72 lines
2.1 KiB
Rust
72 lines
2.1 KiB
Rust
// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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use std::fmt;
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use std::cell::Cell;
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/// A write-once variable. When constructed, it is empty, and
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/// can only be set once.
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///
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/// Ivars ensure that data that can only be initialised once. A full
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/// implementation is used for concurrency and blocks on a read of an
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/// unfulfilled value. This implementation is more minimal and panics
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/// if you attempt to read the value before it has been set. It is also
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/// not `Sync`, but may be extended in the future to be usable as a true
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/// concurrency type.
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///
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/// The `T: Copy` bound is not strictly needed, but it is required by
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/// Cell (so removing it would require using UnsafeCell), and it
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/// suffices for the current purposes.
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#[derive(PartialEq)]
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pub struct Ivar<T: Copy> {
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data: Cell<Option<T>>,
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}
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impl<T: Copy> Ivar<T> {
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pub fn new() -> Ivar<T> {
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Ivar { data: Cell::new(None) }
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}
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pub fn get(&self) -> Option<T> {
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self.data.get()
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}
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pub fn fulfill(&self, value: T) {
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assert!(self.data.get().is_none(), "Value already set!");
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self.data.set(Some(value));
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}
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pub fn is_fulfilled(&self) -> bool {
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self.data.get().is_some()
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}
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pub fn unwrap(&self) -> T {
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self.get().unwrap()
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}
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}
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impl<T: Copy + fmt::Debug> fmt::Debug for Ivar<T> {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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match self.get() {
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Some(val) => write!(f, "Ivar({:?})", val),
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None => f.write_str("Ivar(<unfulfilled>)"),
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}
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}
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}
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impl<T: Copy> Clone for Ivar<T> {
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fn clone(&self) -> Ivar<T> {
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match self.get() {
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Some(val) => Ivar { data: Cell::new(Some(val)) },
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None => Ivar::new(),
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}
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}
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}
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