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