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Reform power_of_two methods for perf increase & semantic change to consider 0 not a power of 2.

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Vec panics when attempting to reserve capacity > int::MAX (uint::MAX / 2).
This commit is contained in:
Aaron Liblong
2014-12-08 01:03:35 -05:00
parent 99d6956c3b
commit f6328b60da
4 changed files with 43 additions and 37 deletions
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27
src/libcore/num/mod.rs
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@@ -673,35 +673,30 @@ signed_int_impl! { int }
#[unstable = "recently settled as part of numerics reform"]
pub trait UnsignedInt: Int {
/// Returns `true` iff `self == 2^k` for some `k`.
#[inline]
fn is_power_of_two(self) -> bool {
(self - Int::one()) & self == Int::zero()
(self - Int::one()) & self == Int::zero() && !(self == Int::zero())
}
/// Returns the smallest power of two greater than or equal to `self`.
/// Unspecified behavior on overflow.
#[inline]
fn next_power_of_two(self) -> Self {
let halfbits = size_of::<Self>() * 4;
let mut tmp = self - Int::one();
let mut shift = 1u;
while shift <= halfbits {
tmp = tmp | (tmp >> shift);
shift = shift << 1u;
}
tmp + Int::one()
let bits = size_of::<Self>() * 8;
let one: Self = Int::one();
one << ((bits - (self - one).leading_zeros()) % bits)
}
/// Returns the smallest power of two greater than or equal to `n`. If the
/// next power of two is greater than the type's maximum value, `None` is
/// returned, otherwise the power of two is wrapped in `Some`.
fn checked_next_power_of_two(self) -> Option<Self> {
let halfbits = size_of::<Self>() * 4;
let mut tmp = self - Int::one();
let mut shift = 1u;
while shift <= halfbits {
tmp = tmp | (tmp >> shift);
shift = shift << 1u;
let npot = self.next_power_of_two();
if npot >= self {
Some(npot)
} else {
None
}
tmp.checked_add(Int::one())
}
}