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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="An implementation of the Fowler–Noll–Vo hash function."><title>fnv - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><link rel="stylesheet" href="../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../static.files/rustdoc-ac92e1bbe349e143.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="fnv" data-themes="" data-resource-suffix="" data-rustdoc-version="1.76.0 (07dca489a 2024-02-04)" data-channel="1.76.0" data-search-js="search-2b6ce74ff89ae146.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../static.files/storage-f2adc0d6ca4d09fb.js"></script><script defer src="../crates.js"></script><script defer src="../static.files/main-305769736d49e732.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-feafe1bb7466e4bd.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-16x16-8b506e7a72182f1c.png"><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc mod crate"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">☰</button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../fnv/index.html">fnv</a><span class="version">1.0.7</span></h2></div><div class="sidebar-elems"><ul class="block">
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<li><a id="all-types" href="all.html">All Items</a></li></ul><section><ul class="block"><li><a href="#structs">Structs</a></li><li><a href="#types">Type Aliases</a></li></ul></section></div></nav><div class="sidebar-resizer"></div>
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<main><div class="width-limiter"><nav class="sub"><form class="search-form"><span></span><div id="sidebar-button" tabindex="-1"><a href="../fnv/all.html" title="show sidebar"></a></div><input class="search-input" name="search" aria-label="Run search in the documentation" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" tabindex="-1"><a href="../help.html" title="help">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1>Crate <a class="mod" href="#">fnv</a><button id="copy-path" title="Copy item path to clipboard"><img src="../static.files/clipboard-7571035ce49a181d.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="src" href="../src/fnv/lib.rs.html#1-367">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><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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<h3 id="about"><a href="#about">About</a></h3>
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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="https://cglab.ca/~abeinges/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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<h3 id="using-fnv-in-a-hashmap"><a href="#using-fnv-in-a-hashmap">Using FNV in a <code>HashMap</code></a></h3>
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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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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>fnv::FnvHashMap;
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<span class="kw">let </span><span class="kw-2">mut </span>map = FnvHashMap::default();
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map.insert(<span class="number">1</span>, <span class="string">"one"</span>);
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map.insert(<span class="number">2</span>, <span class="string">"two"</span>);
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map = FnvHashMap::with_capacity_and_hasher(<span class="number">10</span>, Default::default());
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map.insert(<span class="number">1</span>, <span class="string">"one"</span>);
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map.insert(<span class="number">2</span>, <span class="string">"two"</span>);</code></pre></div>
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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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<h3 id="using-fnv-in-a-hashset"><a href="#using-fnv-in-a-hashset">Using FNV in a <code>HashSet</code></a></h3>
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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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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>fnv::FnvHashSet;
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<span class="kw">let </span><span class="kw-2">mut </span>set = FnvHashSet::default();
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set.insert(<span class="number">1</span>);
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set.insert(<span class="number">2</span>);
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set = FnvHashSet::with_capacity_and_hasher(<span class="number">10</span>, Default::default());
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set.insert(<span class="number">1</span>);
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set.insert(<span class="number">2</span>);</code></pre></div>
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</div></details><h2 id="structs" class="section-header"><a href="#structs">Structs</a></h2><ul class="item-table"><li><div class="item-name"><a class="struct" href="struct.FnvHasher.html" title="struct fnv::FnvHasher">FnvHasher</a></div><div class="desc docblock-short">An implementation of the Fowler–Noll–Vo hash function.</div></li></ul><h2 id="types" class="section-header"><a href="#types">Type Aliases</a></h2><ul class="item-table"><li><div class="item-name"><a class="type" href="type.FnvBuildHasher.html" title="type fnv::FnvBuildHasher">FnvBuildHasher</a></div><div class="desc docblock-short">A builder for default FNV hashers.</div></li><li><div class="item-name"><a class="type" href="type.FnvHashMap.html" title="type fnv::FnvHashMap">FnvHashMap</a></div><div class="desc docblock-short">A <code>HashMap</code> using a default FNV hasher.</div></li><li><div class="item-name"><a class="type" href="type.FnvHashSet.html" title="type fnv::FnvHashSet">FnvHashSet</a></div><div class="desc docblock-short">A <code>HashSet</code> using a default FNV hasher.</div></li></ul></section></div></main></body></html>
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