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    // Copyright 2016 Amanieu d'Antras
    //
    // Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
    // http://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.

    //! Per-object thread-local storage
    //!
    //! This library provides the `ThreadLocal` type which allows a separate copy of
    //! an object to be used for each thread. This allows for per-object
    //! thread-local storage, unlike the standard library's `thread_local!` macro
    //! which only allows static thread-local storage.
    //!
    //! Per-thread objects are not destroyed when a thread exits. Instead, objects
    //! are only destroyed when the `ThreadLocal` containing them is destroyed.
    //!
    //! A `CachedThreadLocal` type is also provided which wraps a `ThreadLocal` but
    //! also uses a special fast path for the first thread that writes into it. The
    //! fast path has very low overhead (<1ns per access) while keeping the same
    //! performance as `ThreadLocal` for other threads.
    //!
    //! Note that since thread IDs are recycled when a thread exits, it is possible
    //! for one thread to retrieve the object of another thread. Since this can only
    //! occur after a thread has exited this does not lead to any race conditions.
    //!
    //! # Example
    //!
    //! ```rust
    //! use thread_local::ThreadLocal;
    //! let tls: ThreadLocal<u32> = ThreadLocal::new();
    //! assert_eq!(tls.get(), None);
    //! assert_eq!(tls.get_or(|| Box::new(5)), &5);
    //! assert_eq!(tls.get(), Some(&5));
    //! ```

    #![warn(missing_docs)]

    extern crate thread_id;

    use std::sync::atomic::{AtomicPtr, AtomicUsize, Ordering};
    use std::sync::Mutex;
    use std::marker::PhantomData;
    use std::cell::UnsafeCell;
    use std::mem;

    // Option::unchecked_unwrap
    trait UncheckedOptionExt<T> {
        unsafe fn unchecked_unwrap(self) -> T;
    }
    impl<T> UncheckedOptionExt<T> for Option<T> {
        unsafe fn unchecked_unwrap(self) -> T {
            #[inline]
            unsafe fn unreachable() -> ! {
                enum Void {}
                match *(1 as *const Void) {}
            }
            match self {
                Some(x) => x,
                None => unreachable(),
            }
        }
    }

    // BoxExt::{from_raw,into_raw}
    trait BoxExt<T: ?Sized> {
        fn into_raw(b: Self) -> *mut T;
        unsafe fn from_raw(raw: *mut T) -> Self;
    }
    impl<T: ?Sized> BoxExt<T> for Box<T> {
        fn into_raw(mut b: Self) -> *mut T {
            let ptr: *mut T = &mut *b;
            mem::forget(b);
            ptr
        }
        unsafe fn from_raw(raw: *mut T) -> Self {
            mem::transmute(raw)
        }
    }

    /// Thread-local variable wrapper
    ///
    /// See the [module-level documentation](index.html) for more.
    pub struct ThreadLocal<T: ?Sized + Send> {
        // Pointer to the current top-level hash table
        table: AtomicPtr<Table<T>>,

        // Lock used to guard against concurrent modifications. This is only taken
        // while writing to the table, not when reading from it.
        lock: Mutex<usize>,

        // PhantomData to indicate that we logically own T
        marker: PhantomData<T>,
    }

    struct Table<T: ?Sized + Send> {
        // Hash entries for the table
        entries: Box<[TableEntry<T>]>,

        // Number of bits used for the hash function
        hash_bits: usize,

        // Previous table, half the size of the current one
        prev: Option<Box<Table<T>>>,
    }

    struct TableEntry<T: ?Sized + Send> {
        // Current owner of this entry, or 0 if this is an empty entry
        owner: AtomicUsize,

        // The object associated with this entry. This is only ever accessed by the
        // owner of the entry.
        data: UnsafeCell<Option<Box<T>>>,
    }

    // ThreadLocal is always Sync, even if T isn't
    unsafe impl<T: ?Sized + Send> Sync for ThreadLocal<T> {}

    impl<T: ?Sized + Send> Default for ThreadLocal<T> {
        fn default() -> ThreadLocal<T> {
            ThreadLocal::new()
        }
    }

    impl<T: ?Sized + Send> Drop for ThreadLocal<T> {
        fn drop(&mut self) {
            unsafe {
                let _: Box<Table<T>> = BoxExt::from_raw(self.table.load(Ordering::Relaxed));
            }
        }
    }

    // Implementation of Clone for TableEntry, needed to make vec![] work
    impl<T: ?Sized + Send> Clone for TableEntry<T> {
        fn clone(&self) -> TableEntry<T> {
            TableEntry {
                owner: AtomicUsize::new(0),
                data: UnsafeCell::new(None),
            }
        }
    }

    // Hash function for the thread id
    #[cfg(target_pointer_width = "32")]
    #[inline]
    fn hash(id: usize, bits: usize) -> usize {
        id.wrapping_mul(0x9E3779B9) >> (32 - bits)
    }
    #[cfg(target_pointer_width = "64")]
    #[inline]
    fn hash(id: usize, bits: usize) -> usize {
        id.wrapping_mul(0x9E3779B97F4A7C15) >> (64 - bits)
    }

    impl<T: ?Sized + Send> ThreadLocal<T> {
        /// Creates a new empty `ThreadLocal`.
        pub fn new() -> ThreadLocal<T> {
            let entry = TableEntry {
                owner: AtomicUsize::new(0),
                data: UnsafeCell::new(None),
            };
            let table = Table {
                entries: vec![entry; 2].into_boxed_slice(),
                hash_bits: 1,
                prev: None,
            };
            ThreadLocal {
                table: AtomicPtr::new(BoxExt::into_raw(Box::new(table))),
                lock: Mutex::new(0),
                marker: PhantomData,
            }
        }

        /// Returns the element for the current thread, if it exists.
        pub fn get(&self) -> Option<&T> {
            let id = thread_id::get();
            self.get_fast(id)
        }

        /// Returns the element for the current thread, or creates it if it doesn't
        /// exist.
        pub fn get_or<F>(&self, create: F) -> &T
            where F: FnOnce() -> Box<T>
        {
            let id = thread_id::get();
            match self.get_fast(id) {
                Some(x) => x,
                None => self.insert(id, create(), true),
            }
        }

        // Simple hash table lookup function
        fn lookup(id: usize, table: &Table<T>) -> Option<&UnsafeCell<Option<Box<T>>>> {
            // Because we use a Mutex to prevent concurrent modifications (but not
            // reads) of the hash table, we can avoid any memory barriers here. No
            // elements between our hash bucket and our value can have been modified
            // since we inserted our thread-local value into the table.
            for entry in table.entries.iter().cycle().skip(hash(id, table.hash_bits)) {
                let owner = entry.owner.load(Ordering::Relaxed);
                if owner == id {
                    return Some(&entry.data);
                }
                if owner == 0 {
                    return None;
                }
            }
            unreachable!();
        }

        // Fast path: try to find our thread in the top-level hash table
        fn get_fast(&self, id: usize) -> Option<&T> {
            let table = unsafe { &*self.table.load(Ordering::Relaxed) };
            match Self::lookup(id, table) {
                Some(x) => unsafe { Some((*x.get()).as_ref().unchecked_unwrap()) },
                None => self.get_slow(id, table),
            }
        }

        // Slow path: try to find our thread in the other hash tables, and then
        // move it to the top-level hash table.
        #[cold]
        fn get_slow(&self, id: usize, table_top: &Table<T>) -> Option<&T> {
            let mut current = &table_top.prev;
            while let Some(ref table) = *current {
                if let Some(x) = Self::lookup(id, table) {
                    let data = unsafe { (*x.get()).take().unchecked_unwrap() };
                    return Some(self.insert(id, data, false));
                }
                current = &table.prev;
            }
            None
        }

        #[cold]
        fn insert(&self, id: usize, data: Box<T>, new: bool) -> &T {
            // Lock the Mutex to ensure only a single thread is modify the hash
            // table at once.
            let mut count = self.lock.lock().unwrap();
            if new {
                *count += 1;
            }
            let table_raw = self.table.load(Ordering::Relaxed);
            let table = unsafe { &*table_raw };

            // If the current top-level hash table is more than 75% full, add a new
            // level with 2x the capacity. Elements will be moved up to the new top
            // level table as they are accessed.
            let table = if *count > table.entries.len() * 3 / 4 {
                let entry = TableEntry {
                    owner: AtomicUsize::new(0),
                    data: UnsafeCell::new(None),
                };
                let new_table = BoxExt::into_raw(Box::new(Table {
                    entries: vec![entry; table.entries.len() * 2].into_boxed_slice(),
                    hash_bits: table.hash_bits + 1,
                    prev: unsafe { Some(BoxExt::from_raw(table_raw)) },
                }));
                self.table.store(new_table, Ordering::Release);
                unsafe { &*new_table }
            } else {
                table
            };

            // Insert the new element into the top-level hash table
            for entry in table.entries.iter().cycle().skip(hash(id, table.hash_bits)) {
                let owner = entry.owner.load(Ordering::Relaxed);
                if owner == 0 {
                    unsafe {
                        entry.owner.store(id, Ordering::Relaxed);
                        *entry.data.get() = Some(data);
                        return (*entry.data.get()).as_ref().unchecked_unwrap();
                    }
                }
                if owner == id {
                    // This can happen if create() inserted a value into this
                    // ThreadLocal between our calls to get_fast() and insert(). We
                    // just return the existing value and drop the newly-allocated
                    // Box.
                    unsafe {
                        return (*entry.data.get()).as_ref().unchecked_unwrap();
                    }
                }
            }
            unreachable!();
        }
    }

    impl<T: Send + Default> ThreadLocal<T> {
        /// Returns the element for the current thread, or creates a default one if
        /// it doesn't exist.
        pub fn get_default(&self) -> &T {
            self.get_or(|| Box::new(T::default()))
        }
    }

    /// Wrapper around `ThreadLocal` which adds a fast path for a single thread.
    ///
    /// This has the same API as `ThreadLocal`, but will register the first thread
    /// that sets a value as its owner. All accesses by the owner will go through
    /// a special fast path which is much faster than the normal `ThreadLocal` path.
    pub struct CachedThreadLocal<T: ?Sized + Send> {
        owner: AtomicUsize,
        local: UnsafeCell<Option<Box<T>>>,
        global: ThreadLocal<T>,
    }

    // CachedThreadLocal is always Sync, even if T isn't
    unsafe impl<T: ?Sized + Send> Sync for CachedThreadLocal<T> {}

    impl<T: ?Sized + Send> Default for CachedThreadLocal<T> {
        fn default() -> CachedThreadLocal<T> {
            CachedThreadLocal::new()
        }
    }

    impl<T: ?Sized + Send> CachedThreadLocal<T> {
        /// Creates a new empty `CachedThreadLocal`.
        pub fn new() -> CachedThreadLocal<T> {
            CachedThreadLocal {
                owner: AtomicUsize::new(0),
                local: UnsafeCell::new(None),
                global: ThreadLocal::new(),
            }
        }

        /// Returns the element for the current thread, if it exists.
        pub fn get(&self) -> Option<&T> {
            let id = thread_id::get();
            let owner = self.owner.load(Ordering::Relaxed);
            if owner == id {
                return unsafe { Some((*self.local.get()).as_ref().unchecked_unwrap()) };
            }
            if owner == 0 {
                return None;
            }
            self.global.get_fast(id)
        }

        /// Returns the element for the current thread, or creates it if it doesn't
        /// exist.
        #[inline(always)]
        pub fn get_or<F>(&self, create: F) -> &T
            where F: FnOnce() -> Box<T>
        {
            let id = thread_id::get();
            let owner = self.owner.load(Ordering::Relaxed);
            if owner == id {
                return unsafe { (*self.local.get()).as_ref().unchecked_unwrap() };
            }
            self.get_or_slow(id, owner, create)
        }

        #[cold]
        #[inline(never)]
        fn get_or_slow<F>(&self, id: usize, owner: usize, create: F) -> &T
            where F: FnOnce() -> Box<T>
        {
            if owner == 0 && self.owner.compare_and_swap(0, id, Ordering::Relaxed) == 0 {
                unsafe {
                    (*self.local.get()) = Some(create());
                    return (*self.local.get()).as_ref().unchecked_unwrap();
                }
            }
            match self.global.get_fast(id) {
                Some(x) => x,
                None => self.global.insert(id, create(), true),
            }
        }
    }

    impl<T: Send + Default> CachedThreadLocal<T> {
        /// Returns the element for the current thread, or creates a default one if
        /// it doesn't exist.
        pub fn get_default(&self) -> &T {
            self.get_or(|| Box::new(T::default()))
        }
    }

    #[cfg(test)]
    mod tests {
        use std::cell::RefCell;
        use std::sync::Arc;
        use std::sync::atomic::AtomicUsize;
        use std::sync::atomic::Ordering::Relaxed;
        use std::thread;
        use super::{ThreadLocal, CachedThreadLocal};

        fn make_create() -> Arc<Fn() -> Box<usize> + Send + Sync> {
            let count = AtomicUsize::new(0);
            Arc::new(move || Box::new(count.fetch_add(1, Relaxed)))
        }

        #[test]
        fn same_thread() {
            let create = make_create();
            let tls = ThreadLocal::new();
            assert_eq!(None, tls.get());
            assert_eq!(0, *tls.get_or(|| create()));
            assert_eq!(Some(&0), tls.get());
            assert_eq!(0, *tls.get_or(|| create()));
            assert_eq!(Some(&0), tls.get());
            assert_eq!(0, *tls.get_or(|| create()));
            assert_eq!(Some(&0), tls.get());
        }

        #[test]
        fn same_thread_cached() {
            let create = make_create();
            let tls = CachedThreadLocal::new();
            assert_eq!(None, tls.get());
            assert_eq!(0, *tls.get_or(|| create()));
            assert_eq!(Some(&0), tls.get());
            assert_eq!(0, *tls.get_or(|| create()));
            assert_eq!(Some(&0), tls.get());
            assert_eq!(0, *tls.get_or(|| create()));
            assert_eq!(Some(&0), tls.get());
        }

        #[test]
        fn different_thread() {
            let create = make_create();
            let tls = Arc::new(ThreadLocal::new());
            assert_eq!(None, tls.get());
            assert_eq!(0, *tls.get_or(|| create()));
            assert_eq!(Some(&0), tls.get());

            let tls2 = tls.clone();
            let create2 = create.clone();
            thread::spawn(move || {
                assert_eq!(None, tls2.get());
                assert_eq!(1, *tls2.get_or(|| create2()));
                assert_eq!(Some(&1), tls2.get());
            })
                .join()
                .unwrap();

            assert_eq!(Some(&0), tls.get());
            assert_eq!(0, *tls.get_or(|| create()));
        }

        #[test]
        fn different_thread_cached() {
            let create = make_create();
            let tls = Arc::new(CachedThreadLocal::new());
            assert_eq!(None, tls.get());
            assert_eq!(0, *tls.get_or(|| create()));
            assert_eq!(Some(&0), tls.get());

            let tls2 = tls.clone();
            let create2 = create.clone();
            thread::spawn(move || {
                assert_eq!(None, tls2.get());
                assert_eq!(1, *tls2.get_or(|| create2()));
                assert_eq!(Some(&1), tls2.get());
            })
                .join()
                .unwrap();

            assert_eq!(Some(&0), tls.get());
            assert_eq!(0, *tls.get_or(|| create()));
        }

        #[test]
        fn is_sync() {
            fn foo<T: Sync>() {}
            foo::<ThreadLocal<String>>();
            foo::<ThreadLocal<RefCell<String>>>();
            foo::<CachedThreadLocal<String>>();
            foo::<CachedThreadLocal<RefCell<String>>>();
        }
    }

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