<reponame>nurseiit/comm-unist<gh_stars>1-10
# Copyright 2013, <NAME>
#
# Developed for use with the book:
#
#    Data Structures and Algorithms in Python
#    <NAME>, <NAME>, and <NAME>
#    <NAME> & Sons, 2013
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

from m04_trees.p0.priority_queue_base import PriorityQueueBase
from m04_trees.p0.exceptions import Empty


class HeapPriorityQueue(PriorityQueueBase):  # base class defines _Item
    """A min-oriented priority queue implemented with a binary heap."""

    # ------------------------------ nonpublic behaviors ------------------------------
    def _parent(self, j):
        return (j - 1) // 2

    def _left(self, j):
        return 2 * j + 1

    def _right(self, j):
        return 2 * j + 2

    def _has_left(self, j):
        return self._left(j) < len(self._data)  # index beyond end of list?

    def _has_right(self, j):
        return self._right(j) < len(self._data)  # index beyond end of list?

    def _swap(self, i, j):
        """Swap the elements at indices i and j of array."""
        self._data[i], self._data[j] = self._data[j], self._data[i]

    def _upheap(self, j):
        parent = self._parent(j)
        if j > 0 and self._data[j] < self._data[parent]:
            self._swap(j, parent)
            self._upheap(parent)  # recur at position of parent

    def _downheap(self, j):
        if self._has_left(j):
            left = self._left(j)
            small_child = left  # although right may be smaller
            if self._has_right(j):
                right = self._right(j)
                if self._data[right] < self._data[left]:
                    small_child = right
            if self._data[small_child] < self._data[j]:
                self._swap(j, small_child)
                self._downheap(small_child)  # recur at position of small child

    # ------------------------------ public behaviors ------------------------------
    def __init__(self):
        """Create a new empty Priority Queue."""
        self._data = []

    def __len__(self):
        """Return the number of items in the priority queue."""
        return len(self._data)

    def add(self, key, value):
        """Add a key-value pair to the priority queue."""
        self._data.append(self._Item(key, value))
        self._upheap(len(self._data) - 1)  # upheap newly added position

    def min(self):
        """Return but do not remove (k,v) tuple with minimum key.

    Raise Empty exception if empty.
    """
        if self.is_empty():
            raise Empty('Priority queue is empty.')
        item = self._data[0]
        return (item._key, item._value)

    def remove_min(self):
        """Remove and return (k,v) tuple with minimum key.

    Raise Empty exception if empty.
    """
        if self.is_empty():
            raise Empty('Priority queue is empty.')
        self._swap(0, len(self._data) - 1)  # put minimum item at the end
        item = self._data.pop()  # and remove it from the list;
        self._downheap(0)  # then fix new root
        return (item._key, item._value)
