4 releases
0.4.3 | Mar 25, 2023 |
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0.4.2 | Mar 24, 2023 |
0.4.1 | Mar 23, 2023 |
0.4.0 | Mar 23, 2023 |
0.3.0 |
|
#1024 in Algorithms
Used in bbolt-rs
37KB
495 lines
fnv-rs
An implementation of the Fowler–Noll–Vo hash function (FNV-1a) - including 32, 64, 128, 256, 512, & 1024 bit variants.
About
The FNV hash function provides a custom Hasher
implementation that is more
efficient for smaller hash keys, along with other variants that can provide longer hash outputs.
The Rust Standard Library documentation 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.
Examples
If you want to use any of the larger output FNV variants (256, 512, or 1024), make sure you enable the bigint
feature:
[dependencies]
fnv_rs = { version = "0.4.0", features = ["bigint"] }
Hash Method
use fnv_rs::{Fnv64, FnvHasher, FnvHashResult};
let hash = Fnv64::hash(b"Hash this!testing123"); // returns FnvHashResult
println!("{}", hash); // AD2808D0C15A663E
println!("{:X}", hash); // AD2808D0C15A663E
println!("{:x}", hash); // ad2808d0c15a663e
println!("{}", hash.as_hex()); // AD2808D0C15A663E
println!("{:?}", hash.as_bytes()); // [173, 40, 8, 208, 193, 90, 102, 62]
Update Method
use fnv_rs::{Fnv64, FnvHasher, FnvHashResult};
let mut hasher = Fnv64::new();
hasher.update(b"Hash this!");
hasher.update(b"testing123");
let hash = hasher.finalize(); // returns FnvHashResult
println!("{}", hash); // AD2808D0C15A663E
Using FNV in a HashMap
The FnvHashMap
type alias is the easiest way to use the standard library’s
HashMap
with FNV. (This uses FNV with a 64 bit output.)
use fnv_rs::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_rs::FnvHashSet;
let mut set = FnvHashSet::default();
set.insert(1);
set.insert(2);
set = FnvHashSet::with_capacity_and_hasher(10, Default::default());
set.insert(1);
set.insert(2);
Dependencies
~195KB