Add bidirectional search algorithm implementation

graphs/bidirectional_search.py

@@ -0,0 +1,201 @@
+"""
+Bidirectional Search Algorithm.
+
+This algorithm searches from both the source and target nodes simultaneously,
+meeting somewhere in the middle. This approach can significantly reduce the
+search space compared to a traditional one-directional search.
+
+Time Complexity: O(b^(d/2)) where b is the branching factor and d is the depth
+Space Complexity: O(b^(d/2))
+
+https://en.wikipedia.org/wiki/Bidirectional_search
+"""
+
+from collections import deque
+
+
+def expand_search(
+    graph: dict[int, list[int]],
+    queue: deque[int],
+    parents: dict[int, int | None],
+    opposite_direction_parents: dict[int, int | None],
+) -> int | None:
+    if not queue:
+        return None
+
+    current = queue.popleft()
+    for neighbor in graph[current]:
+        if neighbor in parents:
+            continue
+
+        parents[neighbor] = current
+        queue.append(neighbor)
+
+        # Check if this creates an intersection
+        if neighbor in opposite_direction_parents:
+            return neighbor
+
+    return None
+
+
+def construct_path(current: int | None, parents: dict[int, int | None]) -> list[int]:
+    path: list[int] = []
+    while current is not None:
+        path.append(current)
+        current = parents[current]
+    return path
+
+
+def bidirectional_search(
+    graph: dict[int, list[int]], start: int, goal: int
+) -> list[int] | None:
+    """
+    Perform bidirectional search on a graph to find the shortest path.
+
+    Args:
+        graph: A dictionary where keys are nodes and values are lists of adjacent nodes
+        start: The starting node
+        goal: The target node
+
+    Returns:
+        A list representing the path from start to goal, or None if no path exists
+
+    Examples:
+        >>> graph = {
+        ...     0: [1, 2],
+        ...     1: [0, 3, 4],
+        ...     2: [0, 5, 6],
+        ...     3: [1, 7],
+        ...     4: [1, 8],
+        ...     5: [2, 9],
+        ...     6: [2, 10],
+        ...     7: [3, 11],
+        ...     8: [4, 11],
+        ...     9: [5, 11],
+        ...     10: [6, 11],
+        ...     11: [7, 8, 9, 10],
+        ... }
+        >>> bidirectional_search(graph=graph, start=0, goal=11)
+        [0, 1, 3, 7, 11]
+        >>> bidirectional_search(graph=graph, start=5, goal=5)
+        [5]
+        >>> disconnected_graph = {
+        ...     0: [1, 2],
+        ...     1: [0],
+        ...     2: [0],
+        ...     3: [4],
+        ...     4: [3],
+        ... }
+        >>> bidirectional_search(graph=disconnected_graph, start=0, goal=3) is None
+        True
+    """
+    if start == goal:
+        return [start]
+
+    # Check if start and goal are in the graph
+    if start not in graph or goal not in graph:
+        return None
+
+    # Initialize forward and backward search dictionaries
+    # Each maps a node to its parent in the search
+    forward_parents: dict[int, int | None] = {start: None}
+    backward_parents: dict[int, int | None] = {goal: None}
+
+    # Initialize forward and backward search queues
+    forward_queue = deque([start])
+    backward_queue = deque([goal])
+
+    # Intersection node (where the two searches meet)
+    intersection = None
+
+    # Continue until both queues are empty or an intersection is found
+    while forward_queue and backward_queue and intersection is None:
+        # Expand forward search
+        intersection = expand_search(
+            graph=graph,
+            queue=forward_queue,
+            parents=forward_parents,
+            opposite_direction_parents=backward_parents,
+        )
+
+        # If no intersection found, expand backward search
+        if intersection is not None:
+            break
+
+        intersection = expand_search(
+            graph=graph,
+            queue=backward_queue,
+            parents=backward_parents,
+            opposite_direction_parents=forward_parents,
+        )
+
+    # If no intersection found, there's no path
+    if intersection is None:
+        return None
+
+    # Construct path from start to intersection
+    forward_path: list[int] = construct_path(
+        current=intersection, parents=forward_parents
+    )
+    forward_path.reverse()
+
+    # Construct path from intersection to goal
+    backward_path: list[int] = construct_path(
+        current=backward_parents[intersection], parents=backward_parents
+    )
+
+    # Return the complete path
+    return forward_path + backward_path
+
+
+def main() -> None:
+    """
+    Run example of bidirectional search algorithm.
+
+    Examples:
+        >>> main()  # doctest: +NORMALIZE_WHITESPACE
+        Path from 0 to 11: [0, 1, 3, 7, 11]
+        Path from 5 to 5: [5]
+        Path from 0 to 3: None
+    """
+    # Example graph represented as an adjacency list
+    example_graph = {
+        0: [1, 2],
+        1: [0, 3, 4],
+        2: [0, 5, 6],
+        3: [1, 7],
+        4: [1, 8],
+        5: [2, 9],
+        6: [2, 10],
+        7: [3, 11],
+        8: [4, 11],
+        9: [5, 11],
+        10: [6, 11],
+        11: [7, 8, 9, 10],
+    }
+
+    # Test case 1: Path exists
+    start, goal = 0, 11
+    path = bidirectional_search(graph=example_graph, start=start, goal=goal)
+    print(f"Path from {start} to {goal}: {path}")
+
+    # Test case 2: Start and goal are the same
+    start, goal = 5, 5
+    path = bidirectional_search(graph=example_graph, start=start, goal=goal)
+    print(f"Path from {start} to {goal}: {path}")
+
+    # Test case 3: No path exists (disconnected graph)
+    disconnected_graph = {
+        0: [1, 2],
+        1: [0],
+        2: [0],
+        3: [4],
+        4: [3],
+    }
+    start, goal = 0, 3
+    path = bidirectional_search(graph=disconnected_graph, start=start, goal=goal)
+    print(f"Path from {start} to {goal}: {path}")
+
+
+if __name__ == "__main__":
+    main()