binius_field/underlier/

small_uint.rs

// Copyright 2024 Irreducible Inc.

use super::{underlier_with_bit_ops::UnderlierWithBitOps, Random, UnderlierType};
use binius_utils::checked_arithmetics::checked_log_2;
use bytemuck::{NoUninit, Zeroable};
use derive_more::{BitAnd, BitAndAssign, BitOr, BitOrAssign, BitXor, BitXorAssign};
use rand::{
	distributions::{Distribution, Uniform},
	RngCore,
};
use std::{
	fmt::{Debug, Display, LowerHex},
	hash::{Hash, Hasher},
	ops::{Not, Shl, Shr},
};
use subtle::{ConditionallySelectable, ConstantTimeEq};

/// Unsigned type with a size strictly less than 8 bits.
#[derive(
	Default,
	Zeroable,
	Clone,
	Copy,
	PartialEq,
	Eq,
	PartialOrd,
	Ord,
	BitAnd,
	BitAndAssign,
	BitOr,
	BitOrAssign,
	BitXor,
	BitXorAssign,
)]
#[repr(transparent)]
pub struct SmallU<const N: usize>(u8);

impl<const N: usize> SmallU<N> {
	const _CHECK_SIZE: () = {
		assert!(N < 8);
	};

	#[inline(always)]
	pub const fn new(val: u8) -> Self {
		Self(val & Self::ONES.0)
	}

	#[inline(always)]
	pub const fn new_unchecked(val: u8) -> Self {
		Self(val)
	}

	#[inline(always)]
	pub const fn val(&self) -> u8 {
		self.0
	}

	pub fn checked_add(self, rhs: Self) -> Option<Self> {
		self.val()
			.checked_add(rhs.val())
			.and_then(|value| (value < Self::ONES.0).then_some(Self(value)))
	}

	pub fn checked_sub(self, rhs: Self) -> Option<Self> {
		let a = self.val();
		let b = rhs.val();
		(b > a).then_some(Self(b - a))
	}
}

impl<const N: usize> Debug for SmallU<N> {
	fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
		Debug::fmt(&self.val(), f)
	}
}

impl<const N: usize> Display for SmallU<N> {
	fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
		Display::fmt(&self.val(), f)
	}
}

impl<const N: usize> LowerHex for SmallU<N> {
	fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
		LowerHex::fmt(&self.0, f)
	}
}
impl<const N: usize> Hash for SmallU<N> {
	#[inline]
	fn hash<H: Hasher>(&self, state: &mut H) {
		self.val().hash(state);
	}
}

impl<const N: usize> ConstantTimeEq for SmallU<N> {
	fn ct_eq(&self, other: &Self) -> subtle::Choice {
		self.val().ct_eq(&other.val())
	}
}

impl<const N: usize> ConditionallySelectable for SmallU<N> {
	fn conditional_select(a: &Self, b: &Self, choice: subtle::Choice) -> Self {
		Self(u8::conditional_select(&a.0, &b.0, choice))
	}
}

impl<const N: usize> Random for SmallU<N> {
	fn random(mut rng: impl RngCore) -> Self {
		let distr = Uniform::from(0u8..1u8 << N);

		Self(distr.sample(&mut rng))
	}
}

impl<const N: usize> Shr<usize> for SmallU<N> {
	type Output = Self;

	#[inline(always)]
	fn shr(self, rhs: usize) -> Self::Output {
		Self(self.val() >> rhs)
	}
}

impl<const N: usize> Shl<usize> for SmallU<N> {
	type Output = Self;

	#[inline(always)]
	fn shl(self, rhs: usize) -> Self::Output {
		Self(self.val() << rhs) & Self::ONES
	}
}

impl<const N: usize> Not for SmallU<N> {
	type Output = Self;

	fn not(self) -> Self::Output {
		self ^ Self::ONES
	}
}

unsafe impl<const N: usize> NoUninit for SmallU<N> {}

impl<const N: usize> UnderlierType for SmallU<N> {
	const LOG_BITS: usize = checked_log_2(N);
}

impl<const N: usize> UnderlierWithBitOps for SmallU<N> {
	const ZERO: Self = Self(0);
	const ONE: Self = Self(1);
	const ONES: Self = Self((1u8 << N) - 1);

	fn fill_with_bit(val: u8) -> Self {
		Self(u8::fill_with_bit(val)) & Self::ONES
	}
}

impl<const N: usize> From<SmallU<N>> for u8 {
	#[inline(always)]
	fn from(value: SmallU<N>) -> Self {
		value.val()
	}
}

impl<const N: usize> From<SmallU<N>> for u16 {
	#[inline(always)]
	fn from(value: SmallU<N>) -> Self {
		u8::from(value) as _
	}
}

impl<const N: usize> From<SmallU<N>> for u32 {
	#[inline(always)]
	fn from(value: SmallU<N>) -> Self {
		u8::from(value) as _
	}
}

impl<const N: usize> From<SmallU<N>> for u64 {
	#[inline(always)]
	fn from(value: SmallU<N>) -> Self {
		u8::from(value) as _
	}
}

impl<const N: usize> From<SmallU<N>> for usize {
	#[inline(always)]
	fn from(value: SmallU<N>) -> Self {
		u8::from(value) as _
	}
}

impl<const N: usize> From<SmallU<N>> for u128 {
	#[inline(always)]
	fn from(value: SmallU<N>) -> Self {
		u8::from(value) as _
	}
}

impl From<SmallU<1>> for SmallU<2> {
	#[inline(always)]
	fn from(value: SmallU<1>) -> Self {
		Self(value.val())
	}
}

impl From<SmallU<1>> for SmallU<4> {
	#[inline(always)]
	fn from(value: SmallU<1>) -> Self {
		Self(value.val())
	}
}

impl From<SmallU<2>> for SmallU<4> {
	#[inline(always)]
	fn from(value: SmallU<2>) -> Self {
		Self(value.val())
	}
}

pub type U1 = SmallU<1>;
pub type U2 = SmallU<2>;
pub type U4 = SmallU<4>;