code_debug_env/tasks/hard/task_006_graph_cycle.json

{
  "task_id": "task_006_graph_cycle",
  "difficulty": "hard",
  "description": "Fix the bug in has_cycle. It should detect if a directed graph (given as an adjacency dict) contains a cycle. The current implementation incorrectly reports cycles in DAGs because it doesn't distinguish between nodes in the current DFS path vs. nodes that have been fully explored.",
  "buggy_code": "def has_cycle(graph):\n    visited = set()\n\n    def dfs(node):\n        if node in visited:\n            return True\n        visited.add(node)\n        for neighbor in graph.get(node, []):\n            if dfs(neighbor):\n                return True\n        return False\n\n    for node in graph:\n        if node not in visited:\n            if dfs(node):\n                return True\n    return False\n",
  "solution": "def has_cycle(graph):\n    visited = set()\n    rec_stack = set()\n\n    def dfs(node):\n        visited.add(node)\n        rec_stack.add(node)\n        for neighbor in graph.get(node, []):\n            if neighbor not in visited:\n                if dfs(neighbor):\n                    return True\n            elif neighbor in rec_stack:\n                return True\n        rec_stack.discard(node)\n        return False\n\n    for node in graph:\n        if node not in visited:\n            if dfs(node):\n                return True\n    return False\n",
  "test_descriptions": [
    "A simple cycle A->B->C->A should return True",
    "A DAG A->B->C, A->C should return False",
    "A self-loop A->A should return True",
    "An empty graph {} should return False",
    "A disconnected graph with a cycle in one component should return True",
    "A diamond DAG A->B, A->C, B->D, C->D should return False"
  ],
  "test_code": "test_cases = [\n    ({\"A\": [\"B\"], \"B\": [\"C\"], \"C\": [\"A\"]}, True, \"simple_cycle\"),\n    ({\"A\": [\"B\", \"C\"], \"B\": [\"C\"], \"C\": []}, False, \"dag_no_cycle\"),\n    ({\"A\": [\"A\"]}, True, \"self_loop\"),\n    ({}, False, \"empty_graph\"),\n    ({\"A\": [\"B\"], \"B\": [], \"C\": [\"D\"], \"D\": [\"E\"], \"E\": [\"C\"]}, True, \"disconnected_with_cycle\"),\n    ({\"A\": [\"B\", \"C\"], \"B\": [\"D\"], \"C\": [\"D\"], \"D\": []}, False, \"diamond_dag\")\n]\nfor graph, exp, name in test_cases:\n    try:\n        actual = has_cycle(graph)\n        results.append({\"test_name\": name, \"passed\": actual == exp, \"expected\": str(exp), \"actual\": str(actual), \"error\": \"\"})\n    except Exception as e:\n        results.append({\"test_name\": name, \"passed\": False, \"expected\": str(exp), \"actual\": \"\", \"error\": str(e)})",
  "max_steps": 10
}