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// SPDX-FileCopyrightText: 2017 - 2023 Kamila Borowska <kamila@borowska.pw>
// SPDX-FileCopyrightText: 2021 Bruno Corrêa Zimmermann <brunoczim@gmail.com>
// SPDX-FileCopyrightText: 2022 philipp <descpl@yahoo.de>
//
// SPDX-License-Identifier: MIT OR Apache-2.0
use core::cmp::Ordering;
use core::convert::Infallible;
/// Enum mapping type.
///
/// This trait is implemented by `#[derive(Enum)]`.
///
/// 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: Sized {
/// Length of the enum.
const LENGTH: usize;
/// 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;
}
/// Trait associating enum with an array.
///
/// This exists due to limitations of Rust compiler that prevent arrays from using
/// associated constants in structures. The array length must match `LENGTH` of an
/// `Enum`.
pub trait EnumArray<V>: Enum {
/// Representation of an enum map for type `V`.
type Array: Array<V>;
}
/// Array for enum-map storage.
///
/// This trait is inteded for primitive array types (with fixed length).
///
/// # Safety
///
/// The array length needs to match actual storage.
pub unsafe trait Array<V> {
// This is necessary duplication because the length in Enum trait can be
// provided by user and may not be trustworthy for unsafe code.
const LENGTH: usize;
}
unsafe impl<V, const N: usize> Array<V> for [V; N] {
const LENGTH: usize = N;
}
#[doc(hidden)]
#[inline]
pub fn out_of_bounds() -> ! {
panic!("index out of range for Enum::from_usize");
}
impl Enum for bool {
const LENGTH: usize = 2;
#[inline]
fn from_usize(value: usize) -> Self {
match value {
0 => false,
1 => true,
_ => out_of_bounds(),
}
}
#[inline]
fn into_usize(self) -> usize {
usize::from(self)
}
}
impl<T> EnumArray<T> for bool {
type Array = [T; Self::LENGTH];
}
impl Enum for () {
const LENGTH: usize = 1;
#[inline]
fn from_usize(value: usize) -> Self {
match value {
0 => (),
_ => out_of_bounds(),
}
}
#[inline]
fn into_usize(self) -> usize {
0
}
}
impl<T> EnumArray<T> for () {
type Array = [T; Self::LENGTH];
}
impl Enum for u8 {
const LENGTH: usize = 256;
#[inline]
fn from_usize(value: usize) -> Self {
value.try_into().unwrap_or_else(|_| out_of_bounds())
}
#[inline]
fn into_usize(self) -> usize {
usize::from(self)
}
}
impl<T> EnumArray<T> for u8 {
type Array = [T; Self::LENGTH];
}
impl Enum for Infallible {
const LENGTH: usize = 0;
#[inline]
fn from_usize(_: usize) -> Self {
out_of_bounds();
}
#[inline]
fn into_usize(self) -> usize {
match self {}
}
}
impl<T> EnumArray<T> for Infallible {
type Array = [T; Self::LENGTH];
}
impl Enum for Ordering {
const LENGTH: usize = 3;
#[inline]
fn from_usize(value: usize) -> Self {
match value {
0 => Ordering::Less,
1 => Ordering::Equal,
2 => Ordering::Greater,
_ => out_of_bounds(),
}
}
#[inline]
fn into_usize(self) -> usize {
match self {
Ordering::Less => 0,
Ordering::Equal => 1,
Ordering::Greater => 2,
}
}
}
impl<T> EnumArray<T> for Ordering {
type Array = [T; Self::LENGTH];
}
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