This crate provides a map implementation that can make use of a fixed-size backing storage. It enables the compiler to heavily optimize map lookups by translating them into pattern matching over strictly defined enums. Potentially allowing for interesting performance characteristics.
For more information on how to use, see the [documentation].
The following features are available:
serde - Causes [Map] and [Set] to implement [Serialize] and
[Deserialize] if it's implements by their keys and/or values.KeyThe [Key derive] is provided to instruct the fixed-map containers on how
to build optimized storage for a given Key. We also require the key to
implement [Copy] for it to implement Key.
```rust use fixed_map::{Key, Map};
enum Part { One, Two, }
enum Key { Simple, Composite(Part), String(&'static str), Number(u32), Singleton(()), }
let mut map = Map::new();
map.insert(Key::Simple, 1); map.insert(Key::Composite(Part::One), 2); map.insert(Key::String("foo"), 3); map.insert(Key::Number(1), 4); map.insert(Key::Singleton(()), 5);
asserteq!(map.get(Key::Simple), Some(&1)); asserteq!(map.get(Key::Composite(Part::One)), Some(&2)); asserteq!(map.get(Key::Composite(Part::Two)), None); asserteq!(map.get(Key::String("foo")), Some(&3)); asserteq!(map.get(Key::String("bar")), None); asserteq!(map.get(Key::Number(1)), Some(&4)); asserteq!(map.get(Key::Number(2)), None); asserteq!(map.get(Key::Singleton(())), Some(&5)); ```
There are many cases where you want associate a value with a small, fixed number of elements identified by an enum.
For example, let's say you have a game where each room has something in four directions. We can model this relationship between the direction and the item using two enums.
```rust pub enum Dir { North, East, South, West, }
pub enum Item { Bow, Sword, Axe, } ```
The goal is for the performance of fixed-map to be identical to storing the
data linearly in memory like you could by storing the data as an array like
[Option<Item>; N] where each index correspondings to each variant in
Dir.
Doing this yourself could look like this:
```rust
pub enum Dir { North, East, South, West, }
pub enum Item { Bow, Sword, Axe, }
let mut map: [Option
if let Some(item) = &map[Dir::North as usize] { println!("found item: {:?}", item); } ```
But with fixed-map you can do it like this without (hopefully) incurring
any drop in performance:
```rust use fixed_map::{Key, Map};
pub enum Dir { North, East, South, West, }
pub enum Item { Bow, Sword, Axe, }
let mut map = Map::new(); map.insert(Dir::North, Item::Bow);
if let Some(item) = map.get(Dir::North) { println!("found item: {:?}", item); } ```
This crate uses unsafe for its iterators. This is needed because there is no proper way to associate generic lifetimes to associated types.
Instead, we associate the lifetime to the container (Map or Set) which
wraps a set of unsafe derefs over raw pointers.
In the following benchmarks, fixed-map is compared to:
fixed - A fixed_map::Map with a derived Key with N variants.hashbrown] - A high performance hash map. This is only included for
reference.
HashMap::with_capacity(N).array - A simple [Option<Key>; N] array.Note: for all insert benchmarks the underlying map is cloned in each
iteration.
``` fixed/get4 time: [253.97 ps 255.78 ps 257.75 ps] fixed/get8 time: [257.56 ps 259.70 ps 261.82 ps] fixed/get16 time: [253.47 ps 255.78 ps 258.44 ps] fixed/get32 time: [245.90 ps 246.58 ps 247.35 ps]
array/get4 time: [490.51 ps 491.72 ps 493.00 ps] array/get8 time: [514.83 ps 519.88 ps 525.49 ps] array/get16 time: [520.87 ps 528.57 ps 538.60 ps] array/get32 time: [515.18 ps 525.02 ps 537.46 ps]
hashbrown/get4 time: [3.4101 ns 3.4419 ns 3.4792 ns] hashbrown/get8 time: [3.3051 ns 3.3220 ns 3.3437 ns] hashbrown/get16 time: [3.2872 ns 3.2961 ns 3.3058 ns] hashbrown/get32 time: [3.2700 ns 3.2783 ns 3.2869 ns]
fixed/insert4 time: [494.04 ps 495.74 ps 497.61 ps] fixed/insert8 time: [752.24 ps 755.71 ps 760.33 ps] fixed/insert16 time: [1.3142 ns 1.3196 ns 1.3252 ns] fixed/insert32 time: [2.1143 ns 2.1203 ns 2.1259 ns]
array/insert4 time: [500.43 ps 504.79 ps 509.23 ps] array/insert8 time: [767.07 ps 769.24 ps 771.50 ps] array/insert16 time: [1.3343 ns 1.3386 ns 1.3431 ns] array/insert32 time: [2.1609 ns 2.1668 ns 2.1730 ns]
hashbrown/insert4 time: [85.362 ns 86.288 ns 87.712 ns] hashbrown/insert8 time: [96.778 ns 97.102 ns 97.482 ns] hashbrown/insert16 time: [122.12 ns 122.56 ns 123.05 ns] hashbrown/insert32 time: [166.31 ns 167.78 ns 169.47 ns]
fixed/iter4 time: [2.0091 ns 2.0141 ns 2.0191 ns] fixed/iter8 time: [6.4643 ns 6.4953 ns 6.5312 ns] fixed/iter16 time: [46.684 ns 46.839 ns 47.006 ns] fixed/iter32 time: [99.789 ns 100.03 ns 100.28 ns]
array/iter4 time: [6.5876 ns 6.6116 ns 6.6361 ns] array/iter8 time: [6.4052 ns 6.4235 ns 6.4438 ns] array/iter16 time: [7.5715 ns 7.6424 ns 7.7706 ns] array/iter32 time: [10.104 ns 10.174 ns 10.281 ns]
hashbrown/iter4 time: [3.7405 ns 3.7538 ns 3.7672 ns] hashbrown/iter8 time: [4.5252 ns 4.5433 ns 4.5692 ns] hashbrown/iter16 time: [8.5868 ns 8.6118 ns 8.6375 ns] hashbrown/iter32 time: [12.868 ns 12.898 ns 12.928 ns] ```
Most examples are in place to test what kind of assembler they compile to.
To do this, run:
sh
RUSTFLAGS="--emit asm" cargo build --release --example <example>
You should be able to find the assembler generated in the target folder:
sh
ls target/release/examples/