From ac64db94bf1d009a43e7f3729434417bd2e59662 Mon Sep 17 00:00:00 2001
From: Huon Wilson <dbau.pp+github@gmail.com>
Date: Sun, 23 Feb 2014 10:59:23 +1100
Subject: [PATCH] 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
---
 src/libstd/vec_ng.rs | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/src/libstd/vec_ng.rs b/src/libstd/vec_ng.rs
index 2f39adc25d34..52d3405f8c14 100644
--- a/src/libstd/vec_ng.rs
+++ b/src/libstd/vec_ng.rs
@@ -18,6 +18,7 @@ use container::Container;
 use iter::{DoubleEndedIterator, FromIterator, Iterator};
 use libc::{free, c_void};
 use mem::{size_of, move_val_init};
+use num;
 use num::CheckedMul;
 use ops::Drop;
 use option::{None, Option, Some};
@@ -136,6 +137,12 @@ impl<T> Vec<T> {
         self.cap
     }
 
+    pub fn reserve(&mut self, capacity: uint) {
+        if capacity >= self.len {
+            self.reserve_exact(num::next_power_of_two(capacity))
+        }
+    }
+
     pub fn reserve_exact(&mut self, capacity: uint) {
         if capacity >= self.len {
             let size = capacity.checked_mul(&size_of::<T>()).expect("capacity overflow");
@@ -296,7 +303,7 @@ impl<T> Vec<T> {
         let len = self.len();
         assert!(index <= len);
         // space for the new element
-        self.reserve_exact(len + 1);
+        self.reserve(len + 1);
 
         unsafe { // infallible
             // The spot to put the new value