Bug: 171338702

Clone this repo:
  1. 6093558 Merge "Update TEST_MAPPING" am: 5b064ca2d7 am: a6c7a3b74e by Treehugger Robot · 6 months ago main-16k master
  2. a6c7a3b Merge "Update TEST_MAPPING" am: 5b064ca2d7 by Treehugger Robot · 6 months ago
  3. 5b064ca Merge "Update TEST_MAPPING" by Treehugger Robot · 6 months ago
  4. 4c4a29a Update TEST_MAPPING by Matthew Maurer · 6 months ago
  5. 3f253e4 Update TEST_MAPPING am: 8772aab4b8 am: a8c01c923d by Matthew Maurer · 6 months ago


An implementation of the Fowler–Noll–Vo hash function.

Read the documentation


The FNV hash function is a custom Hasher implementation that is more efficient for smaller hash keys.

The Rust FAQ states that while the default Hasher 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, FNV is demonstrably faster.

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).

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.


To include this crate in your program, add the following to your Cargo.toml:

fnv = "1.0.3"

Using FNV in a HashMap

The FnvHashMap type alias is the easiest way to use the standard library’s HashMap with FNV.

use fnv::FnvHashMap;

let mut map = FnvHashMap::default();
map.insert(1, "one");
map.insert(2, "two");

map = FnvHashMap::with_capacity_and_hasher(10, Default::default());
map.insert(1, "one");
map.insert(2, "two");

Note, the standard library’s HashMap::new and HashMap::with_capacity are only implemented for the RandomState hasher, so using Default to get the hasher is the next best option.

Using FNV in a HashSet

Similarly, FnvHashSet is a type alias for the standard library’s HashSet with FNV.

use fnv::FnvHashSet;

let mut set = FnvHashSet::default();

set = FnvHashSet::with_capacity_and_hasher(10, Default::default());