1 // Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! A priority queue implemented with a binary heap.
13 //! Insertion and popping the largest element have `O(log n)` time complexity.
14 //! Checking the largest element is `O(1)`. Converting a vector to a binary heap
15 //! can be done in-place, and has `O(n)` complexity. A binary heap can also be
16 //! converted to a sorted vector in-place, allowing it to be used for an `O(n
17 //! log n)` in-place heapsort.
21 //! This is a larger example that implements [Dijkstra's algorithm][dijkstra]
22 //! to solve the [shortest path problem][sssp] on a [directed graph][dir_graph].
23 //! It shows how to use `BinaryHeap` with custom types.
25 //! [dijkstra]: http://en.wikipedia.org/wiki/Dijkstra%27s_algorithm
26 //! [sssp]: http://en.wikipedia.org/wiki/Shortest_path_problem
27 //! [dir_graph]: http://en.wikipedia.org/wiki/Directed_graph
30 //! use std::cmp::Ordering;
31 //! use std::collections::BinaryHeap;
34 //! #[derive(Copy, Clone, Eq, PartialEq)]
40 //! // The priority queue depends on `Ord`.
41 //! // Explicitly implement the trait so the queue becomes a min-heap
42 //! // instead of a max-heap.
43 //! impl Ord for State {
44 //! fn cmp(&self, other: &State) -> Ordering {
45 //! // Notice that the we flip the ordering here
46 //! other.cost.cmp(&self.cost)
50 //! // `PartialOrd` needs to be implemented as well.
51 //! impl PartialOrd for State {
52 //! fn partial_cmp(&self, other: &State) -> Option<Ordering> {
53 //! Some(self.cmp(other))
57 //! // Each node is represented as an `usize`, for a shorter implementation.
63 //! // Dijkstra's shortest path algorithm.
65 //! // Start at `start` and use `dist` to track the current shortest distance
66 //! // to each node. This implementation isn't memory-efficient as it may leave duplicate
67 //! // nodes in the queue. It also uses `usize::MAX` as a sentinel value,
68 //! // for a simpler implementation.
69 //! fn shortest_path(adj_list: &Vec<Vec<Edge>>, start: usize, goal: usize) -> Option<usize> {
70 //! // dist[node] = current shortest distance from `start` to `node`
71 //! let mut dist: Vec<_> = (0..adj_list.len()).map(|_| usize::MAX).collect();
73 //! let mut heap = BinaryHeap::new();
75 //! // We're at `start`, with a zero cost
77 //! heap.push(State { cost: 0, position: start });
79 //! // Examine the frontier with lower cost nodes first (min-heap)
80 //! while let Some(State { cost, position }) = heap.pop() {
81 //! // Alternatively we could have continued to find all shortest paths
82 //! if position == goal { return Some(cost); }
84 //! // Important as we may have already found a better way
85 //! if cost > dist[position] { continue; }
87 //! // For each node we can reach, see if we can find a way with
88 //! // a lower cost going through this node
89 //! for edge in &adj_list[position] {
90 //! let next = State { cost: cost + edge.cost, position: edge.node };
92 //! // If so, add it to the frontier and continue
93 //! if next.cost < dist[next.position] {
95 //! // Relaxation, we have now found a better way
96 //! dist[next.position] = next.cost;
101 //! // Goal not reachable
106 //! // This is the directed graph we're going to use.
107 //! // The node numbers correspond to the different states,
108 //! // and the edge weights symbolize the cost of moving
109 //! // from one node to another.
110 //! // Note that the edges are one-way.
113 //! // +-----------------+
116 //! // 0 -----> 1 -----> 3 ---> 4
120 //! // +------> 2 -------+ |
122 //! // +---------------+
124 //! // The graph is represented as an adjacency list where each index,
125 //! // corresponding to a node value, has a list of outgoing edges.
126 //! // Chosen for its efficiency.
127 //! let graph = vec![
129 //! vec![Edge { node: 2, cost: 10 },
130 //! Edge { node: 1, cost: 1 }],
132 //! vec![Edge { node: 3, cost: 2 }],
134 //! vec![Edge { node: 1, cost: 1 },
135 //! Edge { node: 3, cost: 3 },
136 //! Edge { node: 4, cost: 1 }],
138 //! vec![Edge { node: 0, cost: 7 },
139 //! Edge { node: 4, cost: 2 }],
143 //! assert_eq!(shortest_path(&graph, 0, 1), Some(1));
144 //! assert_eq!(shortest_path(&graph, 0, 3), Some(3));
145 //! assert_eq!(shortest_path(&graph, 3, 0), Some(7));
146 //! assert_eq!(shortest_path(&graph, 0, 4), Some(5));
147 //! assert_eq!(shortest_path(&graph, 4, 0), None);
151 #![allow(missing_docs)]
152 #![stable(feature = "rust1", since = "1.0.0")]
154 use core::iter::FromIterator;
160 use vec::{self, Vec};
162 /// A priority queue implemented with a binary heap.
164 /// This will be a max-heap.
166 /// It is a logic error for an item to be modified in such a way that the
167 /// item's ordering relative to any other item, as determined by the `Ord`
168 /// trait, changes while it is in the heap. This is normally only possible
169 /// through `Cell`, `RefCell`, global state, I/O, or unsafe code.
170 #[stable(feature = "rust1", since = "1.0.0")]
171 pub struct BinaryHeap<T> {
175 #[stable(feature = "rust1", since = "1.0.0")]
176 impl<T: Clone> Clone for BinaryHeap<T> {
177 fn clone(&self) -> Self {
178 BinaryHeap { data: self.data.clone() }
181 fn clone_from(&mut self, source: &Self) {
182 self.data.clone_from(&source.data);
186 #[stable(feature = "rust1", since = "1.0.0")]
187 impl<T: Ord> Default for BinaryHeap<T> {
189 fn default() -> BinaryHeap<T> {
194 #[stable(feature = "binaryheap_debug", since = "1.4.0")]
195 impl<T: fmt::Debug + Ord> fmt::Debug for BinaryHeap<T> {
196 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
197 f.debug_list().entries(self.iter()).finish()
201 impl<T: Ord> BinaryHeap<T> {
202 /// Creates an empty `BinaryHeap` as a max-heap.
207 /// use std::collections::BinaryHeap;
208 /// let mut heap = BinaryHeap::new();
211 #[stable(feature = "rust1", since = "1.0.0")]
212 pub fn new() -> BinaryHeap<T> {
213 BinaryHeap { data: vec![] }
216 /// Creates an empty `BinaryHeap` with a specific capacity.
217 /// This preallocates enough memory for `capacity` elements,
218 /// so that the `BinaryHeap` does not have to be reallocated
219 /// until it contains at least that many values.
224 /// use std::collections::BinaryHeap;
225 /// let mut heap = BinaryHeap::with_capacity(10);
228 #[stable(feature = "rust1", since = "1.0.0")]
229 pub fn with_capacity(capacity: usize) -> BinaryHeap<T> {
230 BinaryHeap { data: Vec::with_capacity(capacity) }
233 /// Returns an iterator visiting all values in the underlying vector, in
239 /// use std::collections::BinaryHeap;
240 /// let heap = BinaryHeap::from(vec![1, 2, 3, 4]);
242 /// // Print 1, 2, 3, 4 in arbitrary order
243 /// for x in heap.iter() {
244 /// println!("{}", x);
247 #[stable(feature = "rust1", since = "1.0.0")]
248 pub fn iter(&self) -> Iter<T> {
249 Iter { iter: self.data.iter() }
252 /// Returns the greatest item in the binary heap, or `None` if it is empty.
257 /// use std::collections::BinaryHeap;
258 /// let mut heap = BinaryHeap::new();
259 /// assert_eq!(heap.peek(), None);
264 /// assert_eq!(heap.peek(), Some(&5));
267 #[stable(feature = "rust1", since = "1.0.0")]
268 pub fn peek(&self) -> Option<&T> {
272 /// Returns the number of elements the binary heap can hold without reallocating.
277 /// use std::collections::BinaryHeap;
278 /// let mut heap = BinaryHeap::with_capacity(100);
279 /// assert!(heap.capacity() >= 100);
282 #[stable(feature = "rust1", since = "1.0.0")]
283 pub fn capacity(&self) -> usize {
287 /// Reserves the minimum capacity for exactly `additional` more elements to be inserted in the
288 /// given `BinaryHeap`. Does nothing if the capacity is already sufficient.
290 /// Note that the allocator may give the collection more space than it requests. Therefore
291 /// capacity can not be relied upon to be precisely minimal. Prefer `reserve` if future
292 /// insertions are expected.
296 /// Panics if the new capacity overflows `usize`.
301 /// use std::collections::BinaryHeap;
302 /// let mut heap = BinaryHeap::new();
303 /// heap.reserve_exact(100);
304 /// assert!(heap.capacity() >= 100);
307 #[stable(feature = "rust1", since = "1.0.0")]
308 pub fn reserve_exact(&mut self, additional: usize) {
309 self.data.reserve_exact(additional);
312 /// Reserves capacity for at least `additional` more elements to be inserted in the
313 /// `BinaryHeap`. The collection may reserve more space to avoid frequent reallocations.
317 /// Panics if the new capacity overflows `usize`.
322 /// use std::collections::BinaryHeap;
323 /// let mut heap = BinaryHeap::new();
324 /// heap.reserve(100);
325 /// assert!(heap.capacity() >= 100);
328 #[stable(feature = "rust1", since = "1.0.0")]
329 pub fn reserve(&mut self, additional: usize) {
330 self.data.reserve(additional);
333 /// Discards as much additional capacity as possible.
334 #[stable(feature = "rust1", since = "1.0.0")]
335 pub fn shrink_to_fit(&mut self) {
336 self.data.shrink_to_fit();
339 /// Removes the greatest item from the binary heap and returns it, or `None` if it
345 /// use std::collections::BinaryHeap;
346 /// let mut heap = BinaryHeap::from(vec![1, 3]);
348 /// assert_eq!(heap.pop(), Some(3));
349 /// assert_eq!(heap.pop(), Some(1));
350 /// assert_eq!(heap.pop(), None);
352 #[stable(feature = "rust1", since = "1.0.0")]
353 pub fn pop(&mut self) -> Option<T> {
354 self.data.pop().map(|mut item| {
355 if !self.is_empty() {
356 swap(&mut item, &mut self.data[0]);
357 self.sift_down_to_bottom(0);
363 /// Pushes an item onto the binary heap.
368 /// use std::collections::BinaryHeap;
369 /// let mut heap = BinaryHeap::new();
374 /// assert_eq!(heap.len(), 3);
375 /// assert_eq!(heap.peek(), Some(&5));
377 #[stable(feature = "rust1", since = "1.0.0")]
378 pub fn push(&mut self, item: T) {
379 let old_len = self.len();
380 self.data.push(item);
381 self.sift_up(0, old_len);
384 /// Pushes an item onto the binary heap, then pops the greatest item off the queue in
385 /// an optimized fashion.
390 /// #![feature(binary_heap_extras)]
392 /// use std::collections::BinaryHeap;
393 /// let mut heap = BinaryHeap::new();
397 /// assert_eq!(heap.push_pop(3), 5);
398 /// assert_eq!(heap.push_pop(9), 9);
399 /// assert_eq!(heap.len(), 2);
400 /// assert_eq!(heap.peek(), Some(&3));
402 #[unstable(feature = "binary_heap_extras",
403 reason = "needs to be audited",
405 pub fn push_pop(&mut self, mut item: T) -> T {
406 match self.data.get_mut(0) {
410 swap(&mut item, top);
421 /// Pops the greatest item off the binary heap, then pushes an item onto the queue in
422 /// an optimized fashion. The push is done regardless of whether the binary heap
428 /// #![feature(binary_heap_extras)]
430 /// use std::collections::BinaryHeap;
431 /// let mut heap = BinaryHeap::new();
433 /// assert_eq!(heap.replace(1), None);
434 /// assert_eq!(heap.replace(3), Some(1));
435 /// assert_eq!(heap.len(), 1);
436 /// assert_eq!(heap.peek(), Some(&3));
438 #[unstable(feature = "binary_heap_extras",
439 reason = "needs to be audited",
441 pub fn replace(&mut self, mut item: T) -> Option<T> {
442 if !self.is_empty() {
443 swap(&mut item, &mut self.data[0]);
452 /// Consumes the `BinaryHeap` and returns the underlying vector
453 /// in arbitrary order.
458 /// use std::collections::BinaryHeap;
459 /// let heap = BinaryHeap::from(vec![1, 2, 3, 4, 5, 6, 7]);
460 /// let vec = heap.into_vec();
462 /// // Will print in some order
464 /// println!("{}", x);
467 #[stable(feature = "binary_heap_extras_15", since = "1.5.0")]
468 pub fn into_vec(self) -> Vec<T> {
472 /// Consumes the `BinaryHeap` and returns a vector in sorted
473 /// (ascending) order.
478 /// use std::collections::BinaryHeap;
480 /// let mut heap = BinaryHeap::from(vec![1, 2, 4, 5, 7]);
484 /// let vec = heap.into_sorted_vec();
485 /// assert_eq!(vec, [1, 2, 3, 4, 5, 6, 7]);
487 #[stable(feature = "binary_heap_extras_15", since = "1.5.0")]
488 pub fn into_sorted_vec(mut self) -> Vec<T> {
489 let mut end = self.len();
492 self.data.swap(0, end);
493 self.sift_down_range(0, end);
498 // The implementations of sift_up and sift_down use unsafe blocks in
499 // order to move an element out of the vector (leaving behind a
500 // hole), shift along the others and move the removed element back into the
501 // vector at the final location of the hole.
502 // The `Hole` type is used to represent this, and make sure
503 // the hole is filled back at the end of its scope, even on panic.
504 // Using a hole reduces the constant factor compared to using swaps,
505 // which involves twice as many moves.
506 fn sift_up(&mut self, start: usize, pos: usize) {
508 // Take out the value at `pos` and create a hole.
509 let mut hole = Hole::new(&mut self.data, pos);
511 while hole.pos() > start {
512 let parent = (hole.pos() - 1) / 2;
513 if hole.element() <= hole.get(parent) {
516 hole.move_to(parent);
521 /// Take an element at `pos` and move it down the heap,
522 /// while its children are larger.
523 fn sift_down_range(&mut self, pos: usize, end: usize) {
525 let mut hole = Hole::new(&mut self.data, pos);
526 let mut child = 2 * pos + 1;
528 let right = child + 1;
529 // compare with the greater of the two children
530 if right < end && !(hole.get(child) > hole.get(right)) {
533 // if we are already in order, stop.
534 if hole.element() >= hole.get(child) {
538 child = 2 * hole.pos() + 1;
543 fn sift_down(&mut self, pos: usize) {
544 let len = self.len();
545 self.sift_down_range(pos, len);
548 /// Take an element at `pos` and move it all the way down the heap,
549 /// then sift it up to its position.
551 /// Note: This is faster when the element is known to be large / should
552 /// be closer to the bottom.
553 fn sift_down_to_bottom(&mut self, mut pos: usize) {
554 let end = self.len();
557 let mut hole = Hole::new(&mut self.data, pos);
558 let mut child = 2 * pos + 1;
560 let right = child + 1;
561 // compare with the greater of the two children
562 if right < end && !(hole.get(child) > hole.get(right)) {
566 child = 2 * hole.pos() + 1;
570 self.sift_up(start, pos);
573 /// Returns the length of the binary heap.
574 #[stable(feature = "rust1", since = "1.0.0")]
575 pub fn len(&self) -> usize {
579 /// Checks if the binary heap is empty.
580 #[stable(feature = "rust1", since = "1.0.0")]
581 pub fn is_empty(&self) -> bool {
585 /// Clears the binary heap, returning an iterator over the removed elements.
587 /// The elements are removed in arbitrary order.
589 #[stable(feature = "drain", since = "1.6.0")]
590 pub fn drain(&mut self) -> Drain<T> {
591 Drain { iter: self.data.drain(..) }
594 /// Drops all items from the binary heap.
595 #[stable(feature = "rust1", since = "1.0.0")]
596 pub fn clear(&mut self) {
601 /// Hole represents a hole in a slice i.e. an index without valid value
602 /// (because it was moved from or duplicated).
603 /// In drop, `Hole` will restore the slice by filling the hole
604 /// position with the value that was originally removed.
605 struct Hole<'a, T: 'a> {
607 /// `elt` is always `Some` from new until drop.
612 impl<'a, T> Hole<'a, T> {
613 /// Create a new Hole at index `pos`.
614 fn new(data: &'a mut [T], pos: usize) -> Self {
616 let elt = ptr::read(&data[pos]);
626 fn pos(&self) -> usize {
630 /// Return a reference to the element removed
632 fn element(&self) -> &T {
633 self.elt.as_ref().unwrap()
636 /// Return a reference to the element at `index`.
638 /// Panics if the index is out of bounds.
640 /// Unsafe because index must not equal pos.
642 unsafe fn get(&self, index: usize) -> &T {
643 debug_assert!(index != self.pos);
647 /// Move hole to new location
649 /// Unsafe because index must not equal pos.
651 unsafe fn move_to(&mut self, index: usize) {
652 debug_assert!(index != self.pos);
653 let index_ptr: *const _ = &self.data[index];
654 let hole_ptr = &mut self.data[self.pos];
655 ptr::copy_nonoverlapping(index_ptr, hole_ptr, 1);
660 impl<'a, T> Drop for Hole<'a, T> {
662 // fill the hole again
665 ptr::write(&mut self.data[pos], self.elt.take().unwrap());
670 /// `BinaryHeap` iterator.
671 #[stable(feature = "rust1", since = "1.0.0")]
672 pub struct Iter<'a, T: 'a> {
673 iter: slice::Iter<'a, T>,
676 // FIXME(#19839) Remove in favor of `#[derive(Clone)]`
677 #[stable(feature = "rust1", since = "1.0.0")]
678 impl<'a, T> Clone for Iter<'a, T> {
679 fn clone(&self) -> Iter<'a, T> {
680 Iter { iter: self.iter.clone() }
684 #[stable(feature = "rust1", since = "1.0.0")]
685 impl<'a, T> Iterator for Iter<'a, T> {
689 fn next(&mut self) -> Option<&'a T> {
694 fn size_hint(&self) -> (usize, Option<usize>) {
695 self.iter.size_hint()
699 #[stable(feature = "rust1", since = "1.0.0")]
700 impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
702 fn next_back(&mut self) -> Option<&'a T> {
703 self.iter.next_back()
707 #[stable(feature = "rust1", since = "1.0.0")]
708 impl<'a, T> ExactSizeIterator for Iter<'a, T> {}
710 /// An iterator that moves out of a `BinaryHeap`.
711 #[stable(feature = "rust1", since = "1.0.0")]
712 pub struct IntoIter<T> {
713 iter: vec::IntoIter<T>,
716 #[stable(feature = "rust1", since = "1.0.0")]
717 impl<T> Iterator for IntoIter<T> {
721 fn next(&mut self) -> Option<T> {
726 fn size_hint(&self) -> (usize, Option<usize>) {
727 self.iter.size_hint()
731 #[stable(feature = "rust1", since = "1.0.0")]
732 impl<T> DoubleEndedIterator for IntoIter<T> {
734 fn next_back(&mut self) -> Option<T> {
735 self.iter.next_back()
739 #[stable(feature = "rust1", since = "1.0.0")]
740 impl<T> ExactSizeIterator for IntoIter<T> {}
742 /// An iterator that drains a `BinaryHeap`.
743 #[stable(feature = "drain", since = "1.6.0")]
744 pub struct Drain<'a, T: 'a> {
745 iter: vec::Drain<'a, T>,
748 #[stable(feature = "rust1", since = "1.0.0")]
749 impl<'a, T: 'a> Iterator for Drain<'a, T> {
753 fn next(&mut self) -> Option<T> {
758 fn size_hint(&self) -> (usize, Option<usize>) {
759 self.iter.size_hint()
763 #[stable(feature = "rust1", since = "1.0.0")]
764 impl<'a, T: 'a> DoubleEndedIterator for Drain<'a, T> {
766 fn next_back(&mut self) -> Option<T> {
767 self.iter.next_back()
771 #[stable(feature = "rust1", since = "1.0.0")]
772 impl<'a, T: 'a> ExactSizeIterator for Drain<'a, T> {}
774 #[stable(feature = "rust1", since = "1.0.0")]
775 impl<T: Ord> From<Vec<T>> for BinaryHeap<T> {
776 fn from(vec: Vec<T>) -> BinaryHeap<T> {
777 let mut heap = BinaryHeap { data: vec };
778 let mut n = heap.len() / 2;
787 #[stable(feature = "rust1", since = "1.0.0")]
788 impl<T> From<BinaryHeap<T>> for Vec<T> {
789 fn from(heap: BinaryHeap<T>) -> Vec<T> {
794 #[stable(feature = "rust1", since = "1.0.0")]
795 impl<T: Ord> FromIterator<T> for BinaryHeap<T> {
796 fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> BinaryHeap<T> {
797 BinaryHeap::from(iter.into_iter().collect::<Vec<_>>())
801 #[stable(feature = "rust1", since = "1.0.0")]
802 impl<T: Ord> IntoIterator for BinaryHeap<T> {
804 type IntoIter = IntoIter<T>;
806 /// Creates a consuming iterator, that is, one that moves each value out of
807 /// the binary heap in arbitrary order. The binary heap cannot be used
808 /// after calling this.
813 /// use std::collections::BinaryHeap;
814 /// let heap = BinaryHeap::from(vec![1, 2, 3, 4]);
816 /// // Print 1, 2, 3, 4 in arbitrary order
817 /// for x in heap.into_iter() {
818 /// // x has type i32, not &i32
819 /// println!("{}", x);
822 fn into_iter(self) -> IntoIter<T> {
823 IntoIter { iter: self.data.into_iter() }
827 #[stable(feature = "rust1", since = "1.0.0")]
828 impl<'a, T> IntoIterator for &'a BinaryHeap<T> where T: Ord {
830 type IntoIter = Iter<'a, T>;
832 fn into_iter(self) -> Iter<'a, T> {
837 #[stable(feature = "rust1", since = "1.0.0")]
838 impl<T: Ord> Extend<T> for BinaryHeap<T> {
839 fn extend<I: IntoIterator<Item = T>>(&mut self, iterable: I) {
840 let iter = iterable.into_iter();
841 let (lower, _) = iter.size_hint();
851 #[stable(feature = "extend_ref", since = "1.2.0")]
852 impl<'a, T: 'a + Ord + Copy> Extend<&'a T> for BinaryHeap<T> {
853 fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
854 self.extend(iter.into_iter().cloned());