std: Add Vec.reserve for rounding-up reservation.
`.reserve_exact` can cause pathological O(n^2) behaviour, so providing a `.reserve` that ensures that capacity doubles (if you step 1, 2, ..., n) is more efficient. cc #11949
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@ -18,6 +18,7 @@ use container::Container;
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use iter::{DoubleEndedIterator, FromIterator, Iterator};
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use libc::{free, c_void};
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use mem::{size_of, move_val_init};
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use num;
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use num::CheckedMul;
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use ops::Drop;
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use option::{None, Option, Some};
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@ -136,6 +137,12 @@ impl<T> Vec<T> {
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self.cap
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}
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pub fn reserve(&mut self, capacity: uint) {
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if capacity >= self.len {
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self.reserve_exact(num::next_power_of_two(capacity))
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}
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}
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pub fn reserve_exact(&mut self, capacity: uint) {
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if capacity >= self.len {
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let size = capacity.checked_mul(&size_of::<T>()).expect("capacity overflow");
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@ -296,7 +303,7 @@ impl<T> Vec<T> {
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let len = self.len();
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assert!(index <= len);
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// space for the new element
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self.reserve_exact(len + 1);
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self.reserve(len + 1);
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unsafe { // infallible
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// The spot to put the new value
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