num-traits/src/pow.rs

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use core::ops::Mul;
use {One, CheckedMul};
2018-02-19 20:17:41 +00:00
/// Binary operator for raising a value to a power.
pub trait Pow<RHS> {
/// The result after applying the operator.
type Output;
/// Returns `self` to the power `rhs`.
///
/// # Examples
///
/// ```
/// use num_traits::Pow;
/// assert_eq!(10.pow(2), 100);
/// ```
fn pow(self, rhs: RHS) -> Self::Output;
}
macro_rules! pow_impl {
($t:ty, $rhs:ty, $method:ident) => {
impl Pow<$rhs> for $t {
type Output = $t;
#[inline]
fn pow(self, rhs: $rhs) -> $t {
<$t>::$method(self, rhs)
}
}
impl<'a> Pow<&'a $rhs> for $t {
type Output = $t;
#[inline]
fn pow(self, rhs: &'a $rhs) -> $t {
<$t>::$method(self, *rhs)
}
}
impl<'a> Pow<$rhs> for &'a $t {
type Output = $t;
#[inline]
fn pow(self, rhs: $rhs) -> $t {
<$t>::$method(*self, rhs)
}
}
impl<'a, 'b> Pow<&'a $rhs> for &'b $t {
type Output = $t;
#[inline]
fn pow(self, rhs: &'a $rhs) -> $t {
<$t>::$method(*self, *rhs)
}
}
}
}
pow_impl!(u8, u32, pow);
pow_impl!(i8, u32, pow);
pow_impl!(u16, u32, pow);
pow_impl!(i16, u32, pow);
pow_impl!(u32, u32, pow);
pow_impl!(i32, u32, pow);
pow_impl!(u64, u32, pow);
pow_impl!(i64, u32, pow);
pow_impl!(usize, u32, pow);
pow_impl!(isize, u32, pow);
#[cfg(feature = "std")]
mod float_impls {
use super::Pow;
pow_impl!(f32, i32, powi);
pow_impl!(f64, i32, powi);
pow_impl!(f32, f32, powf);
pow_impl!(f64, f64, powf);
}
/// Raises a value to the power of exp, using exponentiation by squaring.
///
/// # Example
///
/// ```rust
/// use num_traits::pow;
///
/// assert_eq!(pow(2i8, 4), 16);
/// assert_eq!(pow(6u8, 3), 216);
/// ```
#[inline]
pub fn pow<T: Clone + One + Mul<T, Output = T>>(mut base: T, mut exp: usize) -> T {
if exp == 0 { return T::one() }
while exp & 1 == 0 {
base = base.clone() * base;
exp >>= 1;
}
if exp == 1 { return base }
let mut acc = base.clone();
while exp > 1 {
exp >>= 1;
base = base.clone() * base;
if exp & 1 == 1 {
acc = acc * base.clone();
}
}
acc
}
/// Raises a value to the power of exp, returning `None` if an overflow occurred.
///
/// Otherwise same as the `pow` function.
///
/// # Example
///
/// ```rust
/// use num_traits::checked_pow;
///
/// assert_eq!(checked_pow(2i8, 4), Some(16));
/// assert_eq!(checked_pow(7i8, 8), None);
/// assert_eq!(checked_pow(7u32, 8), Some(5_764_801));
/// ```
#[inline]
pub fn checked_pow<T: Clone + One + CheckedMul>(mut base: T, mut exp: usize) -> Option<T> {
if exp == 0 { return Some(T::one()) }
macro_rules! optry {
( $ expr : expr ) => {
if let Some(val) = $expr { val } else { return None }
}
}
while exp & 1 == 0 {
base = optry!(base.checked_mul(&base));
exp >>= 1;
}
if exp == 1 { return Some(base) }
let mut acc = base.clone();
while exp > 1 {
exp >>= 1;
base = optry!(base.checked_mul(&base));
if exp & 1 == 1 {
acc = optry!(acc.checked_mul(&base));
}
}
Some(acc)
}