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    use std::{
        collections::HashSet,
        convert::{TryFrom, TryInto},
        fmt,
        rc::Rc,
    };

    use actix_web::{
        dev::RequestHead,
        error::Result,
        http::{
            header::{self, HeaderMap, HeaderName, HeaderValue},
            Method,
        },
    };
    use once_cell::sync::Lazy;
    use smallvec::SmallVec;

    use crate::{AllOrSome, CorsError};

    #[derive(Clone)]
    pub(crate) struct OriginFn {
        #[allow(clippy::type_complexity)]
        pub(crate) boxed_fn: Rc<dyn Fn(&HeaderValue, &RequestHead) -> bool>,
    }

    impl Default for OriginFn {
        /// Dummy default for use in tiny_vec. Do not use.
        fn default() -> Self {
            let boxed_fn: Rc<dyn Fn(&_, &_) -> _> = Rc::new(|_origin, _req_head| false);
            Self { boxed_fn }
        }
    }

    impl fmt::Debug for OriginFn {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str("origin_fn")
        }
    }

    /// Try to parse header value as HTTP method.
    pub(crate) fn header_value_try_into_method(hdr: &HeaderValue) -> Option<Method> {
        hdr.to_str()
            .ok()
            .and_then(|meth| Method::try_from(meth).ok())
    }

    #[derive(Debug, Clone)]
    pub(crate) struct Inner {
        pub(crate) allowed_origins: AllOrSome<HashSet<HeaderValue>>,
        pub(crate) allowed_origins_fns: SmallVec<[OriginFn; 4]>,

        pub(crate) allowed_methods: HashSet<Method>,
        pub(crate) allowed_methods_baked: Option<HeaderValue>,

        pub(crate) allowed_headers: AllOrSome<HashSet<HeaderName>>,
        pub(crate) allowed_headers_baked: Option<HeaderValue>,

        /// `All` will echo back `Access-Control-Request-Header` list.
        pub(crate) expose_headers: AllOrSome<HashSet<HeaderName>>,
        pub(crate) expose_headers_baked: Option<HeaderValue>,

        pub(crate) max_age: Option<usize>,
        pub(crate) preflight: bool,
        pub(crate) send_wildcard: bool,
        pub(crate) supports_credentials: bool,
        #[cfg(feature = "draft-private-network-access")]
        pub(crate) allow_private_network_access: bool,
        pub(crate) vary_header: bool,
        pub(crate) block_on_origin_mismatch: bool,
    }

    static EMPTY_ORIGIN_SET: Lazy<HashSet<HeaderValue>> = Lazy::new(HashSet::new);

    impl Inner {
        /// The bool returned in Ok(_) position indicates whether the `Access-Control-Allow-Origin`
        /// header should be added to the response or not.
        pub(crate) fn validate_origin(&self, req: &RequestHead) -> Result<bool, CorsError> {
            // return early if all origins are allowed or get ref to allowed origins set
            #[allow(clippy::mutable_key_type)]
            let allowed_origins = match &self.allowed_origins {
                AllOrSome::All if self.allowed_origins_fns.is_empty() => return Ok(true),
                AllOrSome::Some(allowed_origins) => allowed_origins,
                // only function origin validators are defined
                _ => &EMPTY_ORIGIN_SET,
            };

            // get origin header and try to parse as string
            match req.headers().get(header::ORIGIN) {
                // origin header exists and is a string
                Some(origin) => {
                    if allowed_origins.contains(origin) || self.validate_origin_fns(origin, req) {
                        Ok(true)
                    } else if self.block_on_origin_mismatch {
                        Err(CorsError::OriginNotAllowed)
                    } else {
                        Ok(false)
                    }
                }

                // origin header is missing
                // note: with our implementation, the origin header is required for OPTIONS request or
                // else this would be unreachable
                None => Err(CorsError::MissingOrigin),
            }
        }

        /// Accepts origin if _ANY_ functions return true. Only called when Origin exists.
        fn validate_origin_fns(&self, origin: &HeaderValue, req: &RequestHead) -> bool {
            self.allowed_origins_fns
                .iter()
                .any(|origin_fn| (origin_fn.boxed_fn)(origin, req))
        }

        /// Only called if origin exists and always after it's validated.
        pub(crate) fn access_control_allow_origin(&self, req: &RequestHead) -> Option<HeaderValue> {
            let origin = req.headers().get(header::ORIGIN);

            match self.allowed_origins {
                AllOrSome::All => {
                    if self.send_wildcard {
                        Some(HeaderValue::from_static("*"))
                    } else {
                        // see note below about why `.cloned()` is correct
                        origin.cloned()
                    }
                }

                AllOrSome::Some(_) => {
                    // since origin (if it exists) is known to be allowed if this method is called
                    // then cloning the option is all that is required to be used as an echoed back
                    // header value (or omitted if None)
                    origin.cloned()
                }
            }
        }

        /// Use in preflight checks and therefore operates on header list in
        /// `Access-Control-Request-Headers` not the actual header set.
        pub(crate) fn validate_allowed_method(&self, req: &RequestHead) -> Result<(), CorsError> {
            // extract access control header and try to parse as method
            let request_method = req
                .headers()
                .get(header::ACCESS_CONTROL_REQUEST_METHOD)
                .map(header_value_try_into_method);

            match request_method {
                // method valid and allowed
                Some(Some(method)) if self.allowed_methods.contains(&method) => Ok(()),

                // method valid but not allowed
                Some(Some(_)) => Err(CorsError::MethodNotAllowed),

                // method invalid
                Some(_) => Err(CorsError::BadRequestMethod),

                // method missing so this is not a preflight request
                None => Err(CorsError::MissingRequestMethod),
            }
        }

        pub(crate) fn validate_allowed_headers(&self, req: &RequestHead) -> Result<(), CorsError> {
            // return early if all headers are allowed or get ref to allowed origins set
            #[allow(clippy::mutable_key_type)]
            let allowed_headers = match &self.allowed_headers {
                AllOrSome::All => return Ok(()),
                AllOrSome::Some(allowed_headers) => allowed_headers,
            };

            // extract access control header as string
            // header format should be comma separated header names
            let request_headers = req
                .headers()
                .get(header::ACCESS_CONTROL_REQUEST_HEADERS)
                .map(|hdr| hdr.to_str());

            match request_headers {
                // header list is valid string
                Some(Ok(headers)) => {
                    // the set is ephemeral we take care not to mutate the
                    // inserted keys so this lint exception is acceptable
                    #[allow(clippy::mutable_key_type)]
                    let mut request_headers = HashSet::with_capacity(8);

                    // try to convert each header name in the comma-separated list
                    for hdr in headers.split(',') {
                        match hdr.trim().try_into() {
                            Ok(hdr) => request_headers.insert(hdr),
                            Err(_) => return Err(CorsError::BadRequestHeaders),
                        };
                    }

                    // header list must contain 1 or more header name
                    if request_headers.is_empty() {
                        return Err(CorsError::BadRequestHeaders);
                    }

                    // request header list must be a subset of allowed headers
                    if !request_headers.is_subset(allowed_headers) {
                        return Err(CorsError::HeadersNotAllowed);
                    }

                    Ok(())
                }

                // header list is not a string
                Some(Err(_)) => Err(CorsError::BadRequestHeaders),

                // header list missing
                None => Ok(()),
            }
        }
    }

    /// Add CORS related request headers to response's Vary header.
    ///
    /// See <https://fetch.spec.whatwg.org/#cors-protocol-and-http-caches>.
    pub(crate) fn add_vary_header(headers: &mut HeaderMap) {
        let value = match headers.get(header::VARY) {
            Some(hdr) => {
                let mut val: Vec<u8> = Vec::with_capacity(hdr.len() + 71);
                val.extend(hdr.as_bytes());
                val.extend(b", Origin, Access-Control-Request-Method, Access-Control-Request-Headers");

                #[cfg(feature = "draft-private-network-access")]
                val.extend(b", Access-Control-Request-Private-Network");

                val.try_into().unwrap()
            }

            #[cfg(feature = "draft-private-network-access")]
            None => HeaderValue::from_static(
                "Origin, Access-Control-Request-Method, Access-Control-Request-Headers, \
                Access-Control-Request-Private-Network",
            ),

            #[cfg(not(feature = "draft-private-network-access"))]
            None => HeaderValue::from_static(
                "Origin, Access-Control-Request-Method, Access-Control-Request-Headers",
            ),
        };

        headers.insert(header::VARY, value);
    }

    #[cfg(test)]
    mod test {
        use std::rc::Rc;

        use actix_web::{
            dev::Transform,
            http::{
                header::{self, HeaderValue},
                Method, StatusCode,
            },
            test::{self, TestRequest},
        };

        use crate::Cors;

        fn val_as_str(val: &HeaderValue) -> &str {
            val.to_str().unwrap()
        }

        #[actix_web::test]
        async fn test_validate_not_allowed_origin() {
            let cors = Cors::default()
                .allowed_origin("https://www.example.com")
                .block_on_origin_mismatch(true)
                .new_transform(test::ok_service())
                .await
                .unwrap();

            let req = TestRequest::get()
                .insert_header((header::ORIGIN, "https://www.unknown.com"))
                .insert_header((header::ACCESS_CONTROL_REQUEST_HEADERS, "DNT"))
                .to_srv_request();

            assert!(cors.inner.validate_origin(req.head()).is_err());
            assert!(cors.inner.validate_allowed_method(req.head()).is_err());
            assert!(cors.inner.validate_allowed_headers(req.head()).is_err());
        }

        #[actix_web::test]
        async fn test_preflight() {
            let mut cors = Cors::default()
                .allow_any_origin()
                .send_wildcard()
                .max_age(3600)
                .allowed_methods(vec![Method::GET, Method::OPTIONS, Method::POST])
                .allowed_headers(vec![header::AUTHORIZATION, header::ACCEPT])
                .allowed_header(header::CONTENT_TYPE)
                .new_transform(test::ok_service())
                .await
                .unwrap();

            let req = TestRequest::default()
                .method(Method::OPTIONS)
                .insert_header(("Origin", "https://www.example.com"))
                .insert_header((header::ACCESS_CONTROL_REQUEST_HEADERS, "X-Not-Allowed"))
                .to_srv_request();

            assert!(cors.inner.validate_allowed_method(req.head()).is_err());
            assert!(cors.inner.validate_allowed_headers(req.head()).is_err());
            let resp = test::call_service(&cors, req).await;
            assert_eq!(resp.status(), StatusCode::OK);

            let req = TestRequest::default()
                .method(Method::OPTIONS)
                .insert_header(("Origin", "https://www.example.com"))
                .insert_header((header::ACCESS_CONTROL_REQUEST_METHOD, "put"))
                .to_srv_request();

            assert!(cors.inner.validate_allowed_method(req.head()).is_err());
            assert!(cors.inner.validate_allowed_headers(req.head()).is_ok());

            let req = TestRequest::default()
                .method(Method::OPTIONS)
                .insert_header(("Origin", "https://www.example.com"))
                .insert_header((header::ACCESS_CONTROL_REQUEST_METHOD, "POST"))
                .insert_header((
                    header::ACCESS_CONTROL_REQUEST_HEADERS,
                    "AUTHORIZATION,ACCEPT",
                ))
                .to_srv_request();

            let resp = test::call_service(&cors, req).await;
            assert_eq!(
                Some(&b"*"[..]),
                resp.headers()
                    .get(header::ACCESS_CONTROL_ALLOW_ORIGIN)
                    .map(HeaderValue::as_bytes)
            );
            assert_eq!(
                Some(&b"3600"[..]),
                resp.headers()
                    .get(header::ACCESS_CONTROL_MAX_AGE)
                    .map(HeaderValue::as_bytes)
            );

            let hdr = resp
                .headers()
                .get(header::ACCESS_CONTROL_ALLOW_HEADERS)
                .map(val_as_str)
                .unwrap();
            assert!(hdr.contains("authorization"));
            assert!(hdr.contains("accept"));
            assert!(hdr.contains("content-type"));

            let methods = resp
                .headers()
                .get(header::ACCESS_CONTROL_ALLOW_METHODS)
                .unwrap()
                .to_str()
                .unwrap();
            assert!(methods.contains("POST"));
            assert!(methods.contains("GET"));
            assert!(methods.contains("OPTIONS"));

            Rc::get_mut(&mut cors.inner).unwrap().preflight = false;

            let req = TestRequest::default()
                .method(Method::OPTIONS)
                .insert_header(("Origin", "https://www.example.com"))
                .insert_header((header::ACCESS_CONTROL_REQUEST_METHOD, "POST"))
                .insert_header((
                    header::ACCESS_CONTROL_REQUEST_HEADERS,
                    "AUTHORIZATION,ACCEPT",
                ))
                .to_srv_request();

            let resp = test::call_service(&cors, req).await;
            assert_eq!(resp.status(), StatusCode::OK);
        }

        #[actix_web::test]
        async fn allow_fn_origin_equals_head_origin() {
            let cors = Cors::default()
                .allowed_origin_fn(|origin, head| {
                    let head_origin = head
                        .headers()
                        .get(header::ORIGIN)
                        .expect("unwrapping origin header should never fail in allowed_origin_fn");
                    assert!(origin == head_origin);
                    true
                })
                .allow_any_method()
                .allow_any_header()
                .new_transform(test::status_service(StatusCode::NO_CONTENT))
                .await
                .unwrap();

            let req = TestRequest::default()
                .method(Method::OPTIONS)
                .insert_header(("Origin", "https://www.example.com"))
                .insert_header((header::ACCESS_CONTROL_REQUEST_METHOD, "POST"))
                .to_srv_request();
            let resp = test::call_service(&cors, req).await;
            assert_eq!(resp.status(), StatusCode::OK);

            let req = TestRequest::default()
                .method(Method::GET)
                .insert_header(("Origin", "https://www.example.com"))
                .to_srv_request();
            let resp = test::call_service(&cors, req).await;
            assert_eq!(resp.status(), StatusCode::NO_CONTENT);
        }
    }