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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 `fnv` crate."><meta name="keywords" content="rust, rustlang, rust-lang, fnv"><title>fnv - 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 mod"><!--[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><p class='location'>Crate fnv</p><div class="sidebar-elems"><div class="block items"><ul><li><a href="#structs">Structs</a></li><li><a href="#types">Type Definitions</a></li></ul></div><p class='location'></p><script>window.sidebarCurrent = {name: 'fnv', ty: 'mod', relpath: '../'};</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'>Crate <a class="mod" href=''>fnv</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/fnv/lib.rs.html#1-349' title='goto source code'>[src]</a></span></h1><div class='docblock'><p>An implementation of the <a href="http://www.isthe.com/chongo/tech/comp/fnv/index.html">Fowler–Noll–Vo hash function</a>.</p>
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<h2 id="about" class="section-header"><a href="#about">About</a></h2>
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<p>The FNV hash function is a custom <code>Hasher</code> implementation that is more
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efficient for smaller hash keys.</p>
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<p><a href="https://www.rust-lang.org/en-US/faq.html#why-are-rusts-hashmaps-slow">The Rust FAQ states that</a> while the default <code>Hasher</code> implementation,
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SipHash, is good in many cases, it is notably slower than other algorithms
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with short keys, such as when you have a map of integers to other values.
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In cases like these, <a href="http://cglab.ca/%7Eabeinges/blah/hash-rs/">FNV is demonstrably faster</a>.</p>
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<p>Its disadvantages are that it performs badly on larger inputs, and
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provides no protection against collision attacks, where a malicious user
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can craft specific keys designed to slow a hasher down. Thus, it is
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important to profile your program to ensure that you are using small hash
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keys, and be certain that your program could not be exposed to malicious
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inputs (including being a networked server).</p>
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<p>The Rust compiler itself uses FNV, as it is not worried about
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denial-of-service attacks, and can assume that its inputs are going to be
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small—a perfect use case for FNV.</p>
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<h2 id="using-fnv-in-a-hashmap" class="section-header"><a href="#using-fnv-in-a-hashmap">Using FNV in a <code>HashMap</code></a></h2>
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<p>The <code>FnvHashMap</code> type alias is the easiest way to use the standard library’s
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<code>HashMap</code> with FNV.</p>
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<pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">fnv</span>::<span class="ident">FnvHashMap</span>;
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">map</span> <span class="op">=</span> <span class="ident">FnvHashMap</span>::<span class="ident">default</span>();
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<span class="ident">map</span>.<span class="ident">insert</span>(<span class="number">1</span>, <span class="string">"one"</span>);
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<span class="ident">map</span>.<span class="ident">insert</span>(<span class="number">2</span>, <span class="string">"two"</span>);
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<span class="ident">map</span> <span class="op">=</span> <span class="ident">FnvHashMap</span>::<span class="ident">with_capacity_and_hasher</span>(<span class="number">10</span>, <span class="ident">Default</span>::<span class="ident">default</span>());
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<span class="ident">map</span>.<span class="ident">insert</span>(<span class="number">1</span>, <span class="string">"one"</span>);
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<span class="ident">map</span>.<span class="ident">insert</span>(<span class="number">2</span>, <span class="string">"two"</span>);</pre>
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<p>Note, the standard library’s <code>HashMap::new</code> and <code>HashMap::with_capacity</code>
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are only implemented for the <code>RandomState</code> hasher, so using <code>Default</code> to
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get the hasher is the next best option.</p>
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<h2 id="using-fnv-in-a-hashset" class="section-header"><a href="#using-fnv-in-a-hashset">Using FNV in a <code>HashSet</code></a></h2>
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<p>Similarly, <code>FnvHashSet</code> is a type alias for the standard library’s <code>HashSet</code>
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with FNV.</p>
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<pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">fnv</span>::<span class="ident">FnvHashSet</span>;
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">set</span> <span class="op">=</span> <span class="ident">FnvHashSet</span>::<span class="ident">default</span>();
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<span class="ident">set</span>.<span class="ident">insert</span>(<span class="number">1</span>);
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<span class="ident">set</span>.<span class="ident">insert</span>(<span class="number">2</span>);
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<span class="ident">set</span> <span class="op">=</span> <span class="ident">FnvHashSet</span>::<span class="ident">with_capacity_and_hasher</span>(<span class="number">10</span>, <span class="ident">Default</span>::<span class="ident">default</span>());
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<span class="ident">set</span>.<span class="ident">insert</span>(<span class="number">1</span>);
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<span class="ident">set</span>.<span class="ident">insert</span>(<span class="number">2</span>);</pre>
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</div><h2 id='structs' class='section-header'><a href="#structs">Structs</a></h2>
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<table>
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<tr class=' module-item'>
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<td><a class="struct" href="struct.FnvHasher.html"
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title='struct fnv::FnvHasher'>FnvHasher</a></td>
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<td class='docblock-short'>
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<p>An implementation of the Fowler–Noll–Vo hash function.</p>
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</td>
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</tr></table><h2 id='types' class='section-header'><a href="#types">Type Definitions</a></h2>
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<table>
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<tr class=' module-item'>
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<td><a class="type" href="type.FnvBuildHasher.html"
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title='type fnv::FnvBuildHasher'>FnvBuildHasher</a></td>
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<td class='docblock-short'>
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<p>A builder for default FNV hashers.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="type" href="type.FnvHashMap.html"
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title='type fnv::FnvHashMap'>FnvHashMap</a></td>
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<td class='docblock-short'>
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<p>A <code>HashMap</code> using a default FNV hasher.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="type" href="type.FnvHashSet.html"
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title='type fnv::FnvHashSet'>FnvHashSet</a></td>
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<td class='docblock-short'>
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<p>A <code>HashSet</code> using a default FNV hasher.</p>
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</td>
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