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deque: Move the shorter part when growing

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The deque is split at the marker lo, or logical index 0. Move the
shortest part (split by lo) when growing. This way add_front is just as
fast as add_back, on average.
This commit is contained in:
blake2-ppc 2013-07-06 05:42:45 +02:00
parent 75015c36f9
commit 8a3267672c
1 changed files with 25 additions and 7 deletions
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32
src/libextra/deque.rs
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@ -108,7 +108,7 @@ impl<T> Deque<T> {
/// Prepend an element to the deque
pub fn add_front(&mut self, t: T) {
if self.nelts == self.elts.len() {
grow(self.nelts, self.lo, &mut self.elts);
grow(self.nelts, &mut self.lo, &mut self.elts);
}
if self.lo == 0u {
self.lo = self.elts.len() - 1u;
@ -120,7 +120,7 @@ impl<T> Deque<T> {
/// Append an element to the deque
pub fn add_back(&mut self, t: T) {
if self.nelts == self.elts.len() {
grow(self.nelts, self.lo, &mut self.elts);
grow(self.nelts, &mut self.lo, &mut self.elts);
}
let hi = self.raw_index(self.nelts);
self.elts[hi] = Some(t);
@ -230,18 +230,36 @@ iterator!{impl DequeMutRevIterator -> &'self mut T, get_mut_ref, -1}
/// Grow is only called on full elts, so nelts is also len(elts), unlike
/// elsewhere.
fn grow<T>(nelts: uint, lo: uint, elts: &mut ~[Option<T>]) {
fn grow<T>(nelts: uint, loptr: &mut uint, elts: &mut ~[Option<T>]) {
assert_eq!(nelts, elts.len());
let newlen = elts.capacity() * 2;
let lo = *loptr;
let newlen = nelts * 2;
elts.reserve(newlen);
/* fill with None */
for uint::range(elts.len(), elts.capacity()) |_| {
elts.push(None);
}
/* move the former wraparound to the new half */
for uint::range(0, lo) |i| {
elts.swap(i, nelts + i);
/*
Move the shortest half into the newly reserved area.
lo ---->|
nelts ----------->|
[o o o|o o o o o]
A [. . .|o o o o o o o o|. . . . .]
B [o o o|. . . . . . . .|o o o o o]
*/
assert!(newlen - nelts/2 >= nelts);
if lo <= (nelts - lo) { // A
for uint::range(0, lo) |i| {
elts.swap(i, nelts + i);
}
} else { // B
for uint::range(lo, nelts) |i| {
elts.swap(i, newlen - nelts + i);
}
*loptr += newlen - nelts;
}
}