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```
```//! Definition of the Shared combinator, a future that is cloneable,
//! and can be polled in multiple threads.
//!
//! # Examples
//!
//! ```
//! use futures::future::*;
//!
//! let future = ok::<_, bool>(6);
//! let shared1 = future.shared();
//! let shared2 = shared1.clone();
//! assert_eq!(6, *shared1.wait().unwrap());
//! assert_eq!(6, *shared2.wait().unwrap());
//! ```

use {Future, Poll, Async};
use executor::{self, Notify, Spawn};

use std::{error, fmt, mem, ops};
use std::cell::UnsafeCell;
use std::sync::{Arc, Mutex};
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::SeqCst;
use std::collections::HashMap;

/// A future that is cloneable and can be polled in multiple threads.
/// Use `Future::shared()` method to convert any future into a `Shared` future.
#[must_use = "futures do nothing unless polled"]
pub struct Shared<F: Future> {
inner: Arc<Inner<F>>,
waiter: usize,
}

impl<F> fmt::Debug for Shared<F>
where F: Future + fmt::Debug,
F::Item: fmt::Debug,
F::Error: fmt::Debug,
{
fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
fmt.debug_struct("Shared")
.field("inner", &self.inner)
.field("waiter", &self.waiter)
.finish()
}
}

struct Inner<F: Future> {
next_clone_id: AtomicUsize,
future: UnsafeCell<Option<Spawn<F>>>,
result: UnsafeCell<Option<Result<SharedItem<F::Item>, SharedError<F::Error>>>>,
notifier: Arc<Notifier>,
}

struct Notifier {
state: AtomicUsize,
}

const IDLE: usize = 0;
const POLLING: usize = 1;
const REPOLL: usize = 2;
const COMPLETE: usize = 3;
const POISONED: usize = 4;

pub fn new<F: Future>(future: F) -> Shared<F> {
Shared {
inner: Arc::new(Inner {
next_clone_id: AtomicUsize::new(1),
notifier: Arc::new(Notifier {
state: AtomicUsize::new(IDLE),
waiters: Mutex::new(HashMap::new()),
}),
future: UnsafeCell::new(Some(executor::spawn(future))),
result: UnsafeCell::new(None),
}),
waiter: 0,
}
}

impl<F> Shared<F> where F: Future {
// TODO: make this private
#[deprecated(since = "0.1.12", note = "use `Future::shared` instead")]
#[cfg(feature = "with-deprecated")]
#[doc(hidden)]
pub fn new(future: F) -> Self {
new(future)
}

/// If any clone of this `Shared` has completed execution, returns its result immediately
/// without blocking. Otherwise, returns None without triggering the work represented by
/// this `Shared`.
pub fn peek(&self) -> Option<Result<SharedItem<F::Item>, SharedError<F::Error>>> {
COMPLETE => {
Some(unsafe { self.clone_result() })
}
POISONED => panic!("inner future panicked during poll"),
_ => None,
}
}

fn set_waiter(&mut self) {
let mut waiters = self.inner.notifier.waiters.lock().unwrap();
}

unsafe fn clone_result(&self) -> Result<SharedItem<F::Item>, SharedError<F::Error>> {
match *self.inner.result.get() {
Some(Ok(ref item)) => Ok(SharedItem { item: item.item.clone() }),
Some(Err(ref e)) => Err(SharedError { error: e.error.clone() }),
_ => unreachable!(),
}
}

fn complete(&self) {
unsafe { *self.inner.future.get() = None };
self.inner.notifier.state.store(COMPLETE, SeqCst);
self.inner.notifier.notify(0);
}
}

impl<F> Future for Shared<F>
where F: Future
{
type Item = SharedItem<F::Item>;
type Error = SharedError<F::Error>;

fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
self.set_waiter();

match self.inner.notifier.state.compare_and_swap(IDLE, POLLING, SeqCst) {
IDLE => {
// Lock acquired, fall through
}
POLLING | REPOLL => {
// Another task is currently polling, at this point we just want
// to ensure that our task handle is currently registered

}
COMPLETE => {
}
POISONED => panic!("inner future panicked during poll"),
_ => unreachable!(),
}

loop {
struct Reset<'a>(&'a AtomicUsize);

impl<'a> Drop for Reset<'a> {
fn drop(&mut self) {

self.0.store(POISONED, SeqCst);
}
}
}

let _reset = Reset(&self.inner.notifier.state);

// Poll the future
let res = unsafe {
(*self.inner.future.get()).as_mut().unwrap()
.poll_future_notify(&self.inner.notifier, 0)
};
match res {
// Not ready, try to release the handle
match self.inner.notifier.state.compare_and_swap(POLLING, IDLE, SeqCst) {
POLLING => {
// Success
}
REPOLL => {
// Gotta poll again!
let prev = self.inner.notifier.state.swap(POLLING, SeqCst);
assert_eq!(prev, REPOLL);
}
_ => unreachable!(),
}

}
unsafe {
(*self.inner.result.get()) = Some(Ok(SharedItem { item: Arc::new(i) }));
}

break;
}
Err(e) => {
unsafe {
(*self.inner.result.get()) = Some(Err(SharedError { error: Arc::new(e) }));
}

break;
}
}
}

self.complete();
}
}

impl<F> Clone for Shared<F> where F: Future {
fn clone(&self) -> Self {

Shared {
inner: self.inner.clone(),
waiter: next_clone_id,
}
}
}

impl<F> Drop for Shared<F> where F: Future {
fn drop(&mut self) {
let mut waiters = self.inner.notifier.waiters.lock().unwrap();
waiters.remove(&self.waiter);
}
}

impl Notify for Notifier {
fn notify(&self, _id: usize) {
self.state.compare_and_swap(POLLING, REPOLL, SeqCst);

let waiters = mem::replace(&mut *self.waiters.lock().unwrap(), HashMap::new());

for (_, waiter) in waiters {
waiter.notify();
}
}
}

// The `F` is synchronized by a lock, so `F` doesn't need
// to be `Sync`. However, its `Item` or `Error` are exposed
// through an `Arc` but not lock, so they must be `Send + Sync`.
unsafe impl<F> Send for Inner<F>
where F: Future + Send,
F::Item: Send + Sync,
F::Error: Send + Sync,
{}

unsafe impl<F> Sync for Inner<F>
where F: Future + Send,
F::Item: Send + Sync,
F::Error: Send + Sync,
{}

impl<F> fmt::Debug for Inner<F>
where F: Future + fmt::Debug,
F::Item: fmt::Debug,
F::Error: fmt::Debug,
{
fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
fmt.debug_struct("Inner")
.finish()
}
}

/// A wrapped item of the original future that is cloneable and implements Deref
/// for ease of use.
#[derive(Clone, Debug)]
pub struct SharedItem<T> {
item: Arc<T>,
}

impl<T> ops::Deref for SharedItem<T> {
type Target = T;

fn deref(&self) -> &T {
&self.item.as_ref()
}
}

/// A wrapped error of the original future that is cloneable and implements Deref
/// for ease of use.
#[derive(Clone, Debug)]
pub struct SharedError<E> {
error: Arc<E>,
}

impl<E> ops::Deref for SharedError<E> {
type Target = E;

fn deref(&self) -> &E {
&self.error.as_ref()
}
}

impl<E> fmt::Display for SharedError<E>
where E: fmt::Display,
{
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
self.error.fmt(f)
}
}

impl<E> error::Error for SharedError<E>
where E: error::Error,
{
fn description(&self) -> &str {
self.error.description()
}

fn cause(&self) -> Option<&error::Error> {
self.error.cause()
}
}
```