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/// Fused multiply-add. Computes `(self * a) + b` with only one rounding
/// error, yielding a more accurate result than an unfused multiply-add.
///
/// Using `mul_add` can be more performant than an unfused multiply-add if
/// the target architecture has a dedicated `fma` CPU instruction.
///
/// Note that `A` and `B` are `Self` by default, but this is not mandatory.
///
/// # Example
///
/// ```
/// use std::f32;
///
/// let m = 10.0_f32;
/// let x = 4.0_f32;
/// let b = 60.0_f32;
///
/// // 100.0
/// let abs_difference = (m.mul_add(x, b) - (m*x + b)).abs();
///
/// assert!(abs_difference <= 100.0 * f32::EPSILON);
/// ```
pub trait MulAdd<A = Self, B = Self> {
/// The resulting type after applying the fused multiply-add.
type Output;
/// Performs the fused multiply-add operation `(self * a) + b`
fn mul_add(self, a: A, b: B) -> Self::Output;
}
/// The fused multiply-add assignment operation `*self = (*self * a) + b`
pub trait MulAddAssign<A = Self, B = Self> {
/// Performs the fused multiply-add assignment operation `*self = (*self * a) + b`
fn mul_add_assign(&mut self, a: A, b: B);
}
#[cfg(any(feature = "std", feature = "libm"))]
impl MulAdd<f32, f32> for f32 {
type Output = Self;
#[inline]
fn mul_add(self, a: Self, b: Self) -> Self::Output {
<Self as crate::Float>::mul_add(self, a, b)
}
}
#[cfg(any(feature = "std", feature = "libm"))]
impl MulAdd<f64, f64> for f64 {
type Output = Self;
#[inline]
fn mul_add(self, a: Self, b: Self) -> Self::Output {
<Self as crate::Float>::mul_add(self, a, b)
}
}
macro_rules! mul_add_impl {
($trait_name:ident for $($t:ty)*) => {$(
impl $trait_name for $t {
type Output = Self;
#[inline]
fn mul_add(self, a: Self, b: Self) -> Self::Output {
(self * a) + b
}
}
)*}
}
mul_add_impl!(MulAdd for isize i8 i16 i32 i64 i128);
mul_add_impl!(MulAdd for usize u8 u16 u32 u64 u128);
#[cfg(any(feature = "std", feature = "libm"))]
impl MulAddAssign<f32, f32> for f32 {
#[inline]
fn mul_add_assign(&mut self, a: Self, b: Self) {
*self = <Self as crate::Float>::mul_add(*self, a, b)
}
}
#[cfg(any(feature = "std", feature = "libm"))]
impl MulAddAssign<f64, f64> for f64 {
#[inline]
fn mul_add_assign(&mut self, a: Self, b: Self) {
*self = <Self as crate::Float>::mul_add(*self, a, b)
}
}
macro_rules! mul_add_assign_impl {
($trait_name:ident for $($t:ty)*) => {$(
impl $trait_name for $t {
#[inline]
fn mul_add_assign(&mut self, a: Self, b: Self) {
*self = (*self * a) + b
}
}
)*}
}
mul_add_assign_impl!(MulAddAssign for isize i8 i16 i32 i64 i128);
mul_add_assign_impl!(MulAddAssign for usize u8 u16 u32 u64 u128);
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn mul_add_integer() {
macro_rules! test_mul_add {
($($t:ident)+) => {
$(
{
let m: $t = 2;
let x: $t = 3;
let b: $t = 4;
assert_eq!(MulAdd::mul_add(m, x, b), (m*x + b));
}
)+
};
}
test_mul_add!(usize u8 u16 u32 u64 isize i8 i16 i32 i64);
}
#[test]
#[cfg(feature = "std")]
fn mul_add_float() {
macro_rules! test_mul_add {
($($t:ident)+) => {
$(
{
use core::$t;
let m: $t = 12.0;
let x: $t = 3.4;
let b: $t = 5.6;
let abs_difference = (MulAdd::mul_add(m, x, b) - (m*x + b)).abs();
assert!(abs_difference <= 46.4 * $t::EPSILON);
}
)+
};
}
test_mul_add!(f32 f64);
}
}