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    // Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    use std::fmt;
    use std::ops::{Deref, DerefMut, Drop};
    use std::sync::Mutex;

    /// A very simple memory pool for managing cached state.
    ///
    /// This was motivated by a singular purpose: reduce the allocation overhead
    /// of matching engines.
    ///
    /// With a pool, the matching engines need to allocate state each time they
    /// are invoked. If a regex is used once to check for a match and never again,
    /// then this is OK. But if a regex is used many times over, then not
    /// re-allocating the engine's state is a huge win. (A regex is commonly
    /// used many times, for example, with `find_iter`, `captures_iter` or
    /// `replace_all`.)
    ///
    /// We use inherited mutability and ensure that each thread gets its own
    /// state. There is no limit on the number of states that are created. If a
    /// thread requests one and one isn't available, a new one is created.
    pub struct Pool<T> {
        stack: Mutex<Vec<T>>,
        create: CreateFn<T>,
    }

    /// The type of the function used to create resources if none exist.
    pub type CreateFn<T> = Box<Fn() -> T + Send + Sync>;

    /// A guard the provides access to a value pulled from the pool.
    #[derive(Debug)]
    pub struct PoolGuard<'a, T: 'a> {
        pool: &'a Pool<T>,
        val: Option<T>,
    }

    impl<T> Pool<T> {
        /// Create a new pool.
        ///
        /// When a caller requests a resource from the pool and one does not
        /// exist, then `create` is called to allocate a new resource for the
        /// caller.
        ///
        /// It is up to the caller to put the resource back into the pool for
        /// future reuse.
        ///
        /// All resources are created lazily/on-demand.
        pub fn new(create: CreateFn<T>) -> Pool<T> {
            Pool {
                stack: Mutex::new(vec![]),
                create: create,
            }
        }

        /// Request a resource from the pool.
        ///
        /// If no resources are available, a new one is created.
        ///
        /// Once the guard is dropped, the resource is returned to the pool.
        pub fn get(&self) -> PoolGuard<T> {
            let mut stack = self.stack.lock().unwrap();
            match stack.pop() {
                None => PoolGuard { pool: self, val: Some((self.create)()) },
                Some(v) => PoolGuard { pool: self, val: Some(v) },
            }
        }

        /// Add a resource to the pool.
        ///
        /// This makes the resource available for use with `get`.
        fn put(&self, v: T) {
            let mut stack = self.stack.lock().unwrap();
            stack.push(v);
        }
    }

    impl<'a, T> Deref for PoolGuard<'a, T> {
        type Target = T;
        fn deref(&self) -> &T { self.val.as_ref().unwrap() }
    }

    impl<'a, T> DerefMut for PoolGuard<'a, T> {
        fn deref_mut(&mut self) -> &mut T { self.val.as_mut().unwrap() }
    }

    impl<'a, T> Drop for PoolGuard<'a, T> {
        fn drop(&mut self) {
            let val = self.val.take().unwrap();
            self.pool.put(val);
        }
    }

    impl<T: fmt::Debug> fmt::Debug for Pool<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            let stack = self.stack.lock();
            let stack = stack.unwrap();
            stack.fmt(f)
        }
    }

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