;

        impl<S> Middleware<S> for MiddlewareOne {
            fn start(&self, _: &HttpRequest<S>) -> Result<Started, Error> {
                Err(ErrorInternalServerError("middleware error"))
            }
        }

        #[test]
        fn test_middleware_start_error() {
            let mut srv = test::TestServer::new(move |app| {
                app.middleware(
                    ErrorHandlers::new()
                        .handler(StatusCode::INTERNAL_SERVER_ERROR, render_500),
                ).middleware(MiddlewareOne)
                    .handler(|_| HttpResponse::Ok())
            });

            let request = srv.get().finish().unwrap();
            let response = srv.execute(request.send()).unwrap();
            assert_eq!(response.headers().get(CONTENT_TYPE).unwrap(), "0001");
        }
    }

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