Adding skiplist

pydatastructs/linear_data_structures/__init__.py

@@ -17,7 +17,8 @@
     SinglyLinkedList,
     DoublyLinkedList,
     SinglyCircularLinkedList,
-    DoublyCircularLinkedList
+    DoublyCircularLinkedList,
+    SkipList
 )
 __all__.extend(linked_lists.__all__)
 

pydatastructs/linear_data_structures/linked_lists.py

@@ -1,10 +1,12 @@
-from pydatastructs.utils.misc_util import _check_type, LinkedListNode
+from pydatastructs.utils.misc_util import _check_type, LinkedListNode, SkipNode
+import math, random
 
 __all__ = [
     'SinglyLinkedList',
     'DoublyLinkedList',
     'SinglyCircularLinkedList',
-    'DoublyCircularLinkedList'
+    'DoublyCircularLinkedList',
+    'SkipList'
 ]
 
 class LinkedList(object):
@@ -219,12 +221,12 @@ class DoublyLinkedList(LinkedList):
     >>> dll.append(5)
     >>> dll.appendleft(2)
     >>> str(dll)
-    "['2', '6', '5']"
+    "['(2, None)', '(6, None)', '(5, None)']"
     >>> dll[0].key = 7.2
     >>> dll.extract(1).key
     6
     >>> str(dll)
-    "['7.2', '5']"
+    "['(7.2, None)', '(5, None)']"
 
     References
     ==========
@@ -290,6 +292,11 @@ def insert_at(self, index, key, data=None):
         if self.size == 1:
             self.head, self.tail = \
                 new_node, new_node
+        elif index == self.size - 1:
+            new_node.prev = self.tail
+            new_node.next = self.tail.next
+            self.tail.next = new_node
+            self.tail = new_node
         else:
             counter = 0
             current_node = self.head
@@ -354,12 +361,12 @@ class SinglyLinkedList(LinkedList):
     >>> sll.append(5)
     >>> sll.appendleft(2)
     >>> str(sll)
-    "['2', '6', '5']"
+    "['(2, None)', '(6, None)', '(5, None)']"
     >>> sll[0].key = 7.2
     >>> sll.extract(1).key
     6
     >>> str(sll)
-    "['7.2', '5']"
+    "['(7.2, None)', '(5, None)']"
 
     References
     ==========
@@ -409,6 +416,10 @@ def insert_at(self, index, key, data=None):
         if self.size == 1:
             self.head, self.tail = \
                 new_node, new_node
+        elif index == self.size - 1:
+            new_node.next = self.tail.next
+            self.tail.next = new_node
+            self.tail = new_node
         else:
             counter = 0
             current_node = self.head
@@ -469,12 +480,12 @@ class SinglyCircularLinkedList(SinglyLinkedList):
     >>> scll.append(5)
     >>> scll.appendleft(2)
     >>> str(scll)
-    "['2', '6', '5']"
+    "['(2, None)', '(6, None)', '(5, None)']"
     >>> scll[0].key = 7.2
     >>> scll.extract(1).key
     6
     >>> str(scll)
-    "['7.2', '5']"
+    "['(7.2, None)', '(5, None)']"
 
     References
     ==========
@@ -528,12 +539,12 @@ class DoublyCircularLinkedList(DoublyLinkedList):
     >>> dcll.append(5)
     >>> dcll.appendleft(2)
     >>> str(dcll)
-    "['2', '6', '5']"
+    "['(2, None)', '(6, None)', '(5, None)']"
     >>> dcll[0].key = 7.2
     >>> dcll.extract(1).key
     6
     >>> str(dcll)
-    "['7.2', '5']"
+    "['(7.2, None)', '(5, None)']"
 
     References
     ==========
@@ -581,3 +592,181 @@ def extract(self, index):
         elif index == 0:
             self.tail.next = self.head
         return node
+
+class SkipList(object):
+    """
+    Represents Skip List
+
+    Examples
+    ========
+
+    >>> from pydatastructs import SkipList
+    >>> sl = SkipList()
+    >>> sl.insert(6)
+    >>> sl.insert(1)
+    >>> sl.insert(3)
+    >>> node = sl.extract(1)
+    >>> str(node)
+    '(1, None)'
+    >>> sl.insert(4)
+    >>> sl.insert(2)
+    >>> sl.search(4)
+    True
+    >>> sl.search(10)
+    False
+
+    """
+
+    __slots__ = ['head', 'tail', '_levels', '_num_nodes', 'seed']
+
+    def __new__(cls):
+        obj = object.__new__(cls)
+        obj.head, obj.tail = None, None
+        obj._num_nodes = 0
+        obj._levels = 0
+        obj._add_level()
+        return obj
+
+    @classmethod
+    def methods(cls):
+        return ['__new__', 'levels', 'search',
+                'extract', '__str__', 'size']
+
+    def _add_level(self):
+        self.tail = SkipNode(math.inf, next=None, down=self.tail)
+        self.head = SkipNode(-math.inf, next=self.tail, down=self.head)
+        self._levels += 1
+
+    @property
+    def levels(self):
+        """
+        Returns the number of levels in the
+        current skip list.
+        """
+        return self._levels
+
+    def _search(self, key) -> list:
+        path = []
+        node = self.head
+        while node:
+            if node.next.key >= key:
+                path.append(node)
+                node = node.down
+            else:
+                node = node.next
+        return path
+
+    def search(self, key) -> bool:
+        return self._search(key)[-1].next.key == key
+
+    def insert(self, key, data=None):
+        """
+        Inserts a new node to the skip list.
+
+        Parameters
+        ==========
+
+        key
+            Any valid identifier to uniquely
+            identify the node in the linked list.
+
+        data
+            Any valid data to be stored in the node.
+        """
+        path = self._search(key)
+        tip = path[-1]
+        below = SkipNode(key=key, data=data, next=tip.next)
+        tip.next = below
+        total_level = self._levels
+        level = 1
+        while random.getrandbits(1) % 2 == 0 and level <= total_level:
+            if level == total_level:
+                self._add_level()
+                prev = self.head
+            else:
+                prev = path[total_level - 1 - level]
+            below = SkipNode(key=key, data=None, next=prev.next, down=below)
+            prev.next = below
+            level += 1
+        self._num_nodes += 1
+
+    @property
+    def size(self):
+        return self._num_nodes
+
+    def extract(self, key):
+        """
+        Extracts the node with the given key in the skip list.
+
+        Parameters
+        ==========
+
+        key
+            The key of the node under consideration.
+
+        Returns
+        =======
+
+        return_node: SkipNode
+            The node with given key.
+        """
+        path = self._search(key)
+        tip = path[-1]
+        if tip.next.key != key:
+            raise KeyError('Node with key %s is not there in %s'%(key, self))
+        return_node = SkipNode(tip.next.key, tip.next.data)
+        total_level = self._levels
+        level = total_level - 1
+        while level >= 0 and path[level].next.key == key:
+            path[level].next = path[level].next.next
+            level -= 1
+        walk = self.head
+        while walk is not None:
+            if walk.next is self.tail:
+                self._levels -= 1
+                self.head = walk.down
+                self.tail = self.tail.down
+                walk = walk.down
+            else:
+                break
+        self._num_nodes -= 1
+        if self._levels == 0:
+            self._add_level()
+        return return_node
+
+    def __str__(self):
+        node2row = {}
+        node2col = {}
+        walk = self.head
+        curr_level = self._levels - 1
+        while walk is not None:
+            curr_node = walk
+            col = 0
+            while curr_node is not None:
+                if curr_node.key != math.inf and curr_node.key != -math.inf:
+                    node2row[curr_node] = curr_level
+                    if walk.down is None:
+                        node2col[curr_node.key] = col
+                    col += 1
+                curr_node = curr_node.next
+            walk = walk.down
+            curr_level -= 1
+        print(self._num_nodes, self._levels)
+        sl_mat = [[str(None) for _ in range(self._num_nodes)] for _ in range(self._levels)]
+        walk = self.head
+        while walk is not None:
+            curr_node = walk
+            while curr_node is not None:
+                if curr_node in node2row:
+                    row = node2row[curr_node]
+                    col = node2col[curr_node.key]
+                    sl_mat[row][col] = str(curr_node)
+                curr_node = curr_node.next
+            walk = walk.down
+        sl_str = ""
+        for level_list in sl_mat[::-1]:
+            for node_str in level_list:
+                sl_str += node_str + " "
+            if len(sl_str) > 0:
+                sl_str += "\n"
+        return sl_str

pydatastructs/linear_data_structures/tests/test_linked_lists.py

@@ -1,4 +1,4 @@
-from pydatastructs.linear_data_structures import DoublyLinkedList, SinglyLinkedList, SinglyCircularLinkedList, DoublyCircularLinkedList
+from pydatastructs.linear_data_structures import DoublyLinkedList, SinglyLinkedList, SinglyCircularLinkedList, DoublyCircularLinkedList, SkipList
 from pydatastructs.utils.raises_util import raises
 import copy, random
 
@@ -22,7 +22,9 @@ def test_DoublyLinkedList():
     assert dll.search(3) == dll[-2]
     assert dll.search(-1) is None
     dll[-2].key = 0
-    assert str(dll) == "['7', '5', '1', '6', '1.1', '0', '9']"
+    assert str(dll) == ("['(7, None)', '(5, None)', '(1, None)', "
+                        "'(6, None)', '(1.1, None)', '(0, None)', "
+                        "'(9, None)']")
     assert len(dll) == 7
     assert raises(IndexError, lambda: dll.insert_at(8, None))
     assert raises(IndexError, lambda: dll.extract(20))
@@ -57,7 +59,8 @@ def test_SinglyLinkedList():
     assert sll.popleft().key == 2
     assert sll.popright().key == 6
     sll[-2].key = 0
-    assert str(sll) == "['2', '4', '1', '0', '9']"
+    assert str(sll) == ("['(2, None)', '(4, None)', '(1, None)', "
+                        "'(0, None)', '(9, None)']")
     assert len(sll) == 5
     assert raises(IndexError, lambda: sll.insert_at(6, None))
     assert raises(IndexError, lambda: sll.extract(20))
@@ -93,7 +96,8 @@ def test_SinglyCircularLinkedList():
     assert scll.popright().key == 6
     assert scll.search(-1) is None
     scll[-2].key = 0
-    assert str(scll) == "['2', '4', '1', '0', '9']"
+    assert str(scll) == ("['(2, None)', '(4, None)', '(1, None)', "
+                         "'(0, None)', '(9, None)']")
     assert len(scll) == 5
     assert raises(IndexError, lambda: scll.insert_at(6, None))
     assert raises(IndexError, lambda: scll.extract(20))
@@ -130,7 +134,9 @@ def test_DoublyCircularLinkedList():
     assert dcll.popleft().key == 2
     assert dcll.popright().key == 4
     dcll[-2].key = 0
-    assert str(dcll) == "['7', '5', '1', '6', '1', '0', '9']"
+    assert str(dcll) == ("['(7, None)', '(5, None)', '(1, None)', "
+                         "'(6, None)', '(1, None)', '(0, None)', "
+                         "'(9, None)']")
     assert len(dcll) == 7
     assert raises(IndexError, lambda: dcll.insert_at(8, None))
     assert raises(IndexError, lambda: dcll.extract(20))
@@ -148,3 +154,40 @@ def test_DoublyCircularLinkedList():
         dcll_copy.extract(index)
     assert str(dcll_copy) == "[]"
     assert raises(ValueError, lambda: dcll_copy.extract(1))
+
+def test_SkipList():
+    random.seed(0)
+    sl = SkipList()
+    sl.insert(2)
+    sl.insert(10)
+    sl.insert(92)
+    sl.insert(1)
+    sl.insert(4)
+    sl.insert(27)
+    sl.extract(10)
+    assert str(sl) == ("(1, None) None None None None \n"
+                       "(1, None) None None None None \n"
+                       "(1, None) (2, None) (4, None) (27, None) (92, None) \n")
+    assert raises(KeyError, lambda: sl.extract(15))
+    assert sl.search(1) is True
+    assert sl.search(47) is False
+
+    sl = SkipList()
+
+    for a in range(0, 20, 2):
+        sl.insert(a)
+    assert sl.search(16) is True
+    for a in range(4, 20, 4):
+        sl.extract(a)
+    assert sl.search(10) is True
+    for a in range(4, 20, 4):
+        sl.insert(a)
+    for a in range(0, 20, 2):
+        sl.extract(a)
+    assert sl.search(3) is False
+
+    li = SkipList()
+    li.insert(1)
+    li.insert(2)
+    assert li.levels == 1
+    assert li.size == 2

pydatastructs/miscellaneous_data_structures/tests/test_queue.py

@@ -54,7 +54,8 @@ def test_LinkedListQueue():
     q1 = Queue(implementation='linked_list', items = [0, 1])
     q1.append(2)
     q1.append(3)
-    assert str(q1) == "['0', '1', '2', '3']"
+    assert str(q1) == ("['(0, None)', '(1, None)', "
+                       "'(2, None)', '(3, None)']")
     assert len(q1) == 4
     assert q1.popleft().key == 0
     assert q1.popleft().key == 1
@@ -83,7 +84,7 @@ def test_LinkedListQueue():
     q1 = Queue(implementation='linked_list', items = [0, 1], double_ended=True)
     q1.appendleft(2)
     q1.append(3)
-    assert str(q1) == "['2', '0', '1', '3']"
+    assert str(q1) == "['(2, None)', '(0, None)', '(1, None)', '(3, None)']"
     assert len(q1) == 4
     assert q1.popleft().key == 2
     assert q1.pop().key == 3