backend/app/algorithms/bfs.py

"""Breadth-First Search algorithm."""
from typing import Tuple, Optional, List, Generator, TYPE_CHECKING

if TYPE_CHECKING:
    from ..core.generic_search import GenericSearch

from ..core.node import SearchNode
from ..core.frontier import QueueFrontier
from ..models.state import PathResult, SearchStep


def bfs_search(
    problem: 'GenericSearch',
    visualize: bool = False
) -> Tuple[PathResult, Optional[List[SearchStep]]]:
    """
    Breadth-first search using FIFO queue.

    Finds path with minimum number of steps (not minimum cost).
    Complete and optimal for unweighted graphs.

    Args:
        problem: The search problem to solve
        visualize: If True, collect visualization steps

    Returns:
        Tuple of (PathResult, Optional[List[SearchStep]])
    """
    frontier = QueueFrontier()
    start = problem.initial_state()
    start_node = SearchNode(state=start, path_cost=0, depth=0)
    frontier.push(start_node)

    explored: set = set()
    nodes_expanded = 0
    steps: List[SearchStep] = [] if visualize else None

    while not frontier.is_empty():
        node = frontier.pop()

        # Record step for visualization
        if visualize:
            steps.append(SearchStep(
                step_number=nodes_expanded,
                current_node=node.state,
                action=node.action,
                frontier=frontier.get_states(),
                explored=list(explored),
                current_path=node.get_path(),
                path_cost=node.path_cost
            ))

        # Goal test after pop (standard BFS)
        if problem.goal_test(node.state):
            return PathResult(
                plan=node.get_solution(),
                cost=node.path_cost,
                nodes_expanded=nodes_expanded,
                path=node.get_path()
            ), steps

        # Skip if already explored
        if node.state in explored:
            continue

        explored.add(node.state)
        nodes_expanded += 1

        # Expand node
        for action in problem.actions(node.state):
            child_state = problem.result(node.state, action)
            if child_state not in explored and not frontier.contains_state(child_state):
                step_cost = problem.step_cost(node.state, action, child_state)
                child = SearchNode(
                    state=child_state,
                    parent=node,
                    action=action,
                    path_cost=node.path_cost + step_cost,
                    depth=node.depth + 1
                )
                frontier.push(child)

    # No solution found
    return PathResult(
        plan="",
        cost=float('inf'),
        nodes_expanded=nodes_expanded,
        path=[]
    ), steps


def bfs_search_generator(
    problem: 'GenericSearch'
) -> Generator[SearchStep, None, PathResult]:
    """
    Generator version of BFS that yields steps during execution.

    Args:
        problem: The search problem to solve

    Yields:
        SearchStep objects

    Returns:
        Final PathResult
    """
    frontier = QueueFrontier()
    start = problem.initial_state()
    start_node = SearchNode(state=start, path_cost=0, depth=0)
    frontier.push(start_node)

    explored: set = set()
    nodes_expanded = 0

    while not frontier.is_empty():
        node = frontier.pop()

        yield SearchStep(
            step_number=nodes_expanded,
            current_node=node.state,
            action=node.action,
            frontier=frontier.get_states(),
            explored=list(explored),
            current_path=node.get_path(),
            path_cost=node.path_cost
        )

        if problem.goal_test(node.state):
            return PathResult(
                plan=node.get_solution(),
                cost=node.path_cost,
                nodes_expanded=nodes_expanded,
                path=node.get_path()
            )

        if node.state in explored:
            continue

        explored.add(node.state)
        nodes_expanded += 1

        for action in problem.actions(node.state):
            child_state = problem.result(node.state, action)
            if child_state not in explored and not frontier.contains_state(child_state):
                step_cost = problem.step_cost(node.state, action, child_state)
                child = SearchNode(
                    state=child_state,
                    parent=node,
                    action=action,
                    path_cost=node.path_cost + step_cost,
                    depth=node.depth + 1
                )
                frontier.push(child)

    return PathResult(
        plan="",
        cost=float('inf'),
        nodes_expanded=nodes_expanded,
        path=[]
    )