bjoernager/rust
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Simplify helper functions

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Based on @scottmcm 's suggestion
This commit is contained in:
Irfan Hudda 2017-05-31 22:25:15 +05:30
parent 93219a2627
commit 18fadb61c4
2 changed files with 17 additions and 16 deletions
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28
src/libcore/num/mod.rs
View file

@ -15,7 +15,6 @@
use convert::TryFrom; use convert::TryFrom;
use fmt; use fmt;
use intrinsics; use intrinsics;
use mem::size_of;
use str::FromStr; use str::FromStr;
/// Provides intentionally-wrapped arithmetic on `T`. /// Provides intentionally-wrapped arithmetic on `T`.
@ -2327,26 +2326,25 @@ macro_rules! uint_impl {
(self.wrapping_sub(1)) & self == 0 && !(self == 0) (self.wrapping_sub(1)) & self == 0 && !(self == 0)
} }
// Returns one less than next greater power of two.
// (For 8u8 next greater power of two is 16u8 and for 6u8 it is 8u8)
//
// 8u8.round_up_to_one_less_than_a_power_of_two() == 15
// 6u8.round_up_to_one_less_than_a_power_of_two() == 7
fn round_up_to_one_less_than_a_power_of_two(self) -> Self {
let bits = size_of::<Self>() as u32 * 8;
let z = self.leading_zeros();
(if z == bits { 0 as Self } else { !0 }).wrapping_shr(z)
}
// Returns one less than next power of two. // Returns one less than next power of two.
// (For 8u8 next power of two is 8u8 and for 6u8 it is 8u8) // (For 8u8 next power of two is 8u8 and for 6u8 it is 8u8)
// //
// 8u8.one_less_than_next_power_of_two() == 7 // 8u8.one_less_than_next_power_of_two() == 7
// 6u8.one_less_than_next_power_of_two() == 7 // 6u8.one_less_than_next_power_of_two() == 7
//
// This method cannot overflow, as in the `next_power_of_two`
// overflow cases it instead ends up returning the maximum value
// of the type, and can return 0 for 0.
fn one_less_than_next_power_of_two(self) -> Self { fn one_less_than_next_power_of_two(self) -> Self {
self.wrapping_sub(1) if self <= 1 { return 0; }
.round_up_to_one_less_than_a_power_of_two()
.wrapping_add(if self == 0 { 1 } else { 0 }) // Because `p > 0`, it cannot consist entirely of leading zeros.
// That means the shift is always in-bounds, and some processors
// (such as intel pre-haswell) have more efficient ctlz
// intrinsics when the argument is non-zero.
let p = self - 1;
let z = p.leading_zeros();
<$SelfT>::max_value() >> z
} }
/// Returns the smallest power of two greater than or equal to `self`. /// Returns the smallest power of two greater than or equal to `self`.

5
src/libstd/num.rs
View file

@ -176,7 +176,10 @@ mod tests {
fn $test_name() { fn $test_name() {
#![test] #![test]
assert_eq!((0 as $T).checked_next_power_of_two(), Some(1)); assert_eq!((0 as $T).checked_next_power_of_two(), Some(1));
assert!(($T::MAX / 2).checked_next_power_of_two().is_some()); let smax = $T::MAX >> 1;
assert_eq!(smax.checked_next_power_of_two(), Some(smax+1));
assert_eq!((smax + 1).checked_next_power_of_two(), Some(smax + 1));
assert_eq!((smax + 2).checked_next_power_of_two(), None);
assert_eq!(($T::MAX - 1).checked_next_power_of_two(), None); assert_eq!(($T::MAX - 1).checked_next_power_of_two(), None);
assert_eq!($T::MAX.checked_next_power_of_two(), None); assert_eq!($T::MAX.checked_next_power_of_two(), None);
let mut next_power = 1; let mut next_power = 1;