| <!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."><meta name="keywords" content="rust, rustlang, rust-lang, fnv"><title>fnv - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../normalize.css"><link rel="stylesheet" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../ayu.css" disabled><link rel="stylesheet" href="../dark.css" disabled><link rel="stylesheet" href="../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../storage.js"></script><script defer src="../crates.js"></script><script defer src="../main.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../favicon.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><a class="sidebar-logo" href="../fnv/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../fnv/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><h2 class="location"><a href="#">Crate fnv</a></h2><div class="sidebar-elems"><ul class="block"><li class="version">Version 1.0.7</li><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 Definitions</a></li></ul></section></div></nav><main><div class="width-limiter"><nav class="sub"><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1 class="fqn">Crate <a class="mod" href="#">fnv</a><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../clipboard.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="srclink" href="../src/fnv/lib.rs.html#1-367">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><details class="rustdoc-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> |
| <h3 id="about"><a href="#about">About</a></h3> |
| <p>The FNV hash function is a custom <code>Hasher</code> implementation that is more |
| efficient for smaller hash keys.</p> |
| <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, |
| SipHash, is good in many cases, it is notably slower than other algorithms |
| with short keys, such as when you have a map of integers to other values. |
| In cases like these, <a href="https://cglab.ca/~abeinges/blah/hash-rs/">FNV is demonstrably faster</a>.</p> |
| <p>Its disadvantages are that it performs badly on larger inputs, and |
| provides no protection against collision attacks, where a malicious user |
| can craft specific keys designed to slow a hasher down. Thus, it is |
| important to profile your program to ensure that you are using small hash |
| keys, and be certain that your program could not be exposed to malicious |
| inputs (including being a networked server).</p> |
| <p>The Rust compiler itself uses FNV, as it is not worried about |
| denial-of-service attacks, and can assume that its inputs are going to be |
| small—a perfect use case for FNV.</p> |
| <h3 id="using-fnv-in-a-hashmap"><a href="#using-fnv-in-a-hashmap">Using FNV in a <code>HashMap</code></a></h3> |
| <p>The <code>FnvHashMap</code> type alias is the easiest way to use the standard library’s |
| <code>HashMap</code> with FNV.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>fnv::FnvHashMap; |
| |
| <span class="kw">let </span><span class="kw-2">mut </span>map = FnvHashMap::default(); |
| map.insert(<span class="number">1</span>, <span class="string">"one"</span>); |
| map.insert(<span class="number">2</span>, <span class="string">"two"</span>); |
| |
| map = FnvHashMap::with_capacity_and_hasher(<span class="number">10</span>, Default::default()); |
| map.insert(<span class="number">1</span>, <span class="string">"one"</span>); |
| map.insert(<span class="number">2</span>, <span class="string">"two"</span>);</code></pre></div> |
| <p>Note, the standard library’s <code>HashMap::new</code> and <code>HashMap::with_capacity</code> |
| are only implemented for the <code>RandomState</code> hasher, so using <code>Default</code> to |
| get the hasher is the next best option.</p> |
| <h3 id="using-fnv-in-a-hashset"><a href="#using-fnv-in-a-hashset">Using FNV in a <code>HashSet</code></a></h3> |
| <p>Similarly, <code>FnvHashSet</code> is a type alias for the standard library’s <code>HashSet</code> |
| with FNV.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>fnv::FnvHashSet; |
| |
| <span class="kw">let </span><span class="kw-2">mut </span>set = FnvHashSet::default(); |
| set.insert(<span class="number">1</span>); |
| set.insert(<span class="number">2</span>); |
| |
| set = FnvHashSet::with_capacity_and_hasher(<span class="number">10</span>, Default::default()); |
| set.insert(<span class="number">1</span>); |
| set.insert(<span class="number">2</span>);</code></pre></div> |
| </div></details><h2 id="structs" class="small-section-header"><a href="#structs">Structs</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.FnvHasher.html" title="fnv::FnvHasher struct">FnvHasher</a></div><div class="item-right docblock-short">An implementation of the Fowler–Noll–Vo hash function.</div></div></div><h2 id="types" class="small-section-header"><a href="#types">Type Definitions</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="type" href="type.FnvBuildHasher.html" title="fnv::FnvBuildHasher type">FnvBuildHasher</a></div><div class="item-right docblock-short">A builder for default FNV hashers.</div></div><div class="item-row"><div class="item-left module-item"><a class="type" href="type.FnvHashMap.html" title="fnv::FnvHashMap type">FnvHashMap</a></div><div class="item-right docblock-short">A <code>HashMap</code> using a default FNV hasher.</div></div><div class="item-row"><div class="item-left module-item"><a class="type" href="type.FnvHashSet.html" title="fnv::FnvHashSet type">FnvHashSet</a></div><div class="item-right docblock-short">A <code>HashSet</code> using a default FNV hasher.</div></div></div></section></div></main><div id="rustdoc-vars" data-root-path="../" data-current-crate="fnv" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
