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// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License.  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied.  See the License for the
// specific language governing permissions and limitations
// under the License.

use std::fmt::Debug;
use std::fmt::Formatter;
use std::future::Future;
use std::sync::Arc;

use futures::FutureExt;

use super::*;
use crate::raw::BoxedStaticFuture;
use crate::raw::MaybeSend;

/// Executor that runs futures in background.
///
/// Executor is created by users and used by opendal. So it's by design that Executor only
/// expose constructor methods.
///
/// Executor will run futures in background and return a `Task` as handle to the future. Users
/// can call `task.await` to wait for the future to complete or drop the `Task` to cancel it.
#[derive(Clone)]
pub struct Executor {
    executor: Arc<dyn Execute>,
}

impl Debug for Executor {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        write!(f, "Executor")
    }
}

impl Default for Executor {
    fn default() -> Self {
        Self::new()
    }
}

impl Executor {
    /// Create a default executor.
    ///
    /// The default executor is enabled by feature flags. If no feature flags enabled, the default
    /// executor will always return error if users try to perform concurrent tasks.
    pub fn new() -> Self {
        #[cfg(feature = "executors-tokio")]
        {
            Self::with(executors::TokioExecutor::default())
        }
        #[cfg(not(feature = "executors-tokio"))]
        {
            Self::with(())
        }
    }

    /// Create a new executor with given execute impl.
    pub fn with(exec: impl Execute) -> Self {
        Self {
            executor: Arc::new(exec),
        }
    }

    /// Return the inner executor.
    pub(crate) fn into_inner(self) -> Arc<dyn Execute> {
        self.executor
    }

    /// Return a future that will be resolved after the given timeout.
    pub(crate) fn timeout(&self) -> Option<BoxedStaticFuture<()>> {
        self.executor.timeout()
    }

    /// Run given future in background immediately.
    pub(crate) fn execute<F>(&self, f: F) -> Task<F::Output>
    where
        F: Future + MaybeSend + 'static,
        F::Output: MaybeSend + 'static,
    {
        let (fut, handle) = f.remote_handle();
        self.executor.execute(Box::pin(fut));
        Task::new(handle)
    }
}