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Source code

use std::mem::{swap, ManuallyDrop};
use std::ops::{Deref, DerefMut};
use std::ptr;

pub struct BinaryHeap<T> {
    data: Vec<T>,
}

pub struct PeekMut<'a, T: 'a + Ord> {
    heap: &'a mut BinaryHeap<T>,
    sift: bool,
}

impl<T: Ord> Drop for PeekMut<'_, T> {
    fn drop(&mut self) {
        if self.sift {
            self.heap.sift_down(0);
        }
    }
}

impl<T: Ord> Deref for PeekMut<'_, T> {
    type Target = T;
    fn deref(&self) -> &T {
        debug_assert!(!self.heap.is_empty());
        // SAFE: PeekMut is only instantiated for non-empty heaps
        unsafe { self.heap.data.get_unchecked(0) }
    }
}

impl<T: Ord> DerefMut for PeekMut<'_, T> {
    fn deref_mut(&mut self) -> &mut T {
        debug_assert!(!self.heap.is_empty());
        self.sift = true;
        // SAFE: PeekMut is only instantiated for non-empty heaps
        unsafe { self.heap.data.get_unchecked_mut(0) }
    }
}

impl<'a, T: Ord> PeekMut<'a, T> {
    pub fn pop(mut this: PeekMut<'a, T>) -> T {
        let value = this.heap.pop().unwrap();
        this.sift = false;
        value
    }
}

impl<T: Ord> BinaryHeap<T> {
    pub fn peek_mut(&mut self) -> Option<PeekMut<'_, T>> {
        if self.is_empty() { None } else { Some(PeekMut { heap: self, sift: false }) }
    }

pub fn pop(&mut self) -> Option<T> {
        self.data.pop().map(|mut item| {
            if !self.is_empty() {
                swap(&mut item, &mut self.data[0]);
                self.sift_down_to_bottom(0);
            }
            item
        })
    }

fn sift_up(&mut self, start: usize, pos: usize) -> usize {
        unsafe {
            // Take out the value at `pos` and create a hole.
            let mut hole = Hole::new(&mut self.data, pos);

while hole.pos() > start {
                let parent = (hole.pos() - 1) / 2;
                if hole.element() <= hole.get(parent) {
                    break;
                }
                hole.move_to(parent);
            }
            hole.pos()
        }
    }

/// Take an element at `pos` and move it down the heap,
    /// while its children are larger.
    fn sift_down_range(&mut self, pos: usize, end: usize) {
        unsafe {
            let mut hole = Hole::new(&mut self.data, pos);
            let mut child = 2 * pos + 1;
            while child < end {
                let right = child + 1;
                // compare with the greater of the two children
                if right < end && hole.get(child) <= hole.get(right) {
                    child = right;
                }
                // if we are already in order, stop.
                if hole.element() >= hole.get(child) {
                    break;
                }
                hole.move_to(child);
                child = 2 * hole.pos() + 1;
            }
        }
    }

fn sift_down(&mut self, pos: usize) {
        let len = self.len();
        self.sift_down_range(pos, len);
    }

fn sift_down_to_bottom(&mut self, mut pos: usize) {
        let end = self.len();
        let start = pos;
        unsafe {
            let mut hole = Hole::new(&mut self.data, pos);
            let mut child = 2 * pos + 1;
            while child < end {
                let right = child + 1;
                // compare with the greater of the two children
                if right < end && hole.get(child) <= hole.get(right) {
                    child = right;
                }
                hole.move_to(child);
                child = 2 * hole.pos() + 1;
            }
            pos = hole.pos;
        }
        self.sift_up(start, pos);
    }
}

impl<T> BinaryHeap<T> {
    pub fn len(&self) -> usize {
        self.data.len()
    }

pub fn is_empty(&self) -> bool {
        self.len() == 0
    }
}

/// Hole represents a hole in a slice i.e., an index without valid value
/// (because it was moved from or duplicated).
/// In drop, `Hole` will restore the slice by filling the hole
/// position with the value that was originally removed.
struct Hole<'a, T: 'a> {
    data: &'a mut [T],
    elt: ManuallyDrop<T>,
    pos: usize,
}

impl<'a, T> Hole<'a, T> {
    #[inline]
    unsafe fn new(data: &'a mut [T], pos: usize) -> Self {
        debug_assert!(pos < data.len());
        // SAFE: pos should be inside the slice
        let elt = unsafe { ptr::read(data.get_unchecked(pos)) };
        Hole { data, elt: ManuallyDrop::new(elt), pos }
    }

#[inline]
    fn pos(&self) -> usize {
        self.pos
    }

#[inline]
    fn element(&self) -> &T {
        &self.elt
    }

#[inline]
    unsafe fn get(&self, index: usize) -> &T {
        debug_assert!(index != self.pos);
        debug_assert!(index < self.data.len());
        unsafe { self.data.get_unchecked(index) }
    }

#[inline]
    unsafe fn move_to(&mut self, index: usize) {
        debug_assert!(index != self.pos);
        debug_assert!(index < self.data.len());
        unsafe {
            let index_ptr: *const _ = self.data.get_unchecked(index);
            let hole_ptr = self.data.get_unchecked_mut(self.pos);
            ptr::copy_nonoverlapping(index_ptr, hole_ptr, 1);
        }
        self.pos = index;
    }
}

impl<T> Drop for Hole<'_, T> {
    #[inline]
    fn drop(&mut self) {
        // fill the hole again
        unsafe {
            let pos = self.pos;
            ptr::copy_nonoverlapping(&*self.elt, self.data.get_unchecked_mut(pos), 1);
        }
    }
}

pub fn push_if_below_max(heap: &mut BinaryHeap<i32>, x: i32) {
    if let Some(mut max) = heap.peek_mut() {
        if x < *max {
            *max = x;
        }
    }
}