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</pre><pre class="rust"><code><span class="comment">// Copyright 2019 Developers of the Rand project.
//
// Licensed under the Apache License, Version 2.0 &lt;LICENSE-APACHE or
// https://www.apache.org/licenses/LICENSE-2.0&gt; or the MIT license
// &lt;LICENSE-MIT or https://opensource.org/licenses/MIT&gt;, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
</span><span class="attribute">#![allow(dead_code)]
</span><span class="kw">use </span>core::sync::atomic::{AtomicUsize, Ordering::Relaxed};
<span class="comment">// This structure represents a lazily initialized static usize value. Useful
// when it is preferable to just rerun initialization instead of locking.
// Both unsync_init and sync_init will invoke an init() function until it
// succeeds, then return the cached value for future calls.
//
// Both methods support init() &quot;failing&quot;. If the init() method returns UNINIT,
// that value will be returned as normal, but will not be cached.
//
// Users should only depend on the _value_ returned by init() functions.
// Specifically, for the following init() function:
// fn init() -&gt; usize {
// a();
// let v = b();
// c();
// v
// }
// the effects of c() or writes to shared memory will not necessarily be
// observed and additional synchronization methods with be needed.
</span><span class="kw">pub struct </span>LazyUsize(AtomicUsize);
<span class="kw">impl </span>LazyUsize {
<span class="kw">pub const fn </span>new() -&gt; <span class="self">Self </span>{
<span class="self">Self</span>(AtomicUsize::new(<span class="self">Self</span>::UNINIT))
}
<span class="comment">// The initialization is not completed.
</span><span class="kw">pub const </span>UNINIT: usize = usize::max_value();
<span class="comment">// Runs the init() function at least once, returning the value of some run
// of init(). Multiple callers can run their init() functions in parallel.
// init() should always return the same value, if it succeeds.
</span><span class="kw">pub fn </span>unsync_init(<span class="kw-2">&amp;</span><span class="self">self</span>, init: <span class="kw">impl </span>FnOnce() -&gt; usize) -&gt; usize {
<span class="comment">// Relaxed ordering is fine, as we only have a single atomic variable.
</span><span class="kw">let </span><span class="kw-2">mut </span>val = <span class="self">self</span>.<span class="number">0</span>.load(Relaxed);
<span class="kw">if </span>val == <span class="self">Self</span>::UNINIT {
val = init();
<span class="self">self</span>.<span class="number">0</span>.store(val, Relaxed);
}
val
}
}
<span class="comment">// Identical to LazyUsize except with bool instead of usize.
</span><span class="kw">pub struct </span>LazyBool(LazyUsize);
<span class="kw">impl </span>LazyBool {
<span class="kw">pub const fn </span>new() -&gt; <span class="self">Self </span>{
<span class="self">Self</span>(LazyUsize::new())
}
<span class="kw">pub fn </span>unsync_init(<span class="kw-2">&amp;</span><span class="self">self</span>, init: <span class="kw">impl </span>FnOnce() -&gt; bool) -&gt; bool {
<span class="self">self</span>.<span class="number">0</span>.unsync_init(|| init() <span class="kw">as </span>usize) != <span class="number">0
</span>}
}
</code></pre></div>
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