enum-vec

Efficiently store a vector of enum variants

Documentation

Let's say you have an enum Direction with 4 variants. You only need 2 bits to store the discriminant, but Rust will use the minimum of 1 byte (8 bits). Therefore, when using a Vec<Direction> with 16 elements it will use 16 bytes of memory. However, this crate provides the EnumVec type, which only uses as many bits as needed. So a EnumVec<Direction> with 16 elements will only use 4 bytes of memory.

Implementation

Since Rust doesn't provide a way to count the variants of a type, the enum_like crate defines a trait EnumLike with an associated constant NUM_VARIANTS, and helper methods to convert from usize to T. This trait is implemented for a few common types, like bool and Option<T>, and can be implemented for any type. The implementation can be automated using the enum_like_derive crate, which provides the #[derive(EnumLike)] proc macro.

Example

Add this to your Cargo.toml:

[dependencies]
enum_vec = "0.3"
enum_like = "0.2"
enum_like_derive = "0.1"

And then in src/main.rs:

#[macro_use]
extern crate enum_like_derive;
extern crate enum_like;
extern crate enum_vec;

use enum_vec::EnumVec;

#[derive(Copy, Clone, Debug, EnumLike)]
enum Direction {
    Left, Right, Up, Down,
}

fn main() {
    let mut v = EnumVec::new();
    v.push(Direction::Left);
    v.push(Direction::Right);
    v.push(Direction::Left);
    v.push(Direction::Right);

    for d in v {
        println!("{:?}", d);
    }
}

See examples/src/main.rs for more usage examples.

BitVec

Since an EnumVec is essentially a n-bitvec, you can use it as such.

type BitVec = EnumVec<bool>;
type TwoBitVec = EnumVec<[bool; 2]>;
type TwoBitVec = EnumVec<(bool, bool)>;
type FourBitVec = EnumVec<[bool; 4]>;

Deriving EnumLike

You can automatically derive EnumLike for almost any type, as long as all of its fields are EnumLike.

struct BitField {
    opt_0: bool,
    opt_1: bool,
    opt_2: bool,
    opt_3: bool,
}
enum BitsOrRaw {
    Bits(BitField),
    Raw { opt_01: (bool, bool), opt_23: (bool, bool), },
}

impl EnumLike

You can write a custom EnumLike implementation: the following code allows to create a EnumVec<Digit> where each element is 4 bits, instead of the 8 required by u8.

#[derive(Copy, Clone, Debug, PartialEq, Eq)]
struct Digit {
    x: u8, // x >= 0 && x <= 9
}

unsafe impl EnumLike for Digit {
    const NUM_VARIANTS: usize = 10;
    fn to_discr(self) -> usize {
        self.x as usize
    }
    fn from_discr(x: usize) -> Self {
        let x = x as u8;
        Self { x }
    }
}

This trait is unsafe because other code assumes that to_discr() never returns something >= than NUM_VARIANTS.

Memory efficiency

Since by default each block is 32 bits, the EnumVec is only 100% memory efficient when each element is 1, 2, 4, 8, 16 or 32 bits long. That's because the elements are never split across two blocks: a 15-bit element stored inside a 32-bit block will always use 30 bits and waste the remaining 2. In general, the efficiency can be calculated as 1 - (32 % n) / 32, but it's always equal or better than a normal Vec. However it's equal when n >= 11, so if you have a type with 2048 variants, you should consider using a Vec instead.

n	Vec	EnumVec8	EnumVec16	EnumVec32	EnumVec64	EnumVec128
1	0.125	1	1	1	1	1
2	0.25	1	1	1	1	1
3	0.375	0.75	0.9375	0.9375	0.984375	0.984375
4	0.5	1	1	1	1	1
5	0.625	0.625	0.9375	0.9375	0.9375	0.9765625
6	0.75	0.75	0.75	0.9375	0.9375	0.984375
7	0.875	0.875	0.875	0.875	0.984375	0.984375
8	1	1	1	1	1	1
9	0.5625	0	0.5625	0.84375	0.984375	0.984375
10	0.625	0	0.625	0.9375	0.9375	0.9375
11	0.6875	0	0.6875	0.6875	0.859375	0.9453125

The complete table is available as a python one-liner:

x = [(n, n/8 if n <= 8 else n/16 if n <= 16 else n/32 if n <= 32 else n/64, 1-(8%n)/8, 1-(16%n)/16, 1-(32%n)/32, 1-(64%n)/64, 1-(128%n)/128) for n in range(1, 64+1)]

An EnumVec8 with 8-bit storage blocks cannot be used to store items larger than 8 bits. Similarly, for storing elements larger than 32 bits, the default EnumVec32 is not enough. The maximum size of an item in bits is defined on the EnumLike crate as the number of bits that can fit in one usize. The EnumVec with 128-bit storage is the most memory-efficient option right now, but most of the operations are 2x slower than the other implementations on a tipical 64-bit machine. The 8, 16, 32 and 64-bit versions have similar performance.

The "efficiency limits" of each EnumVecN, the largest item size in bits where it is better than a Vec are the following:

Storage size	Efficiency limit
EnumVec8	4
EnumVec16	4
EnumVec32	11
EnumVec64	22
EnumVec128	42

Customization

To change the default storage just import the EnumVec from an internal module:

use enum_vec::vec_u64::EnumVec;
use enum_vec::vec_u8::EnumVec as EnumVec8;

This will make the EnumVec use 64-bit blocks, improving the memory efficiency, and also add the option to use an EnumVec8 with 8-bit blocks. Note that the enum_vec![] macro will always create an EnumVec, so code like:

let a: EnumVec8 = enum_vec![];

will not compile.

Which storage size to choose?

• Use EnumVec8 to minimize the overhead of small vectors, well actually consider using a SmallEnumVec instead.
• Use EnumVec64 with very large vectors, especially when the element bit size is not a power of 2, as it is more memory efficient in some cases.
• Use EnumVec128 only if memory efficiency is more important than performance.
• Use Vec if performance is more important than memory efficiency.
• Use SmallEnumVec if most of the time you need to store few elements (up to 128 bits).

PackedU8

When the item size is 8 or 16 bits, using a Vec is always a better option. But that's not always easy, as a Vec<[bool; 8]> will use 8 bytes per element instead of 8 bits. To force it to use 8 bits wrap it as Vec<PackedU8<[bool; 8]>>:

use enum_like::PackedU8;

let a = vec![PackedU8::new([true; 8]); 10];

for x in a {
    let x = x.value();
}

SmallEnumVec

There is an experimental SmallEnumVec available at:

use enum_vec::smallvec_u32::EnumVec as SmallEnumVec;

When compiled with the smallvec feature, enabled in Cargo.toml:

enum_vec = { version = "0.3", features = ["smallvec"] }

A SmallEnumVec will use the stack to store the items, and will only allocate when it grows too large. The default right now is to use 4x32 bits of inline storage. This will allow to store 128 1-bit items, 64 2-bit, 32 4-bit, etc.

See the smallvec crate for more information.

Drawbacks

• There is no indexing syntax, since the EnumVec can't return a reference. Use get and set instead.
• You can't use slice methods, like split(), get(range), reverse(), chunk and window iterators, sort(), dedup(), etc. Because there is no deref impl (unlike &Vec which can be used as a &[T]).
• Most operations (push, pop, insert, remove) are about 2 or 3 times slower than the Vec equivalent. Operations like extend, from_slice, or vec![None; 1000]; are even worse.

Benchmarks

Here is a comparison of Vec<T> vs EnumVec<T> when T requires 2 bits of storage.

(commit e8db9c883b82e472e9aefb6087be55dafd76b6a0)

 name                           normal_vec2 ns/iter  enum_vec32_2 ns/iter  diff ns/iter    diff %  speedup 
 ::bench_all                    3                    5                                2    66.67%   x 0.60 
 ::bench_all_small              3                    5                                2    66.67%   x 0.60 
 ::bench_all_worst_case         1,308                41                          -1,267   -96.87%  x 31.90 
 ::bench_all_worst_case_small   19                   5                              -14   -73.68%   x 3.80 
 ::bench_any                    8                    12                               4    50.00%   x 0.67 
 ::bench_any_small              8                    12                               4    50.00%   x 0.67 
 ::bench_any_worst_case         447                  59                            -388   -86.80%   x 7.58 
 ::bench_any_worst_case_small   11                   6                               -5   -45.45%   x 1.83 
 ::bench_extend                 419                  3,793                        3,374   805.25%   x 0.11 
 ::bench_extend_small           48                   108                             60   125.00%   x 0.44 
 ::bench_from_slice             180                  3,237                        3,057  1698.33%   x 0.06 
 ::bench_from_slice_small       27                   79                              52   192.59%   x 0.34 
 ::bench_insert                 8,059                13,154                       5,095    63.22%   x 0.61 
 ::bench_insert_at_zero         16,898               38,729                      21,831   129.19%   x 0.44 
 ::bench_insert_at_zero_small   218                  190                            -28   -12.84%   x 1.15 
 ::bench_insert_small           275                  258                            -17    -6.18%   x 1.07 
 ::bench_iter_all               2,327                4,948                        2,621   112.63%   x 0.47 
 ::bench_macro_from_elem        602                  2,435                        1,833   304.49%   x 0.25 
 ::bench_macro_from_elem_small  28                   80                              52   185.71%   x 0.35 
 ::bench_push                   4,914                7,097                        2,183    44.42%   x 0.69 
 ::bench_push_small             181                  130                            -51   -28.18%   x 1.39 
 ::bench_pushpop                4,390                12,107                       7,717   175.79%   x 0.36 
 ::bench_remove                 5,261                10,823                       5,562   105.72%   x 0.49 
 ::bench_remove_at_zero         15,880               68,593                      52,713   331.95%   x 0.23 
 ::bench_remove_at_zero_small   101                  443                            342   338.61%   x 0.23 
 ::bench_remove_small           103                  207                            104   100.97%   x 0.50 

The only methods which are definitely faster than the Vec equivalent are all and any, which take advantage of the packing to process many elements at once. Some other benchmarks appear faster because of reallocation: a Vec will reallocate when it reaches 1, 2, 4, 8, ... elements but an EnumVec will reallocate every 32/n, 64/n, ... and since in the benchmark n=2 and the number of insertions in "_small" benchmarks defaults to 16, a Vec will reallocate 4 times while an EnumVec will reallocate 1 time.

To run the benchmarks yourself, download the source code and run:

cargo +nightly bench --features smallvec > bench_log
cargo benchcmp normal_vec2 enum_vec32_2 bench_log

You will need to install cargo-benchcmp to be able to easily compare benchmarks. For example, to compare the default 32-bit EnumVec with a 8-bit EnumVec, when dealing with 4-bit elements, run:

cargo benchcmp enum_vec32_4 enum_vec8_4 bench bench_log

See also

enum-set

enum-map

enum-kinds

bit-vec

smallbitvec

smallvec