Added more implementations of priority queue (#205)

Minor changes have been made to interface of BinomialHeap and DHeap along with adding more implementations.

pydatastructs/miscellaneous_data_structures/queue.py

@@ -1,5 +1,6 @@
 from pydatastructs.linear_data_structures import DynamicOneDimensionalArray, SinglyLinkedList
 from pydatastructs.utils.misc_util import NoneType, LinkedListNode, _check_type
+from pydatastructs.trees.heaps import BinaryHeap, BinomialHeap
 from copy import deepcopy as dc
 
 __all__ = [
@@ -186,13 +187,17 @@ class PriorityQueue(object):
         of priority queue.
         The following implementations are supported,
         'linked_list' -> Linked list implementation.
-        Optional, by default, 'linked_list' implementation
+        'binary_heap' -> Binary heap implementation.
+        'binomial_heap' -> Binomial heap implementation.
+            Doesn't support custom comparators, minimum
+            key data is extracted in every pop.
+        Optional, by default, 'binary_heap' implementation
         is used.
     comp: function
         The comparator to be used while comparing priorities.
         Must return a bool object.
-        By default, `lambda u, v: u > v` is used to compare
-        priorities i.e., maximum priority elements are extracted
+        By default, `lambda u, v: u < v` is used to compare
+        priorities i.e., minimum priority elements are extracted
         by pop operation.
 
     Examples
@@ -203,43 +208,65 @@ class PriorityQueue(object):
     >>> pq.push(1, 2)
     >>> pq.push(2, 3)
     >>> pq.pop()
-    2
-    >>> pq2 = PriorityQueue(comp=lambda u, v: u < v)
+    1
+    >>> pq2 = PriorityQueue(comp=lambda u, v: u > v)
     >>> pq2.push(1, 2)
     >>> pq2.push(2, 3)
     >>> pq2.pop()
-    1
+    2
 
     References
     ==========
 
-    .. [1] https://en.wikipedia.org/wiki/Priority_queue#Naive_implementations
+    .. [1] https://en.wikipedia.org/wiki/Priority_queue
     """
 
-    def __new__(cls, implementation='linked_list', **kwargs):
+    def __new__(cls, implementation='binary_heap', **kwargs):
+        comp = kwargs.get("comp", lambda u, v: u < v)
         if implementation == 'linked_list':
-            return LinkedListPriorityQueue(
-                kwargs.get("comp", lambda u, v: u > v)
-            )
+            return LinkedListPriorityQueue(comp)
+        elif implementation == 'binary_heap':
+            return BinaryHeapPriorityQueue(comp)
+        elif implementation == 'binomial_heap':
+            return BinomialHeapPriorityQueue()
+        else:
+            raise NotImplementedError(
+                "%s implementation is not currently supported "
+                "by priority queue.")
 
     def push(self, value, priority):
+        """
+        Pushes the value to the priority queue
+        according to the given priority.
+
+        value
+            Value to be pushed.
+        priority
+            Priority to be given to the value.
+        """
         raise NotImplementedError(
                 "This is an abstract method.")
 
     def pop(self):
+        """
+        Pops out the value from the priority queue.
+        """
         raise NotImplementedError(
             "This is an abstract method.")
 
     @property
     def is_empty(self):
+        """
+        Checks if the priority queue is empty.
+        """
         raise NotImplementedError(
             "This is an abstract method.")
 
 class LinkedListPriorityQueue(PriorityQueue):
 
     __slots__ = ['items', 'comp']
 
-    def __new__(cls, comp=lambda u, v: u > v):
+    def __new__(cls, comp):
         obj = object.__new__(cls)
         obj.items = SinglyLinkedList()
         obj.comp = comp
@@ -266,3 +293,45 @@ def pop(self):
     @property
     def is_empty(self):
         return self.items.size == 0
+
+class BinaryHeapPriorityQueue(PriorityQueue):
+
+    __slots__ = ['items']
+
+    def __new__(cls, comp):
+        obj = object.__new__(cls)
+        obj.items = BinaryHeap()
+        obj.items._comp = comp
+        return obj
+
+    def push(self, value, priority):
+        self.items.insert(priority, value)
+
+    def pop(self):
+        node = self.items.extract()
+        return node.data
+
+    @property
+    def is_empty(self):
+        return self.items.is_empty
+
+class BinomialHeapPriorityQueue(PriorityQueue):
+
+    __slots__ = ['items']
+
+    def __new__(cls):
+        obj = object.__new__(cls)
+        obj.items = BinomialHeap()
+        return obj
+
+    def push(self, value, priority):
+        self.items.insert(priority, value)
+
+    def pop(self):
+        node = self.items.find_minimum()
+        self.items.delete_minimum()
+        return node.data
+
+    @property
+    def is_empty(self):
+        return self.items.is_empty

pydatastructs/miscellaneous_data_structures/tests/test_queue.py

@@ -65,12 +65,15 @@ def test_PriorityQueue():
     assert _check_type(pq1, LinkedListPriorityQueue) is True
     assert raises(NotImplementedError, lambda: Queue(implementation=''))
 
-def test_LinkedListPriorityQueue():
-    pq1 = PriorityQueue(implementation='linked_list')
-    pq1.push(1, 2)
-    pq1.push(2, 3)
-    pq1.push(3, 4)
-    assert pq1.pop() == 3
-    assert pq1.pop() == 2
-    assert pq1.pop() == 1
-    assert raises(IndexError, lambda: pq1.pop())
+def test_ImplementationPriorityQueue():
+    impls = ['linked_list', 'binomial_heap', 'binary_heap']
+    for impl in impls:
+        pq1 = PriorityQueue(implementation=impl)
+        pq1.push(1, 2)
+        pq1.push(2, 3)
+        pq1.push(3, 4)
+        assert pq1.pop() == 1
+        assert pq1.pop() == 2
+        assert pq1.pop() == 3
+        assert pq1.is_empty is True
+        assert raises(IndexError, lambda: pq1.pop())

pydatastructs/trees/heaps.py

@@ -171,7 +171,7 @@ def extract(self):
             If the heap is empty.
         """
         if self._last_pos_filled == -1:
-            return None
+            raise IndexError("Heap is empty.")
         else:
             element_to_be_extracted = TreeNode(self.heap[0].key, self.heap[0].data)
             self._swap(0, self._last_pos_filled)
@@ -194,6 +194,13 @@ def __str__(self):
             to_be_printed[i] = (node.key, node.data, children)
         return str(to_be_printed)
 
+    @property
+    def is_empty(self):
+        """
+        Checks if the heap is empty.
+        """
+        return self.heap._last_pos_filled == -1
+
 
 class BinaryHeap(DHeap):
     """
@@ -456,7 +463,7 @@ def find_minimum(self, **kwargs):
         min_node: BinomialTreeNode
         """
         if self.is_empty:
-            raise ValueError("Binomial heap is empty.")
+            raise IndexError("Binomial heap is empty.")
         min_node = None
         idx, min_idx = 0, None
         for tree in self.root_list:

pydatastructs/trees/tests/test_heaps.py

@@ -9,7 +9,7 @@ def test_BinaryHeap():
 
     max_heap = BinaryHeap(heap_property="max")
 
-    assert max_heap.extract() is None
+    assert raises(IndexError, lambda: max_heap.extract())
 
     max_heap.insert(100, 100)
     max_heap.insert(19, 19)
@@ -57,7 +57,7 @@ def test_BinaryHeap():
                 BinaryHeap(elements = non_TreeNode_elements, heap_property='min'))
 def test_TernaryHeap():
     max_heap = TernaryHeap(heap_property="max")
-    assert max_heap.extract() is None
+    assert raises(IndexError, lambda: max_heap.extract())
     max_heap.insert(100, 100)
     max_heap.insert(19, 19)
     max_heap.insert(36, 36)
@@ -97,7 +97,7 @@ def test_TernaryHeap():
 def test_DHeap():
     assert raises(ValueError, lambda: DHeap(heap_property="none", d=4))
     max_heap = DHeap(heap_property="max", d=5)
-    assert max_heap.extract() is None
+    assert raises(IndexError, lambda: max_heap.extract())
     max_heap.insert(100, 100)
     max_heap.insert(19, 19)
     max_heap.insert(36, 36)
@@ -213,7 +213,7 @@ def bfs(heap):
 
     # Testing BinomialHeap.insert
     heap = BinomialHeap()
-    assert raises(ValueError, lambda: heap.find_minimum())
+    assert raises(IndexError, lambda: heap.find_minimum())
     heap.insert(1, 1)
     heap.insert(3, 3)
     heap.insert(6, 6)