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Add comments to explain helper functions

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This commit is contained in:
Irfan Hudda 2017-05-23 23:04:23 +05:30
parent ed24829985
commit 93219a2627
1 changed files with 13 additions and 5 deletions
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18
src/libcore/num/mod.rs
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@ -2327,12 +2327,22 @@ macro_rules! uint_impl {
(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.
// (For 8u8 next power of two is 8u8 and for 6u8 it is 8u8)
//
// 8u8.one_less_than_next_power_of_two() == 7
// 6u8.one_less_than_next_power_of_two() == 7
fn one_less_than_next_power_of_two(self) -> Self {
self.wrapping_sub(1)
.round_up_to_one_less_than_a_power_of_two()
@ -2340,10 +2350,10 @@ macro_rules! uint_impl {
}
/// Returns the smallest power of two greater than or equal to `self`.
/// When return value overflows, it panics in debug mode and return
/// value is wrapped in release mode.
///
/// More details about overflow behavior can be found in [RFC 560].
/// When return value overflows (i.e. `self > (1 << (N-1))` for type
/// `uN`), it panics in debug mode and return value is wrapped to 0 in
/// release mode (the only situation in which method can return 0).
///
/// # Examples
///
@ -2353,8 +2363,6 @@ macro_rules! uint_impl {
/// assert_eq!(2u8.next_power_of_two(), 2);
/// assert_eq!(3u8.next_power_of_two(), 4);
/// ```
///
/// [RFC 560]: https://github.com/rust-lang/rfcs/blob/master/text/0560-integer-overflow.md
#[stable(feature = "rust1", since = "1.0.0")]
#[inline]
pub fn next_power_of_two(self) -> Self {