<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source to the Rust file `/Users/syrusakbary/.cargo/registry/src/github.com-1ecc6299db9ec823/crossbeam-queue-0.1.2/src/array_queue.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>array_queue.rs.html -- source</title><link rel="stylesheet" type="text/css" href="../../normalize.css"><link rel="stylesheet" type="text/css" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../dark.css"><link rel="stylesheet" type="text/css" href="../../light.css" id="themeStyle"><script src="../../storage.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="shortcut icon" href="../../favicon.ico"><style type="text/css">#crate-search{background-image:url("../../down-arrow.svg");}</style></head><body class="rustdoc source"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../../crossbeam_queue/index.html'><div class='logo-container'><img src='../../rust-logo.png' alt='logo'></div></a></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../../theme.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><a id="settings-menu" href="../../settings.html"><img src="../../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><pre class="line-numbers"><span id="1"> 1</span>
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</pre><div class="example-wrap"><pre class="rust ">
<span class="doccomment">//! The implementation is based on Dmitry Vyukov's bounded MPMC queue.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! Source:</span>
<span class="doccomment">//! - http://www.1024cores.net/home/lock-free-algorithms/queues/bounded-mpmc-queue</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! Copyright & License:</span>
<span class="doccomment">//! - Copyright (c) 2010-2011 Dmitry Vyukov</span>
<span class="doccomment">//! - Simplified BSD License and Apache License, Version 2.0</span>
<span class="doccomment">//! - http://www.1024cores.net/home/code-license</span>
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">cell</span>::<span class="ident">UnsafeCell</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">fmt</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">marker</span>::<span class="ident">PhantomData</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">mem</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">ptr</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">sync</span>::<span class="ident">atomic</span>::{<span class="self">self</span>, <span class="ident">AtomicUsize</span>, <span class="ident">Ordering</span>};
<span class="kw">use</span> <span class="ident">crossbeam_utils</span>::{<span class="ident">Backoff</span>, <span class="ident">CachePadded</span>};
<span class="kw">use</span> <span class="ident">err</span>::{<span class="ident">PopError</span>, <span class="ident">PushError</span>};
<span class="doccomment">/// A slot in a queue.</span>
<span class="kw">struct</span> <span class="ident">Slot</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="doccomment">/// The current stamp.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// If the stamp equals the tail, this node will be next written to. If it equals the head,</span>
<span class="doccomment">/// this node will be next read from.</span>
<span class="ident">stamp</span>: <span class="ident">AtomicUsize</span>,
<span class="doccomment">/// The value in this slot.</span>
<span class="ident">value</span>: <span class="ident">UnsafeCell</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>,
}
<span class="doccomment">/// A bounded multi-producer multi-consumer queue.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This queue allocates a fixed-capacity buffer on construction, which is used to store pushed</span>
<span class="doccomment">/// elements. The queue cannot hold more elements that the buffer allows. Attempting to push an</span>
<span class="doccomment">/// element into a full queue will fail. Having a buffer allocated upfront makes this queue a bit</span>
<span class="doccomment">/// faster than [`SegQueue`].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`SegQueue`]: struct.SegQueue.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use crossbeam_queue::{ArrayQueue, PushError};</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let q = ArrayQueue::new(2);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(q.push('a'), Ok(()));</span>
<span class="doccomment">/// assert_eq!(q.push('b'), Ok(()));</span>
<span class="doccomment">/// assert_eq!(q.push('c'), Err(PushError('c')));</span>
<span class="doccomment">/// assert_eq!(q.pop(), Ok('a'));</span>
<span class="doccomment">/// ```</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">ArrayQueue</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="doccomment">/// The head of the queue.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This value is a "stamp" consisting of an index into the buffer and a lap, but packed into a</span>
<span class="doccomment">/// single `usize`. The lower bits represent the index, while the upper bits represent the lap.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Elements are popped from the head of the queue.</span>
<span class="ident">head</span>: <span class="ident">CachePadded</span><span class="op"><</span><span class="ident">AtomicUsize</span><span class="op">></span>,
<span class="doccomment">/// The tail of the queue.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This value is a "stamp" consisting of an index into the buffer and a lap, but packed into a</span>
<span class="doccomment">/// single `usize`. The lower bits represent the index, while the upper bits represent the lap.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Elements are pushed into the tail of the queue.</span>
<span class="ident">tail</span>: <span class="ident">CachePadded</span><span class="op"><</span><span class="ident">AtomicUsize</span><span class="op">></span>,
<span class="doccomment">/// The buffer holding slots.</span>
<span class="ident">buffer</span>: <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">Slot</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>,
<span class="doccomment">/// The queue capacity.</span>
<span class="ident">cap</span>: <span class="ident">usize</span>,
<span class="doccomment">/// A stamp with the value of `{ lap: 1, index: 0 }`.</span>
<span class="ident">one_lap</span>: <span class="ident">usize</span>,
<span class="doccomment">/// Indicates that dropping an `ArrayQueue<T>` may drop elements of type `T`.</span>
<span class="ident">_marker</span>: <span class="ident">PhantomData</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>,
}
<span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">Send</span><span class="op">></span> <span class="ident">Sync</span> <span class="kw">for</span> <span class="ident">ArrayQueue</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {}
<span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">Send</span><span class="op">></span> <span class="ident">Send</span> <span class="kw">for</span> <span class="ident">ArrayQueue</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {}
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">ArrayQueue</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="doccomment">/// Creates a new bounded queue with the given capacity.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Panics</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Panics if the capacity is zero.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use crossbeam_queue::ArrayQueue;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let q = ArrayQueue::<i32>::new(100);</span>
<span class="doccomment">/// ```</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">new</span>(<span class="ident">cap</span>: <span class="ident">usize</span>) <span class="op">-</span><span class="op">></span> <span class="ident">ArrayQueue</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">cap</span> <span class="op">></span> <span class="number">0</span>, <span class="string">"capacity must be non-zero"</span>);
<span class="comment">// Head is initialized to `{ lap: 0, index: 0 }`.</span>
<span class="comment">// Tail is initialized to `{ lap: 0, index: 0 }`.</span>
<span class="kw">let</span> <span class="ident">head</span> <span class="op">=</span> <span class="number">0</span>;
<span class="kw">let</span> <span class="ident">tail</span> <span class="op">=</span> <span class="number">0</span>;
<span class="comment">// Allocate a buffer of `cap` slots.</span>
<span class="kw">let</span> <span class="ident">buffer</span> <span class="op">=</span> {
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">v</span> <span class="op">=</span> <span class="ident">Vec</span>::<span class="op"><</span><span class="ident">Slot</span><span class="op"><</span><span class="ident">T</span><span class="op">></span><span class="op">></span>::<span class="ident">with_capacity</span>(<span class="ident">cap</span>);
<span class="kw">let</span> <span class="ident">ptr</span> <span class="op">=</span> <span class="ident">v</span>.<span class="ident">as_mut_ptr</span>();
<span class="ident">mem</span>::<span class="ident">forget</span>(<span class="ident">v</span>);
<span class="ident">ptr</span>
};
<span class="comment">// Initialize stamps in the slots.</span>
<span class="kw">for</span> <span class="ident">i</span> <span class="kw">in</span> <span class="number">0</span>..<span class="ident">cap</span> {
<span class="kw">unsafe</span> {
<span class="comment">// Set the stamp to `{ lap: 0, index: i }`.</span>
<span class="kw">let</span> <span class="ident">slot</span> <span class="op">=</span> <span class="ident">buffer</span>.<span class="ident">add</span>(<span class="ident">i</span>);
<span class="ident">ptr</span>::<span class="ident">write</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> (<span class="kw-2">*</span><span class="ident">slot</span>).<span class="ident">stamp</span>, <span class="ident">AtomicUsize</span>::<span class="ident">new</span>(<span class="ident">i</span>));
}
}
<span class="comment">// One lap is the smallest power of two greater than `cap`.</span>
<span class="kw">let</span> <span class="ident">one_lap</span> <span class="op">=</span> (<span class="ident">cap</span> <span class="op">+</span> <span class="number">1</span>).<span class="ident">next_power_of_two</span>();
<span class="ident">ArrayQueue</span> {
<span class="ident">buffer</span>,
<span class="ident">cap</span>,
<span class="ident">one_lap</span>,
<span class="ident">head</span>: <span class="ident">CachePadded</span>::<span class="ident">new</span>(<span class="ident">AtomicUsize</span>::<span class="ident">new</span>(<span class="ident">head</span>)),
<span class="ident">tail</span>: <span class="ident">CachePadded</span>::<span class="ident">new</span>(<span class="ident">AtomicUsize</span>::<span class="ident">new</span>(<span class="ident">tail</span>)),
<span class="ident">_marker</span>: <span class="ident">PhantomData</span>,
}
}
<span class="doccomment">/// Attempts to push an element into the queue.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// If the queue is full, the element is returned back as an error.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use crossbeam_queue::{ArrayQueue, PushError};</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let q = ArrayQueue::new(1);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(q.push(10), Ok(()));</span>
<span class="doccomment">/// assert_eq!(q.push(20), Err(PushError(20)));</span>
<span class="doccomment">/// ```</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">push</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">value</span>: <span class="ident">T</span>) <span class="op">-</span><span class="op">></span> <span class="prelude-ty">Result</span><span class="op"><</span>(), <span class="ident">PushError</span><span class="op"><</span><span class="ident">T</span><span class="op">></span><span class="op">></span> {
<span class="kw">let</span> <span class="ident">backoff</span> <span class="op">=</span> <span class="ident">Backoff</span>::<span class="ident">new</span>();
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">tail</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">tail</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">Relaxed</span>);
<span class="kw">loop</span> {
<span class="comment">// Deconstruct the tail.</span>
<span class="kw">let</span> <span class="ident">index</span> <span class="op">=</span> <span class="ident">tail</span> <span class="op">&</span> (<span class="self">self</span>.<span class="ident">one_lap</span> <span class="op">-</span> <span class="number">1</span>);
<span class="kw">let</span> <span class="ident">lap</span> <span class="op">=</span> <span class="ident">tail</span> <span class="op">&</span> <span class="op">!</span>(<span class="self">self</span>.<span class="ident">one_lap</span> <span class="op">-</span> <span class="number">1</span>);
<span class="comment">// Inspect the corresponding slot.</span>
<span class="kw">let</span> <span class="ident">slot</span> <span class="op">=</span> <span class="kw">unsafe</span> { <span class="kw-2">&</span><span class="kw-2">*</span><span class="self">self</span>.<span class="ident">buffer</span>.<span class="ident">add</span>(<span class="ident">index</span>) };
<span class="kw">let</span> <span class="ident">stamp</span> <span class="op">=</span> <span class="ident">slot</span>.<span class="ident">stamp</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">Acquire</span>);
<span class="comment">// If the tail and the stamp match, we may attempt to push.</span>
<span class="kw">if</span> <span class="ident">tail</span> <span class="op">=</span><span class="op">=</span> <span class="ident">stamp</span> {
<span class="kw">let</span> <span class="ident">new_tail</span> <span class="op">=</span> <span class="kw">if</span> <span class="ident">index</span> <span class="op">+</span> <span class="number">1</span> <span class="op"><</span> <span class="self">self</span>.<span class="ident">cap</span> {
<span class="comment">// Same lap, incremented index.</span>
<span class="comment">// Set to `{ lap: lap, index: index + 1 }`.</span>
<span class="ident">tail</span> <span class="op">+</span> <span class="number">1</span>
} <span class="kw">else</span> {
<span class="comment">// One lap forward, index wraps around to zero.</span>
<span class="comment">// Set to `{ lap: lap.wrapping_add(1), index: 0 }`.</span>
<span class="ident">lap</span>.<span class="ident">wrapping_add</span>(<span class="self">self</span>.<span class="ident">one_lap</span>)
};
<span class="comment">// Try moving the tail.</span>
<span class="kw">match</span> <span class="self">self</span>
.<span class="ident">tail</span>
.<span class="ident">compare_exchange_weak</span>(<span class="ident">tail</span>, <span class="ident">new_tail</span>, <span class="ident">Ordering</span>::<span class="ident">SeqCst</span>, <span class="ident">Ordering</span>::<span class="ident">Relaxed</span>)
{
<span class="prelude-val">Ok</span>(<span class="kw">_</span>) <span class="op">=</span><span class="op">></span> {
<span class="comment">// Write the value into the slot and update the stamp.</span>
<span class="kw">unsafe</span> { <span class="ident">slot</span>.<span class="ident">value</span>.<span class="ident">get</span>().<span class="ident">write</span>(<span class="ident">value</span>); }
<span class="ident">slot</span>.<span class="ident">stamp</span>.<span class="ident">store</span>(<span class="ident">tail</span> <span class="op">+</span> <span class="number">1</span>, <span class="ident">Ordering</span>::<span class="ident">Release</span>);
<span class="kw">return</span> <span class="prelude-val">Ok</span>(());
}
<span class="prelude-val">Err</span>(<span class="ident">t</span>) <span class="op">=</span><span class="op">></span> {
<span class="ident">tail</span> <span class="op">=</span> <span class="ident">t</span>;
<span class="ident">backoff</span>.<span class="ident">spin</span>();
}
}
} <span class="kw">else</span> <span class="kw">if</span> <span class="ident">stamp</span>.<span class="ident">wrapping_add</span>(<span class="self">self</span>.<span class="ident">one_lap</span>) <span class="op">=</span><span class="op">=</span> <span class="ident">tail</span> <span class="op">+</span> <span class="number">1</span> {
<span class="ident">atomic</span>::<span class="ident">fence</span>(<span class="ident">Ordering</span>::<span class="ident">SeqCst</span>);
<span class="kw">let</span> <span class="ident">head</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">head</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">Relaxed</span>);
<span class="comment">// If the head lags one lap behind the tail as well...</span>
<span class="kw">if</span> <span class="ident">head</span>.<span class="ident">wrapping_add</span>(<span class="self">self</span>.<span class="ident">one_lap</span>) <span class="op">=</span><span class="op">=</span> <span class="ident">tail</span> {
<span class="comment">// ...then the queue is full.</span>
<span class="kw">return</span> <span class="prelude-val">Err</span>(<span class="ident">PushError</span>(<span class="ident">value</span>));
}
<span class="ident">backoff</span>.<span class="ident">spin</span>();
<span class="ident">tail</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">tail</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">Relaxed</span>);
} <span class="kw">else</span> {
<span class="comment">// Snooze because we need to wait for the stamp to get updated.</span>
<span class="ident">backoff</span>.<span class="ident">snooze</span>();
<span class="ident">tail</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">tail</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">Relaxed</span>);
}
}
}
<span class="doccomment">/// Attempts to pop an element from the queue.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// If the queue is empty, an error is returned.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use crossbeam_queue::{ArrayQueue, PopError};</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let q = ArrayQueue::new(1);</span>
<span class="doccomment">/// assert_eq!(q.push(10), Ok(()));</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(q.pop(), Ok(10));</span>
<span class="doccomment">/// assert_eq!(q.pop(), Err(PopError));</span>
<span class="doccomment">/// ```</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">pop</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-</span><span class="op">></span> <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">PopError</span><span class="op">></span> {
<span class="kw">let</span> <span class="ident">backoff</span> <span class="op">=</span> <span class="ident">Backoff</span>::<span class="ident">new</span>();
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">head</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">head</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">Relaxed</span>);
<span class="kw">loop</span> {
<span class="comment">// Deconstruct the head.</span>
<span class="kw">let</span> <span class="ident">index</span> <span class="op">=</span> <span class="ident">head</span> <span class="op">&</span> (<span class="self">self</span>.<span class="ident">one_lap</span> <span class="op">-</span> <span class="number">1</span>);
<span class="kw">let</span> <span class="ident">lap</span> <span class="op">=</span> <span class="ident">head</span> <span class="op">&</span> <span class="op">!</span>(<span class="self">self</span>.<span class="ident">one_lap</span> <span class="op">-</span> <span class="number">1</span>);
<span class="comment">// Inspect the corresponding slot.</span>
<span class="kw">let</span> <span class="ident">slot</span> <span class="op">=</span> <span class="kw">unsafe</span> { <span class="kw-2">&</span><span class="kw-2">*</span><span class="self">self</span>.<span class="ident">buffer</span>.<span class="ident">add</span>(<span class="ident">index</span>) };
<span class="kw">let</span> <span class="ident">stamp</span> <span class="op">=</span> <span class="ident">slot</span>.<span class="ident">stamp</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">Acquire</span>);
<span class="comment">// If the the stamp is ahead of the head by 1, we may attempt to pop.</span>
<span class="kw">if</span> <span class="ident">head</span> <span class="op">+</span> <span class="number">1</span> <span class="op">=</span><span class="op">=</span> <span class="ident">stamp</span> {
<span class="kw">let</span> <span class="ident">new</span> <span class="op">=</span> <span class="kw">if</span> <span class="ident">index</span> <span class="op">+</span> <span class="number">1</span> <span class="op"><</span> <span class="self">self</span>.<span class="ident">cap</span> {
<span class="comment">// Same lap, incremented index.</span>
<span class="comment">// Set to `{ lap: lap, index: index + 1 }`.</span>
<span class="ident">head</span> <span class="op">+</span> <span class="number">1</span>
} <span class="kw">else</span> {
<span class="comment">// One lap forward, index wraps around to zero.</span>
<span class="comment">// Set to `{ lap: lap.wrapping_add(1), index: 0 }`.</span>
<span class="ident">lap</span>.<span class="ident">wrapping_add</span>(<span class="self">self</span>.<span class="ident">one_lap</span>)
};
<span class="comment">// Try moving the head.</span>
<span class="kw">match</span> <span class="self">self</span>
.<span class="ident">head</span>
.<span class="ident">compare_exchange_weak</span>(<span class="ident">head</span>, <span class="ident">new</span>, <span class="ident">Ordering</span>::<span class="ident">SeqCst</span>, <span class="ident">Ordering</span>::<span class="ident">Relaxed</span>)
{
<span class="prelude-val">Ok</span>(<span class="kw">_</span>) <span class="op">=</span><span class="op">></span> {
<span class="comment">// Read the value from the slot and update the stamp.</span>
<span class="kw">let</span> <span class="ident">msg</span> <span class="op">=</span> <span class="kw">unsafe</span> { <span class="ident">slot</span>.<span class="ident">value</span>.<span class="ident">get</span>().<span class="ident">read</span>() };
<span class="ident">slot</span>.<span class="ident">stamp</span>.<span class="ident">store</span>(<span class="ident">head</span>.<span class="ident">wrapping_add</span>(<span class="self">self</span>.<span class="ident">one_lap</span>), <span class="ident">Ordering</span>::<span class="ident">Release</span>);
<span class="kw">return</span> <span class="prelude-val">Ok</span>(<span class="ident">msg</span>);
}
<span class="prelude-val">Err</span>(<span class="ident">h</span>) <span class="op">=</span><span class="op">></span> {
<span class="ident">head</span> <span class="op">=</span> <span class="ident">h</span>;
<span class="ident">backoff</span>.<span class="ident">spin</span>();
}
}
} <span class="kw">else</span> <span class="kw">if</span> <span class="ident">stamp</span> <span class="op">=</span><span class="op">=</span> <span class="ident">head</span> {
<span class="ident">atomic</span>::<span class="ident">fence</span>(<span class="ident">Ordering</span>::<span class="ident">SeqCst</span>);
<span class="kw">let</span> <span class="ident">tail</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">tail</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">Relaxed</span>);
<span class="comment">// If the tail equals the head, that means the channel is empty.</span>
<span class="kw">if</span> <span class="ident">tail</span> <span class="op">=</span><span class="op">=</span> <span class="ident">head</span> {
<span class="kw">return</span> <span class="prelude-val">Err</span>(<span class="ident">PopError</span>);
}
<span class="ident">backoff</span>.<span class="ident">spin</span>();
<span class="ident">head</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">head</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">Relaxed</span>);
} <span class="kw">else</span> {
<span class="comment">// Snooze because we need to wait for the stamp to get updated.</span>
<span class="ident">backoff</span>.<span class="ident">snooze</span>();
<span class="ident">head</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">head</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">Relaxed</span>);
}
}
}
<span class="doccomment">/// Returns the capacity of the queue.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use crossbeam_queue::{ArrayQueue, PopError};</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let q = ArrayQueue::<i32>::new(100);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert_eq!(q.capacity(), 100);</span>
<span class="doccomment">/// ```</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">capacity</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-</span><span class="op">></span> <span class="ident">usize</span> {
<span class="self">self</span>.<span class="ident">cap</span>
}
<span class="doccomment">/// Returns `true` if the queue is empty.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use crossbeam_queue::{ArrayQueue, PopError};</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let q = ArrayQueue::new(100);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert!(q.is_empty());</span>
<span class="doccomment">/// q.push(1).unwrap();</span>
<span class="doccomment">/// assert!(!q.is_empty());</span>
<span class="doccomment">/// ```</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">is_empty</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-</span><span class="op">></span> <span class="ident">bool</span> {
<span class="kw">let</span> <span class="ident">head</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">head</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">SeqCst</span>);
<span class="kw">let</span> <span class="ident">tail</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">tail</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">SeqCst</span>);
<span class="comment">// Is the tail lagging one lap behind head?</span>
<span class="comment">// Is the tail equal to the head?</span>
<span class="comment">//</span>
<span class="comment">// Note: If the head changes just before we load the tail, that means there was a moment</span>
<span class="comment">// when the channel was not empty, so it is safe to just return `false`.</span>
<span class="ident">tail</span> <span class="op">=</span><span class="op">=</span> <span class="ident">head</span>
}
<span class="doccomment">/// Returns `true` if the queue is full.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use crossbeam_queue::{ArrayQueue, PopError};</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let q = ArrayQueue::new(1);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// assert!(!q.is_full());</span>
<span class="doccomment">/// q.push(1).unwrap();</span>
<span class="doccomment">/// assert!(q.is_full());</span>
<span class="doccomment">/// ```</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">is_full</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-</span><span class="op">></span> <span class="ident">bool</span> {
<span class="kw">let</span> <span class="ident">tail</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">tail</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">SeqCst</span>);
<span class="kw">let</span> <span class="ident">head</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">head</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">SeqCst</span>);
<span class="comment">// Is the head lagging one lap behind tail?</span>
<span class="comment">//</span>
<span class="comment">// Note: If the tail changes just before we load the head, that means there was a moment</span>
<span class="comment">// when the queue was not full, so it is safe to just return `false`.</span>
<span class="ident">head</span>.<span class="ident">wrapping_add</span>(<span class="self">self</span>.<span class="ident">one_lap</span>) <span class="op">=</span><span class="op">=</span> <span class="ident">tail</span>
}
<span class="doccomment">/// Returns the number of elements in the queue.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use crossbeam_queue::{ArrayQueue, PopError};</span>
<span class="doccomment">///</span>
<span class="doccomment">/// let q = ArrayQueue::new(100);</span>
<span class="doccomment">/// assert_eq!(q.len(), 0);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// q.push(10).unwrap();</span>
<span class="doccomment">/// assert_eq!(q.len(), 1);</span>
<span class="doccomment">///</span>
<span class="doccomment">/// q.push(20).unwrap();</span>
<span class="doccomment">/// assert_eq!(q.len(), 2);</span>
<span class="doccomment">/// ```</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">len</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-</span><span class="op">></span> <span class="ident">usize</span> {
<span class="kw">loop</span> {
<span class="comment">// Load the tail, then load the head.</span>
<span class="kw">let</span> <span class="ident">tail</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">tail</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">SeqCst</span>);
<span class="kw">let</span> <span class="ident">head</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">head</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">SeqCst</span>);
<span class="comment">// If the tail didn't change, we've got consistent values to work with.</span>
<span class="kw">if</span> <span class="self">self</span>.<span class="ident">tail</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">SeqCst</span>) <span class="op">=</span><span class="op">=</span> <span class="ident">tail</span> {
<span class="kw">let</span> <span class="ident">hix</span> <span class="op">=</span> <span class="ident">head</span> <span class="op">&</span> (<span class="self">self</span>.<span class="ident">one_lap</span> <span class="op">-</span> <span class="number">1</span>);
<span class="kw">let</span> <span class="ident">tix</span> <span class="op">=</span> <span class="ident">tail</span> <span class="op">&</span> (<span class="self">self</span>.<span class="ident">one_lap</span> <span class="op">-</span> <span class="number">1</span>);
<span class="kw">return</span> <span class="kw">if</span> <span class="ident">hix</span> <span class="op"><</span> <span class="ident">tix</span> {
<span class="ident">tix</span> <span class="op">-</span> <span class="ident">hix</span>
} <span class="kw">else</span> <span class="kw">if</span> <span class="ident">hix</span> <span class="op">></span> <span class="ident">tix</span> {
<span class="self">self</span>.<span class="ident">cap</span> <span class="op">-</span> <span class="ident">hix</span> <span class="op">+</span> <span class="ident">tix</span>
} <span class="kw">else</span> <span class="kw">if</span> <span class="ident">tail</span> <span class="op">=</span><span class="op">=</span> <span class="ident">head</span> {
<span class="number">0</span>
} <span class="kw">else</span> {
<span class="self">self</span>.<span class="ident">cap</span>
};
}
}
}
}
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Drop</span> <span class="kw">for</span> <span class="ident">ArrayQueue</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">drop</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) {
<span class="comment">// Get the index of the head.</span>
<span class="kw">let</span> <span class="ident">hix</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">head</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">Relaxed</span>) <span class="op">&</span> (<span class="self">self</span>.<span class="ident">one_lap</span> <span class="op">-</span> <span class="number">1</span>);
<span class="comment">// Loop over all slots that hold a message and drop them.</span>
<span class="kw">for</span> <span class="ident">i</span> <span class="kw">in</span> <span class="number">0</span>..<span class="self">self</span>.<span class="ident">len</span>() {
<span class="comment">// Compute the index of the next slot holding a message.</span>
<span class="kw">let</span> <span class="ident">index</span> <span class="op">=</span> <span class="kw">if</span> <span class="ident">hix</span> <span class="op">+</span> <span class="ident">i</span> <span class="op"><</span> <span class="self">self</span>.<span class="ident">cap</span> {
<span class="ident">hix</span> <span class="op">+</span> <span class="ident">i</span>
} <span class="kw">else</span> {
<span class="ident">hix</span> <span class="op">+</span> <span class="ident">i</span> <span class="op">-</span> <span class="self">self</span>.<span class="ident">cap</span>
};
<span class="kw">unsafe</span> {
<span class="self">self</span>.<span class="ident">buffer</span>.<span class="ident">add</span>(<span class="ident">index</span>).<span class="ident">drop_in_place</span>();
}
}
<span class="comment">// Finally, deallocate the buffer, but don't run any destructors.</span>
<span class="kw">unsafe</span> {
<span class="ident">Vec</span>::<span class="ident">from_raw_parts</span>(<span class="self">self</span>.<span class="ident">buffer</span>, <span class="number">0</span>, <span class="self">self</span>.<span class="ident">cap</span>);
}
}
}
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">fmt</span>::<span class="ident">Debug</span> <span class="kw">for</span> <span class="ident">ArrayQueue</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">fmt</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">f</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">fmt</span>::<span class="ident">Formatter</span>) <span class="op">-</span><span class="op">></span> <span class="ident">fmt</span>::<span class="prelude-ty">Result</span> {
<span class="ident">f</span>.<span class="ident">pad</span>(<span class="string">"ArrayQueue { .. }"</span>)
}
}
</pre></div>
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