1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
//! Thread-safe, non-blocking, "first one wins" flavor of `OnceCell`.
//!
//! If two threads race to initialize a type from the `race` module, they
//! don't block, execute initialization function together, but only one of
//! them stores the result.
//!
//! This module does not require `std` feature.

#[cfg(feature = "atomic-polyfill")]
use atomic_polyfill as atomic;
#[cfg(not(feature = "atomic-polyfill"))]
use core::sync::atomic;

use atomic::{AtomicUsize, Ordering};
use core::num::NonZeroUsize;

/// A thread-safe cell which can be written to only once.
#[derive(Default, Debug)]
pub struct OnceNonZeroUsize {
    inner: AtomicUsize,
}

impl OnceNonZeroUsize {
    /// Creates a new empty cell.
    #[inline]
    pub const fn new() -> OnceNonZeroUsize {
        OnceNonZeroUsize { inner: AtomicUsize::new(0) }
    }

    /// Gets the underlying value.
    #[inline]
    pub fn get(&self) -> Option<NonZeroUsize> {
        let val = self.inner.load(Ordering::Acquire);
        NonZeroUsize::new(val)
    }

    /// Sets the contents of this cell to `value`.
    ///
    /// Returns `Ok(())` if the cell was empty and `Err(())` if it was
    /// full.
    #[inline]
    pub fn set(&self, value: NonZeroUsize) -> Result<(), ()> {
        let exchange =
            self.inner.compare_exchange(0, value.get(), Ordering::AcqRel, Ordering::Acquire);
        match exchange {
            Ok(_) => Ok(()),
            Err(_) => Err(()),
        }
    }

    /// Gets the contents of the cell, initializing it with `f` if the cell was
    /// empty.
    ///
    /// If several threads concurrently run `get_or_init`, more than one `f` can
    /// be called. However, all threads will return the same value, produced by
    /// some `f`.
    pub fn get_or_init<F>(&self, f: F) -> NonZeroUsize
    where
        F: FnOnce() -> NonZeroUsize,
    {
        enum Void {}
        match self.get_or_try_init(|| Ok::<NonZeroUsize, Void>(f())) {
            Ok(val) => val,
            Err(void) => match void {},
        }
    }

    /// Gets the contents of the cell, initializing it with `f` if
    /// the cell was empty. If the cell was empty and `f` failed, an
    /// error is returned.
    ///
    /// If several threads concurrently run `get_or_init`, more than one `f` can
    /// be called. However, all threads will return the same value, produced by
    /// some `f`.
    pub fn get_or_try_init<F, E>(&self, f: F) -> Result<NonZeroUsize, E>
    where
        F: FnOnce() -> Result<NonZeroUsize, E>,
    {
        let val = self.inner.load(Ordering::Acquire);
        let res = match NonZeroUsize::new(val) {
            Some(it) => it,
            None => {
                let mut val = f()?.get();
                let exchange =
                    self.inner.compare_exchange(0, val, Ordering::AcqRel, Ordering::Acquire);
                if let Err(old) = exchange {
                    val = old;
                }
                unsafe { NonZeroUsize::new_unchecked(val) }
            }
        };
        Ok(res)
    }
}

/// A thread-safe cell which can be written to only once.
#[derive(Default, Debug)]
pub struct OnceBool {
    inner: OnceNonZeroUsize,
}

impl OnceBool {
    /// Creates a new empty cell.
    #[inline]
    pub const fn new() -> OnceBool {
        OnceBool { inner: OnceNonZeroUsize::new() }
    }

    /// Gets the underlying value.
    #[inline]
    pub fn get(&self) -> Option<bool> {
        self.inner.get().map(OnceBool::from_usize)
    }

    /// Sets the contents of this cell to `value`.
    ///
    /// Returns `Ok(())` if the cell was empty and `Err(())` if it was
    /// full.
    #[inline]
    pub fn set(&self, value: bool) -> Result<(), ()> {
        self.inner.set(OnceBool::to_usize(value))
    }

    /// Gets the contents of the cell, initializing it with `f` if the cell was
    /// empty.
    ///
    /// If several threads concurrently run `get_or_init`, more than one `f` can
    /// be called. However, all threads will return the same value, produced by
    /// some `f`.
    pub fn get_or_init<F>(&self, f: F) -> bool
    where
        F: FnOnce() -> bool,
    {
        OnceBool::from_usize(self.inner.get_or_init(|| OnceBool::to_usize(f())))
    }

    /// Gets the contents of the cell, initializing it with `f` if
    /// the cell was empty. If the cell was empty and `f` failed, an
    /// error is returned.
    ///
    /// If several threads concurrently run `get_or_init`, more than one `f` can
    /// be called. However, all threads will return the same value, produced by
    /// some `f`.
    pub fn get_or_try_init<F, E>(&self, f: F) -> Result<bool, E>
    where
        F: FnOnce() -> Result<bool, E>,
    {
        self.inner.get_or_try_init(|| f().map(OnceBool::to_usize)).map(OnceBool::from_usize)
    }

    #[inline]
    fn from_usize(value: NonZeroUsize) -> bool {
        value.get() == 1
    }
    #[inline]
    fn to_usize(value: bool) -> NonZeroUsize {
        unsafe { NonZeroUsize::new_unchecked(if value { 1 } else { 2 }) }
    }
}

#[cfg(feature = "alloc")]
pub use self::once_box::OnceBox;

#[cfg(feature = "alloc")]
mod once_box {
    use super::atomic::{AtomicPtr, Ordering};
    use core::{marker::PhantomData, ptr};

    use alloc::boxed::Box;

    /// A thread-safe cell which can be written to only once.
    pub struct OnceBox<T> {
        inner: AtomicPtr<T>,
        ghost: PhantomData<Option<Box<T>>>,
    }

    impl<T> core::fmt::Debug for OnceBox<T> {
        fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
            write!(f, "OnceBox({:?})", self.inner.load(Ordering::Relaxed))
        }
    }

    impl<T> Default for OnceBox<T> {
        fn default() -> Self {
            Self::new()
        }
    }

    impl<T> Drop for OnceBox<T> {
        fn drop(&mut self) {
            let ptr = *self.inner.get_mut();
            if !ptr.is_null() {
                drop(unsafe { Box::from_raw(ptr) })
            }
        }
    }

    impl<T> OnceBox<T> {
        /// Creates a new empty cell.
        pub const fn new() -> OnceBox<T> {
            OnceBox { inner: AtomicPtr::new(ptr::null_mut()), ghost: PhantomData }
        }

        /// Gets a reference to the underlying value.
        pub fn get(&self) -> Option<&T> {
            let ptr = self.inner.load(Ordering::Acquire);
            if ptr.is_null() {
                return None;
            }
            Some(unsafe { &*ptr })
        }

        /// Sets the contents of this cell to `value`.
        ///
        /// Returns `Ok(())` if the cell was empty and `Err(value)` if it was
        /// full.
        pub fn set(&self, value: Box<T>) -> Result<(), Box<T>> {
            let ptr = Box::into_raw(value);
            let exchange = self.inner.compare_exchange(
                ptr::null_mut(),
                ptr,
                Ordering::AcqRel,
                Ordering::Acquire,
            );
            if let Err(_) = exchange {
                let value = unsafe { Box::from_raw(ptr) };
                return Err(value);
            }
            Ok(())
        }

        /// Gets the contents of the cell, initializing it with `f` if the cell was
        /// empty.
        ///
        /// If several threads concurrently run `get_or_init`, more than one `f` can
        /// be called. However, all threads will return the same value, produced by
        /// some `f`.
        pub fn get_or_init<F>(&self, f: F) -> &T
        where
            F: FnOnce() -> Box<T>,
        {
            enum Void {}
            match self.get_or_try_init(|| Ok::<Box<T>, Void>(f())) {
                Ok(val) => val,
                Err(void) => match void {},
            }
        }

        /// Gets the contents of the cell, initializing it with `f` if
        /// the cell was empty. If the cell was empty and `f` failed, an
        /// error is returned.
        ///
        /// If several threads concurrently run `get_or_init`, more than one `f` can
        /// be called. However, all threads will return the same value, produced by
        /// some `f`.
        pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
        where
            F: FnOnce() -> Result<Box<T>, E>,
        {
            let mut ptr = self.inner.load(Ordering::Acquire);

            if ptr.is_null() {
                let val = f()?;
                ptr = Box::into_raw(val);
                let exchange = self.inner.compare_exchange(
                    ptr::null_mut(),
                    ptr,
                    Ordering::AcqRel,
                    Ordering::Acquire,
                );
                if let Err(old) = exchange {
                    drop(unsafe { Box::from_raw(ptr) });
                    ptr = old;
                }
            };
            Ok(unsafe { &*ptr })
        }
    }

    unsafe impl<T: Sync + Send> Sync for OnceBox<T> {}

    /// ```compile_fail
    /// struct S(*mut ());
    /// unsafe impl Sync for S {}
    ///
    /// fn share<T: Sync>(_: &T) {}
    /// share(&once_cell::race::OnceBox::<S>::new());
    /// ```
    fn _dummy() {}
}