# [−][src]Crate zwohash

A fast, deterministic, non-cryptographic hash for use in hash tables.

ZwoHash implements a very fast hash algorithm optimized for the use in hash tables. It has low per-hash overhead, which is important when hashing small keys. It is non-cryptographic and deterministic and as such not suited for inserting untrusted user-provided input into hash tables, unless other denial of service countermeasures are taken. As such it covers the same use cases as rustc's FxHash.

Compared to FxHash, ZwoHash provides essentially the same hashing speed while aiming for more uniform outputs. When used in a hash table ZwoHash is almost always able to match the performance of FxHash while outperforming it by quite a bit for some common inputs for which FxHash's output is particularly poor.

The hash algorithm used by ZwoHash is very similar to that of FxHash, both process one `usize`

at a time and perform the same number and kind of operations per `usize`

. ZwoHash though,
replaces the last iteration with a slightly more expensive operation that provides better output
guarantees. The additional overhead (independent of the size of the hashed data) consists of
performing a wide multiplication instead of a truncated multiplication and one additional
subtraction. This is very little overhead, and almost doesn't register when using ZwoHash in a
hash table.

ZwoHash guarantees that any input bit can affect any bit of the output. FxHash does not guarantee this, and even beyond that, ZwoHash's output is more uniform. When used in a hash table, this often reduces the number of collisions and thus the number of required probes for each access. This can result in ZwoHash outperforming FxHash in that setting.

Sometimes, given inputs for which FxHash is especially ill-suited, ZwoHash outperforms FxHash by
a large margin. This includes integer keys that all are a multiple of a power of two, floating
point values with a short base-2 representation, pointers returned from the allocator and other
inputs that only differ in the higher bits of the last processed `usize`

.

## Structs

ZwoHasher | A fast, deterministic, non-cryptographic hash for use in hash tables. |

## Type Definitions

HashMap | A |

HashSet | A |