London | 26-SDC-Mar | Ebrahim Beiati-Asl | Sprint 2 | implement_skip_list

Sprint-2/implement_skip_list/skip_list.py

@@ -0,0 +1,128 @@
+import math
+
+
+class Node:
+    def __init__(self, value, next=None):
+        self.value = value
+        self.next = next
+
+
+class SkipList:
+    def __init__(self):
+        self.head = None
+        self.length = 0
+        self.skips = []  # list of skip pointers, each is a tuple (index, value)
+
+    def _get_node(self, index):
+        """Get the node at the given index."""
+        current = self.head
+        for _ in range(index):
+            if current is None:
+                return None
+            current = current.next
+        return current
+
+    def rebuild_skips(self):
+        """Rebuild the skip pointer so we can jump over sqrt(n) elements."""
+        n = self.length
+        if n == 0:
+            self.skips = []
+            return
+
+        step = int(math.sqrt(n)) or 1
+        self.skips = list(range(0, n, step))
+
+    def _find_position(self, value):
+        """Find the position to insert value using skip pointers."""
+        if self.length == 0:
+            return 0
+
+        # Use skip pointers to find the range where value should be
+        for i in range(len(self.skips) - 1):
+            a = self.skips[i]
+            b = self.skips[i + 1]
+
+            node_a = self._get_node(a)
+            node_b = self._get_node(b)
+
+            if node_a.value <= value < node_b.value:
+                # linear search between a and b
+                idx = a
+                current = node_a
+                while idx < b and current:
+                    if current.value >= value:
+                        return idx
+                    current = current.next
+                    idx += 1
+                return b
+        # Check the last skip pointer
+        start = self.skips[-1]
+        idx = start
+        current = self._get_node(start)
+        while current:
+            if current.value >= value:
+                return idx
+            current = current.next
+            idx += 1
+        return self.length
+
+    def insert(self, value):
+        """Insert value into the skip list, maintaining sorted order."""
+        pos = self._find_position(value)
+
+        if pos < self.length:
+            node_at_pos = self._get_node(pos)
+            if node_at_pos and node_at_pos.value == value:
+                return  # value already exists, do not insert duplicates
+        # Insert the new node
+        new_node = Node(value)
+        if pos == 0:
+            new_node.next = self.head
+            self.head = new_node
+        else:
+            prev_node = self._get_node(pos - 1)
+            new_node.next = prev_node.next
+            prev_node.next = new_node
+        self.length += 1
+        self.rebuild_skips()
+
+    def __contains__(self, value):
+        if self.length == 0:
+            return False
+
+        for i in range(len(self.skips) - 1):
+            a = self.skips[i]
+            b = self.skips[i + 1]
+
+            node_a = self._get_node(a)
+            node_b = self._get_node(b)
+
+            if node_a.value <= value < node_b.value:
+                idx = a
+                current = node_a
+                while idx < b and current:
+                    if current.value == value:
+                        return True
+                    current = current.next
+                    idx += 1
+                return False
+        start = self.skips[-1]
+        idx = start
+        current = self._get_node(start)
+
+        while current:
+            if current.value == value:
+                return True
+            current = current.next
+            idx += 1
+
+        return False
+
+    def to_list(self):
+        """Return the skip list as a regular sorted list."""
+        result = []
+        current = self.head
+        while current:
+            result.append(current.value)
+            current = current.next
+        return result