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<h1 class='fqn'><span class='in-band'>Function <a href='index.html'>itertools</a>::<wbr><a class="fn" href=''>unfold</a></span><span class='out-of-band'><span id='render-detail'>
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[<span class='inner'>−</span>]
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</span><a class='srclink' href='../src/itertools/sources.rs.html#97-104' title='goto source code'>[src]</a></span></h1>
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<div class="important-traits"><div class='tooltip'>ⓘ<span class='tooltiptext'>Important traits for <a class="struct" href="../itertools/structs/struct.Unfold.html" title="struct itertools::structs::Unfold">Unfold</a><St, F></span></div><div class="content hidden"><h3 class="important">Important traits for <a class="struct" href="../itertools/structs/struct.Unfold.html" title="struct itertools::structs::Unfold">Unfold</a><St, F></h3><code class="content"><span class="where fmt-newline">impl<A, St, F> <a class="trait" href="https://doc.rust-lang.org/nightly/core/iter/iterator/trait.Iterator.html" title="trait core::iter::iterator::Iterator">Iterator</a> for <a class="struct" href="../itertools/structs/struct.Unfold.html" title="struct itertools::structs::Unfold">Unfold</a><St, F> <span class="where fmt-newline">where<br> F: <a class="trait" href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnMut.html" title="trait core::ops::function::FnMut">FnMut</a>(<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&mut </a>St) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/option/enum.Option.html" title="enum core::option::Option">Option</a><A>, </span></span><span class="where fmt-newline"> type <a href='https://doc.rust-lang.org/nightly/core/iter/iterator/trait.Iterator.html#associatedtype.Item' class="type">Item</a> = A;</span></code></div></div><pre class='rust fn'>pub fn unfold<A, St, F>(initial_state: St, f: F) -> <a class="struct" href="../itertools/structs/struct.Unfold.html" title="struct itertools::structs::Unfold">Unfold</a><St, F> <span class="where fmt-newline">where<br> F: <a class="trait" href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnMut.html" title="trait core::ops::function::FnMut">FnMut</a>(<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&mut </a>St) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/option/enum.Option.html" title="enum core::option::Option">Option</a><A>, </span></pre><div class='docblock'><p>Creates a new unfold source with the specified closure as the "iterator
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function" and an initial state to eventually pass to the closure</p>
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<p><code>unfold</code> is a general iterator builder: it has a mutable state value,
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and a closure with access to the state that produces the next value.</p>
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<p>This more or less equivalent to a regular struct with an <code>Iterator</code>
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implementation, and is useful for one-off iterators.</p>
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<pre class="rust rust-example-rendered">
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<span class="comment">// an iterator that yields sequential Fibonacci numbers,</span>
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<span class="comment">// and stops at the maximum representable value.</span>
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<span class="kw">use</span> <span class="ident">itertools</span>::<span class="ident">unfold</span>;
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<span class="kw">let</span> (<span class="kw-2">mut</span> <span class="ident">x1</span>, <span class="kw-2">mut</span> <span class="ident">x2</span>) <span class="op">=</span> (<span class="number">1u32</span>, <span class="number">1u32</span>);
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">fibonacci</span> <span class="op">=</span> <span class="ident">unfold</span>((), <span class="kw">move</span> <span class="op">|</span>_<span class="op">|</span> {
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<span class="comment">// Attempt to get the next Fibonacci number</span>
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<span class="kw">let</span> <span class="ident">next</span> <span class="op">=</span> <span class="ident">x1</span>.<span class="ident">saturating_add</span>(<span class="ident">x2</span>);
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<span class="comment">// Shift left: ret <- x1 <- x2 <- next</span>
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<span class="kw">let</span> <span class="ident">ret</span> <span class="op">=</span> <span class="ident">x1</span>;
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<span class="ident">x1</span> <span class="op">=</span> <span class="ident">x2</span>;
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<span class="ident">x2</span> <span class="op">=</span> <span class="ident">next</span>;
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<span class="comment">// If addition has saturated at the maximum, we are finished</span>
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<span class="kw">if</span> <span class="ident">ret</span> <span class="op">==</span> <span class="ident">x1</span> <span class="op">&&</span> <span class="ident">ret</span> <span class="op">></span> <span class="number">1</span> {
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<span class="kw">return</span> <span class="prelude-val">None</span>;
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}
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<span class="prelude-val">Some</span>(<span class="ident">ret</span>)
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});
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<span class="ident">itertools</span>::<span class="ident">assert_equal</span>(<span class="ident">fibonacci</span>.<span class="ident">by_ref</span>().<span class="ident">take</span>(<span class="number">8</span>),
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<span class="macro">vec</span><span class="macro">!</span>[<span class="number">1</span>, <span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">5</span>, <span class="number">8</span>, <span class="number">13</span>, <span class="number">21</span>]);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">fibonacci</span>.<span class="ident">last</span>(), <span class="prelude-val">Some</span>(<span class="number">2_971_215_073</span>))</pre>
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