from collections import deque

from torch.fx import Graph, Node


def _detect_cycles(graph: Graph) -> str:
    current_path: deque[Node] = deque()
    current_path_set: set[Node] = set()
    pending: deque[tuple[Node, Node]] = deque()

    def add_to_current_path(node: Node) -> None:
        current_path.append(node)
        current_path_set.add(node)

    def pop_current_path() -> None:
        node = current_path.pop()
        current_path_set.remove(node)

    def current_path_head() -> Node:
        return current_path[-1]

    for origin in graph.find_nodes(op="placeholder"):
        current_path.clear()
        current_path_set.clear()
        add_to_current_path(origin)
        for child in origin.users:
            pending.append((child, origin))

        while pending:
            cur_node, parent = pending.pop()

            while current_path_head() != parent:
                pop_current_path()

            if cur_node in current_path_set:
                current_path.append(cur_node)
                return f"cycle detected in path: {current_path}"

            add_to_current_path(cur_node)
            for child in cur_node.users:
                pending.append((child, cur_node))

    return "no cycle detected"