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    use crate::runtime::handle::Handle;
    use crate::runtime::{blocking, driver, Callback, Runtime, Spawner};

    use std::fmt;
    use std::io;
    use std::time::Duration;

    /// Builds Tokio Runtime with custom configuration values.
    ///
    /// Methods can be chained in order to set the configuration values. The
    /// Runtime is constructed by calling [`build`].
    ///
    /// New instances of `Builder` are obtained via [`Builder::new_multi_thread`]
    /// or [`Builder::new_current_thread`].
    ///
    /// See function level documentation for details on the various configuration
    /// settings.
    ///
    /// [`build`]: method@Self::build
    /// [`Builder::new_multi_thread`]: method@Self::new_multi_thread
    /// [`Builder::new_current_thread`]: method@Self::new_current_thread
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio::runtime::Builder;
    ///
    /// fn main() {
    ///     // build runtime
    ///     let runtime = Builder::new_multi_thread()
    ///         .worker_threads(4)
    ///         .thread_name("my-custom-name")
    ///         .thread_stack_size(3 * 1024 * 1024)
    ///         .build()
    ///         .unwrap();
    ///
    ///     // use runtime ...
    /// }
    /// ```
    pub struct Builder {
        /// Runtime type
        kind: Kind,

        /// Whether or not to enable the I/O driver
        enable_io: bool,

        /// Whether or not to enable the time driver
        enable_time: bool,

        /// Whether or not the clock should start paused.
        start_paused: bool,

        /// The number of worker threads, used by Runtime.
        ///
        /// Only used when not using the current-thread executor.
        worker_threads: Option<usize>,

        /// Cap on thread usage.
        max_blocking_threads: usize,

        /// Name fn used for threads spawned by the runtime.
        pub(super) thread_name: ThreadNameFn,

        /// Stack size used for threads spawned by the runtime.
        pub(super) thread_stack_size: Option<usize>,

        /// Callback to run after each thread starts.
        pub(super) after_start: Option<Callback>,

        /// To run before each worker thread stops
        pub(super) before_stop: Option<Callback>,

        /// Customizable keep alive timeout for BlockingPool
        pub(super) keep_alive: Option<Duration>,
    }

    pub(crate) type ThreadNameFn = std::sync::Arc<dyn Fn() -> String + Send + Sync + 'static>;

    pub(crate) enum Kind {
        CurrentThread,
        #[cfg(feature = "rt-multi-thread")]
        MultiThread,
    }

    impl Builder {
        /// Returns a new builder with the current thread scheduler selected.
        ///
        /// Configuration methods can be chained on the return value.
        ///
        /// To spawn non-`Send` tasks on the resulting runtime, combine it with a
        /// [`LocalSet`].
        ///
        /// [`LocalSet`]: crate::task::LocalSet
        pub fn new_current_thread() -> Builder {
            Builder::new(Kind::CurrentThread)
        }

        /// Returns a new builder with the multi thread scheduler selected.
        ///
        /// Configuration methods can be chained on the return value.
        #[cfg(feature = "rt-multi-thread")]
        #[cfg_attr(docsrs, doc(cfg(feature = "rt-multi-thread")))]
        pub fn new_multi_thread() -> Builder {
            Builder::new(Kind::MultiThread)
        }

        /// Returns a new runtime builder initialized with default configuration
        /// values.
        ///
        /// Configuration methods can be chained on the return value.
        pub(crate) fn new(kind: Kind) -> Builder {
            Builder {
                kind,

                // I/O defaults to "off"
                enable_io: false,

                // Time defaults to "off"
                enable_time: false,

                // The clock starts not-paused
                start_paused: false,

                // Default to lazy auto-detection (one thread per CPU core)
                worker_threads: None,

                max_blocking_threads: 512,

                // Default thread name
                thread_name: std::sync::Arc::new(|| "tokio-runtime-worker".into()),

                // Do not set a stack size by default
                thread_stack_size: None,

                // No worker thread callbacks
                after_start: None,
                before_stop: None,

                keep_alive: None,
            }
        }

        /// Enables both I/O and time drivers.
        ///
        /// Doing this is a shorthand for calling `enable_io` and `enable_time`
        /// individually. If additional components are added to Tokio in the future,
        /// `enable_all` will include these future components.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::runtime;
        ///
        /// let rt = runtime::Builder::new_multi_thread()
        ///     .enable_all()
        ///     .build()
        ///     .unwrap();
        /// ```
        pub fn enable_all(&mut self) -> &mut Self {
            #[cfg(any(feature = "net", feature = "process", all(unix, feature = "signal")))]
            self.enable_io();
            #[cfg(feature = "time")]
            self.enable_time();

            self
        }

        /// Sets the number of worker threads the `Runtime` will use.
        ///
        /// This can be any number above 0 though it is advised to keep this value
        /// on the smaller side.
        ///
        /// # Default
        ///
        /// The default value is the number of cores available to the system.
        ///
        /// # Panic
        ///
        /// When using the `current_thread` runtime this method will panic, since
        /// those variants do not allow setting worker thread counts.
        ///
        ///
        /// # Examples
        ///
        /// ## Multi threaded runtime with 4 threads
        ///
        /// ```
        /// use tokio::runtime;
        ///
        /// // This will spawn a work-stealing runtime with 4 worker threads.
        /// let rt = runtime::Builder::new_multi_thread()
        ///     .worker_threads(4)
        ///     .build()
        ///     .unwrap();
        ///
        /// rt.spawn(async move {});
        /// ```
        ///
        /// ## Current thread runtime (will only run on the current thread via `Runtime::block_on`)
        ///
        /// ```
        /// use tokio::runtime;
        ///
        /// // Create a runtime that _must_ be driven from a call
        /// // to `Runtime::block_on`.
        /// let rt = runtime::Builder::new_current_thread()
        ///     .build()
        ///     .unwrap();
        ///
        /// // This will run the runtime and future on the current thread
        /// rt.block_on(async move {});
        /// ```
        ///
        /// # Panic
        ///
        /// This will panic if `val` is not larger than `0`.
        pub fn worker_threads(&mut self, val: usize) -> &mut Self {
            assert!(val > 0, "Worker threads cannot be set to 0");
            self.worker_threads = Some(val);
            self
        }

        /// Specifies the limit for additional threads spawned by the Runtime.
        ///
        /// These threads are used for blocking operations like tasks spawned
        /// through [`spawn_blocking`]. Unlike the [`worker_threads`], they are not
        /// always active and will exit if left idle for too long. You can change
        /// this timeout duration with [`thread_keep_alive`].
        ///
        /// The default value is 512.
        ///
        /// # Panic
        ///
        /// This will panic if `val` is not larger than `0`.
        ///
        /// # Upgrading from 0.x
        ///
        /// In old versions `max_threads` limited both blocking and worker threads, but the
        /// current `max_blocking_threads` does not include async worker threads in the count.
        ///
        /// [`spawn_blocking`]: fn@crate::task::spawn_blocking
        /// [`worker_threads`]: Self::worker_threads
        /// [`thread_keep_alive`]: Self::thread_keep_alive
        #[cfg_attr(docsrs, doc(alias = "max_threads"))]
        pub fn max_blocking_threads(&mut self, val: usize) -> &mut Self {
            assert!(val > 0, "Max blocking threads cannot be set to 0");
            self.max_blocking_threads = val;
            self
        }

        /// Sets name of threads spawned by the `Runtime`'s thread pool.
        ///
        /// The default name is "tokio-runtime-worker".
        ///
        /// # Examples
        ///
        /// ```
        /// # use tokio::runtime;
        ///
        /// # pub fn main() {
        /// let rt = runtime::Builder::new_multi_thread()
        ///     .thread_name("my-pool")
        ///     .build();
        /// # }
        /// ```
        pub fn thread_name(&mut self, val: impl Into<String>) -> &mut Self {
            let val = val.into();
            self.thread_name = std::sync::Arc::new(move || val.clone());
            self
        }

        /// Sets a function used to generate the name of threads spawned by the `Runtime`'s thread pool.
        ///
        /// The default name fn is `|| "tokio-runtime-worker".into()`.
        ///
        /// # Examples
        ///
        /// ```
        /// # use tokio::runtime;
        /// # use std::sync::atomic::{AtomicUsize, Ordering};
        ///
        /// # pub fn main() {
        /// let rt = runtime::Builder::new_multi_thread()
        ///     .thread_name_fn(|| {
        ///        static ATOMIC_ID: AtomicUsize = AtomicUsize::new(0);
        ///        let id = ATOMIC_ID.fetch_add(1, Ordering::SeqCst);
        ///        format!("my-pool-{}", id)
        ///     })
        ///     .build();
        /// # }
        /// ```
        pub fn thread_name_fn<F>(&mut self, f: F) -> &mut Self
        where
            F: Fn() -> String + Send + Sync + 'static,
        {
            self.thread_name = std::sync::Arc::new(f);
            self
        }

        /// Sets the stack size (in bytes) for worker threads.
        ///
        /// The actual stack size may be greater than this value if the platform
        /// specifies minimal stack size.
        ///
        /// The default stack size for spawned threads is 2 MiB, though this
        /// particular stack size is subject to change in the future.
        ///
        /// # Examples
        ///
        /// ```
        /// # use tokio::runtime;
        ///
        /// # pub fn main() {
        /// let rt = runtime::Builder::new_multi_thread()
        ///     .thread_stack_size(32 * 1024)
        ///     .build();
        /// # }
        /// ```
        pub fn thread_stack_size(&mut self, val: usize) -> &mut Self {
            self.thread_stack_size = Some(val);
            self
        }

        /// Executes function `f` after each thread is started but before it starts
        /// doing work.
        ///
        /// This is intended for bookkeeping and monitoring use cases.
        ///
        /// # Examples
        ///
        /// ```
        /// # use tokio::runtime;
        ///
        /// # pub fn main() {
        /// let runtime = runtime::Builder::new_multi_thread()
        ///     .on_thread_start(|| {
        ///         println!("thread started");
        ///     })
        ///     .build();
        /// # }
        /// ```
        #[cfg(not(loom))]
        pub fn on_thread_start<F>(&mut self, f: F) -> &mut Self
        where
            F: Fn() + Send + Sync + 'static,
        {
            self.after_start = Some(std::sync::Arc::new(f));
            self
        }

        /// Executes function `f` before each thread stops.
        ///
        /// This is intended for bookkeeping and monitoring use cases.
        ///
        /// # Examples
        ///
        /// ```
        /// # use tokio::runtime;
        ///
        /// # pub fn main() {
        /// let runtime = runtime::Builder::new_multi_thread()
        ///     .on_thread_stop(|| {
        ///         println!("thread stopping");
        ///     })
        ///     .build();
        /// # }
        /// ```
        #[cfg(not(loom))]
        pub fn on_thread_stop<F>(&mut self, f: F) -> &mut Self
        where
            F: Fn() + Send + Sync + 'static,
        {
            self.before_stop = Some(std::sync::Arc::new(f));
            self
        }

        /// Creates the configured `Runtime`.
        ///
        /// The returned `Runtime` instance is ready to spawn tasks.
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::runtime::Builder;
        ///
        /// let rt  = Builder::new_multi_thread().build().unwrap();
        ///
        /// rt.block_on(async {
        ///     println!("Hello from the Tokio runtime");
        /// });
        /// ```
        pub fn build(&mut self) -> io::Result<Runtime> {
            match &self.kind {
                Kind::CurrentThread => self.build_basic_runtime(),
                #[cfg(feature = "rt-multi-thread")]
                Kind::MultiThread => self.build_threaded_runtime(),
            }
        }

        fn get_cfg(&self) -> driver::Cfg {
            driver::Cfg {
                enable_pause_time: match self.kind {
                    Kind::CurrentThread => true,
                    #[cfg(feature = "rt-multi-thread")]
                    Kind::MultiThread => false,
                },
                enable_io: self.enable_io,
                enable_time: self.enable_time,
                start_paused: self.start_paused,
            }
        }

        /// Sets a custom timeout for a thread in the blocking pool.
        ///
        /// By default, the timeout for a thread is set to 10 seconds. This can
        /// be overriden using .thread_keep_alive().
        ///
        /// # Example
        ///
        /// ```
        /// # use tokio::runtime;
        /// # use std::time::Duration;
        ///
        /// # pub fn main() {
        /// let rt = runtime::Builder::new_multi_thread()
        ///     .thread_keep_alive(Duration::from_millis(100))
        ///     .build();
        /// # }
        /// ```
        pub fn thread_keep_alive(&mut self, duration: Duration) -> &mut Self {
            self.keep_alive = Some(duration);
            self
        }

        fn build_basic_runtime(&mut self) -> io::Result<Runtime> {
            use crate::runtime::{BasicScheduler, Kind};

            let (driver, resources) = driver::Driver::new(self.get_cfg())?;

            // And now put a single-threaded scheduler on top of the timer. When
            // there are no futures ready to do something, it'll let the timer or
            // the reactor to generate some new stimuli for the futures to continue
            // in their life.
            let scheduler = BasicScheduler::new(driver);
            let spawner = Spawner::Basic(scheduler.spawner().clone());

            // Blocking pool
            let blocking_pool = blocking::create_blocking_pool(self, self.max_blocking_threads);
            let blocking_spawner = blocking_pool.spawner().clone();

            Ok(Runtime {
                kind: Kind::CurrentThread(scheduler),
                handle: Handle {
                    spawner,
                    io_handle: resources.io_handle,
                    time_handle: resources.time_handle,
                    signal_handle: resources.signal_handle,
                    clock: resources.clock,
                    blocking_spawner,
                },
                blocking_pool,
            })
        }
    }

    cfg_io_driver! {
        impl Builder {
            /// Enables the I/O driver.
            ///
            /// Doing this enables using net, process, signal, and some I/O types on
            /// the runtime.
            ///
            /// # Examples
            ///
            /// ```
            /// use tokio::runtime;
            ///
            /// let rt = runtime::Builder::new_multi_thread()
            ///     .enable_io()
            ///     .build()
            ///     .unwrap();
            /// ```
            pub fn enable_io(&mut self) -> &mut Self {
                self.enable_io = true;
                self
            }
        }
    }

    cfg_time! {
        impl Builder {
            /// Enables the time driver.
            ///
            /// Doing this enables using `tokio::time` on the runtime.
            ///
            /// # Examples
            ///
            /// ```
            /// use tokio::runtime;
            ///
            /// let rt = runtime::Builder::new_multi_thread()
            ///     .enable_time()
            ///     .build()
            ///     .unwrap();
            /// ```
            pub fn enable_time(&mut self) -> &mut Self {
                self.enable_time = true;
                self
            }
        }
    }

    cfg_test_util! {
        impl Builder {
            /// Controls if the runtime's clock starts paused or advancing.
            ///
            /// Pausing time requires the current-thread runtime; construction of
            /// the runtime will panic otherwise.
            ///
            /// # Examples
            ///
            /// ```
            /// use tokio::runtime;
            ///
            /// let rt = runtime::Builder::new_current_thread()
            ///     .enable_time()
            ///     .start_paused(true)
            ///     .build()
            ///     .unwrap();
            /// ```
            pub fn start_paused(&mut self, start_paused: bool) -> &mut Self {
                self.start_paused = start_paused;
                self
            }
        }
    }

    cfg_rt_multi_thread! {
        impl Builder {
            fn build_threaded_runtime(&mut self) -> io::Result<Runtime> {
                use crate::loom::sys::num_cpus;
                use crate::runtime::{Kind, ThreadPool};
                use crate::runtime::park::Parker;

                let core_threads = self.worker_threads.unwrap_or_else(num_cpus);

                let (driver, resources) = driver::Driver::new(self.get_cfg())?;

                let (scheduler, launch) = ThreadPool::new(core_threads, Parker::new(driver));
                let spawner = Spawner::ThreadPool(scheduler.spawner().clone());

                // Create the blocking pool
                let blocking_pool = blocking::create_blocking_pool(self, self.max_blocking_threads + core_threads);
                let blocking_spawner = blocking_pool.spawner().clone();

                // Create the runtime handle
                let handle = Handle {
                    spawner,
                    io_handle: resources.io_handle,
                    time_handle: resources.time_handle,
                    signal_handle: resources.signal_handle,
                    clock: resources.clock,
                    blocking_spawner,
                };

                // Spawn the thread pool workers
                let _enter = crate::runtime::context::enter(handle.clone());
                launch.launch();

                Ok(Runtime {
                    kind: Kind::ThreadPool(scheduler),
                    handle,
                    blocking_pool,
                })
            }
        }
    }

    impl fmt::Debug for Builder {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("Builder")
                .field("worker_threads", &self.worker_threads)
                .field("max_blocking_threads", &self.max_blocking_threads)
                .field(
                    "thread_name",
                    &"<dyn Fn() -> String + Send + Sync + 'static>",
                )
                .field("thread_stack_size", &self.thread_stack_size)
                .field("after_start", &self.after_start.as_ref().map(|_| "..."))
                .field("before_stop", &self.after_start.as_ref().map(|_| "..."))
                .finish()
        }
    }