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<!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="API documentation for the Rust `unfold` fn in crate `itertools`."><meta name="keywords" content="rust, rustlang, rust-lang, unfold"><title>itertools::unfold - Rust</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></head><body class="rustdoc fn"><!--[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><div class="sidebar-elems"><p class='location'><a href='index.html'>itertools</a></p><script>window.sidebarCurrent = {name: 'unfold', ty: 'fn', relpath: ''};</script><script defer src="sidebar-items.js"></script></div></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"><input class="search-input" name="search" autocomplete="off" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><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"><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'><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class='inner'>−</span>]</a></span><a class='srclink' href='../src/itertools/sources.rs.html#96-103' title='goto source code'>[src]</a></span></h1><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">fibonacci</span> <span class="op">=</span> <span class="ident">unfold</span>((<span class="number">1_u32</span>, <span class="number">1_u32</span>), <span class="op">|</span><span class="ident">state</span><span class="op">|</span> {
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<span class="kw">let</span> (<span class="kw-2">ref</span> <span class="kw-2">mut</span> <span class="ident">x1</span>, <span class="kw-2">ref</span> <span class="kw-2">mut</span> <span class="ident">x2</span>) <span class="op">=</span> <span class="kw-2">*</span><span class="ident">state</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="kw-2">*</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="kw-2">*</span><span class="ident">x1</span>;
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<span class="kw-2">*</span><span class="ident">x1</span> <span class="op">=</span> <span class="kw-2">*</span><span class="ident">x2</span>;
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<span class="kw-2">*</span><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="kw-2">*</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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