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82 lines
2.2 KiB
Markdown
82 lines
2.2 KiB
Markdown
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# rust-fnv
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An implementation of the [Fowler–Noll–Vo hash function][chongo].
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### [Read the documentation](https://doc.servo.org/fnv/)
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## About
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The FNV hash function is a custom `Hasher` implementation that is more
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efficient for smaller hash keys.
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[The Rust FAQ states that][faq] while the default `Hasher` 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, [FNV is demonstrably faster][graphs].
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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).
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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.
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## Usage
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To include this crate in your program, add the following to your `Cargo.toml`:
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```toml
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[dependencies]
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fnv = "1.0.3"
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```
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## Using FNV in a HashMap
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The `FnvHashMap` type alias is the easiest way to use the standard library’s
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`HashMap` with FNV.
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```rust
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use fnv::FnvHashMap;
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let mut map = FnvHashMap::default();
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map.insert(1, "one");
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map.insert(2, "two");
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map = FnvHashMap::with_capacity_and_hasher(10, Default::default());
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map.insert(1, "one");
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map.insert(2, "two");
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```
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Note, the standard library’s `HashMap::new` and `HashMap::with_capacity`
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are only implemented for the `RandomState` hasher, so using `Default` to
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get the hasher is the next best option.
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## Using FNV in a HashSet
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Similarly, `FnvHashSet` is a type alias for the standard library’s `HashSet`
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with FNV.
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```rust
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use fnv::FnvHashSet;
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let mut set = FnvHashSet::default();
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set.insert(1);
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set.insert(2);
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set = FnvHashSet::with_capacity_and_hasher(10, Default::default());
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set.insert(1);
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set.insert(2);
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```
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[chongo]: http://www.isthe.com/chongo/tech/comp/fnv/index.html
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[faq]: https://www.rust-lang.org/en-US/faq.html#why-are-rusts-hashmaps-slow
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[graphs]: https://cglab.ca/~abeinges/blah/hash-rs/
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