tinyset contains a few collections that are optimized to scale
in size well for small numbers of elements, while still scaling
well in time (and size) for numbers of elements. We have two set types:
Set is basically interchangeable with HashSet, although it
does require that its elements implement the Copy trait,
since otherwise I would have to learn to write correct unsafe
code, which would be scary. It uses FNV hashing when there are
large numbers of elements.
TinySet is places a stronger requirement on its elements,
which must have trait HasInvalid. This is intended for
elements that are Copy, are Hash, and have an "invalid"
value. For the unsigned integer types, we take their maximum
value to mean invalid. This constraint allows us to save a
bit more space.
Set64 is a set for types that are 64 bits in size or less and are
Copy, intended for essentially integer types. This is our most
efficient type, since it can store values in less space than
std::mem::size_of::<T>(), in the common case that they are small
numbers. It is also essentially as fast as any of the other set
types (faster than many), and can avoid heap allocations
entirely for small sets.
All of these set types will do no heap allocation for small sets of
small elements. TinySet will store up to 16 bytes of elements
before doing any heap allocation, while Set stores sets up to size 8
without allocation. Set64 will store up to 22 bytes of elements,
and if all your elements are small (e.g. 0..22 as u64 it will store
them in as few bytes as possible.
All these sets are similar in speed to fnv::HashSet. Set64 is
usually faster than fnv::HashSet, sometimes by as much as a factor
of 2.
To run the benchmark suite, cd into bench and then run
cargo +nightly run --release
This will give you loads of timings and storage requirements for a wide variety of set types. You will need nightly rust installed with rustup.