1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76

use core::convert::Infallible;

/// Enum mapping type
///
/// This trait is internally used by `#[derive(Enum)]`. `Enum<T>` is
/// implemented by any enum type where V is a generic type representing a
/// value. The purpose of this generic type is to allow providing a value
/// type for arrays, as Rust currently does not support higher kinded types.
///
/// This trait is also implemented by `bool` and `u8`. While `u8` is
/// strictly speaking not an actual enum, there are good reasons to consider
/// it like one, as array of `u8` keys is a relatively common pattern.
pub trait Enum<V>: Sized {
    /// Representation of an enum map for type `V`.
    type Array: Array<V>;
    /// Takes an usize, and returns an element matching `into_usize` function.
    fn from_usize(value: usize) -> Self;
    /// Returns an unique identifier for a value within range of `0..Array::LENGTH`.
    fn into_usize(self) -> usize;
}

pub trait Array<V> {
    const LENGTH: usize;
    fn slice(&self) -> &[V];
    fn slice_mut(&mut self) -> &mut [V];
}

impl<V, const N: usize> Array<V> for [V; N] {
    const LENGTH: usize = N;
    fn slice(&self) -> &[V] {
        self
    }
    fn slice_mut(&mut self) -> &mut [V] {
        self
    }
}

impl<T> Enum<T> for bool {
    type Array = [T; 2];
    #[inline]
    fn from_usize(value: usize) -> Self {
        match value {
            0 => false,
            1 => true,
            _ => unreachable!(),
        }
    }
    #[inline]
    fn into_usize(self) -> usize {
        self as usize
    }
}

impl<T> Enum<T> for u8 {
    type Array = [T; 256];
    #[inline]
    fn from_usize(value: usize) -> Self {
        value as u8
    }
    #[inline]
    fn into_usize(self) -> usize {
        self as usize
    }
}

impl<T> Enum<T> for Infallible {
    type Array = [T; 0];
    #[inline]
    fn from_usize(_: usize) -> Self {
        unreachable!();
    }
    #[inline]
    fn into_usize(self) -> usize {
        match self {}
    }
}