#!/usr/bin/env python3
"""
InsightFlow Database Manager - Phase 3
处理项目、实体、关系的持久化
支持文档类型和多文件融合
"""

import os
import json
import sqlite3
from datetime import datetime
from typing import List, Dict, Optional, Tuple
from dataclasses import dataclass

DB_PATH = os.getenv("DB_PATH", "/app/data/insightflow.db")

@dataclass
class Project:
    id: str
    name: str
    description: str = ""
    created_at: str = ""
    updated_at: str = ""

@dataclass
class Entity:
    id: str
    project_id: str
    name: str
    type: str
    definition: str = ""
    canonical_name: str = ""
    aliases: List[str] = None
    
    def __post_init__(self):
        if self.aliases is None:
            self.aliases = []

@dataclass
class EntityMention:
    id: str
    entity_id: str
    transcript_id: str
    start_pos: int
    end_pos: int
    text_snippet: str
    confidence: float = 1.0

class DatabaseManager:
    def __init__(self, db_path: str = DB_PATH):
        self.db_path = db_path
        os.makedirs(os.path.dirname(db_path), exist_ok=True)
        self.init_db()
    
    def get_conn(self):
        conn = sqlite3.connect(self.db_path)
        conn.row_factory = sqlite3.Row
        return conn
    
    def init_db(self):
        """初始化数据库表"""
        with open(os.path.join(os.path.dirname(__file__), 'schema.sql'), 'r') as f:
            schema = f.read()
        
        conn = self.get_conn()
        conn.executescript(schema)
        conn.commit()
        conn.close()
    
    # Project operations
    def create_project(self, project_id: str, name: str, description: str = "") -> Project:
        conn = self.get_conn()
        now = datetime.now().isoformat()
        conn.execute(
            "INSERT INTO projects (id, name, description, created_at, updated_at) VALUES (?, ?, ?, ?, ?)",
            (project_id, name, description, now, now)
        )
        conn.commit()
        conn.close()
        return Project(id=project_id, name=name, description=description, created_at=now, updated_at=now)
    
    def get_project(self, project_id: str) -> Optional[Project]:
        conn = self.get_conn()
        row = conn.execute("SELECT * FROM projects WHERE id = ?", (project_id,)).fetchone()
        conn.close()
        if row:
            return Project(**dict(row))
        return None
    
    def list_projects(self) -> List[Project]:
        conn = self.get_conn()
        rows = conn.execute("SELECT * FROM projects ORDER BY updated_at DESC").fetchall()
        conn.close()
        return [Project(**dict(r)) for r in rows]
    
    # Entity operations
    def create_entity(self, entity: Entity) -> Entity:
        conn = self.get_conn()
        conn.execute(
            """INSERT INTO entities (id, project_id, name, canonical_name, type, definition, aliases, created_at, updated_at)
               VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)""",
            (entity.id, entity.project_id, entity.name, entity.canonical_name, entity.type,
             entity.definition, json.dumps(entity.aliases), datetime.now().isoformat(), datetime.now().isoformat())
        )
        conn.commit()
        conn.close()
        return entity
    
    def get_entity_by_name(self, project_id: str, name: str) -> Optional[Entity]:
        """通过名称查找实体（用于对齐）"""
        conn = self.get_conn()
        row = conn.execute(
            "SELECT * FROM entities WHERE project_id = ? AND (name = ? OR canonical_name = ? OR aliases LIKE ?)",
            (project_id, name, name, f'%"{name}"%')
        ).fetchone()
        conn.close()
        if row:
            data = dict(row)
            data['aliases'] = json.loads(data['aliases']) if data['aliases'] else []
            return Entity(**data)
        return None
    
    def find_similar_entities(self, project_id: str, name: str, threshold: float = 0.8) -> List[Entity]:
        """查找相似实体（简单实现，生产可用 embedding）"""
        # TODO: 使用 embedding 或模糊匹配
        # 现在简单返回包含相同关键词的实体
        conn = self.get_conn()
        rows = conn.execute(
            "SELECT * FROM entities WHERE project_id = ? AND name LIKE ?",
            (project_id, f"%{name}%")
        ).fetchall()
        conn.close()
        
        entities = []
        for row in rows:
            data = dict(row)
            data['aliases'] = json.loads(data['aliases']) if data['aliases'] else []
            entities.append(Entity(**data))
        return entities
    
    def merge_entities(self, target_id: str, source_id: str) -> Entity:
        """合并两个实体（实体对齐）"""
        conn = self.get_conn()
        
        # 获取两个实体
        target = conn.execute("SELECT * FROM entities WHERE id = ?", (target_id,)).fetchone()
        source = conn.execute("SELECT * FROM entities WHERE id = ?", (source_id,)).fetchone()
        
        if not target or not source:
            conn.close()
            raise ValueError("Entity not found")
        
        # 合并别名
        target_aliases = set(json.loads(target['aliases']) if target['aliases'] else [])
        target_aliases.add(source['name'])
        target_aliases.update(json.loads(source['aliases']) if source['aliases'] else [])
        
        # 更新目标实体
        conn.execute(
            "UPDATE entities SET aliases = ?, updated_at = ? WHERE id = ?",
            (json.dumps(list(target_aliases)), datetime.now().isoformat(), target_id)
        )
        
        # 更新提及记录
        conn.execute(
            "UPDATE entity_mentions SET entity_id = ? WHERE entity_id = ?",
            (target_id, source_id)
        )
        
        # 更新关系 - source 作为 source_entity_id
        conn.execute(
            "UPDATE entity_relations SET source_entity_id = ? WHERE source_entity_id = ?",
            (target_id, source_id)
        )
        
        # 更新关系 - source 作为 target_entity_id
        conn.execute(
            "UPDATE entity_relations SET target_entity_id = ? WHERE target_entity_id = ?",
            (target_id, source_id)
        )
        
        # 删除源实体
        conn.execute("DELETE FROM entities WHERE id = ?", (source_id,))
        
        conn.commit()
        conn.close()
        
        return self.get_entity(target_id)
    
    def get_entity(self, entity_id: str) -> Optional[Entity]:
        conn = self.get_conn()
        row = conn.execute("SELECT * FROM entities WHERE id = ?", (entity_id,)).fetchone()
        conn.close()
        if row:
            data = dict(row)
            data['aliases'] = json.loads(data['aliases']) if data['aliases'] else []
            return Entity(**data)
        return None
    
    def list_project_entities(self, project_id: str) -> List[Entity]:
        conn = self.get_conn()
        rows = conn.execute(
            "SELECT * FROM entities WHERE project_id = ? ORDER BY updated_at DESC",
            (project_id,)
        ).fetchall()
        conn.close()
        
        entities = []
        for row in rows:
            data = dict(row)
            data['aliases'] = json.loads(data['aliases']) if data['aliases'] else []
            entities.append(Entity(**data))
        return entities
    
    # Mention operations
    def add_mention(self, mention: EntityMention) -> EntityMention:
        conn = self.get_conn()
        conn.execute(
            """INSERT INTO entity_mentions (id, entity_id, transcript_id, start_pos, end_pos, text_snippet, confidence)
               VALUES (?, ?, ?, ?, ?, ?, ?)""",
            (mention.id, mention.entity_id, mention.transcript_id, mention.start_pos,
             mention.end_pos, mention.text_snippet, mention.confidence)
        )
        conn.commit()
        conn.close()
        return mention
    
    def get_entity_mentions(self, entity_id: str) -> List[EntityMention]:
        conn = self.get_conn()
        rows = conn.execute(
            "SELECT * FROM entity_mentions WHERE entity_id = ? ORDER BY transcript_id, start_pos",
            (entity_id,)
        ).fetchall()
        conn.close()
        return [EntityMention(**dict(r)) for r in rows]

    # Transcript operations
    def save_transcript(self, transcript_id: str, project_id: str, filename: str, full_text: str, transcript_type: str = "audio"):
        """保存转录记录"""
        conn = self.get_conn()
        now = datetime.now().isoformat()
        conn.execute(
            "INSERT INTO transcripts (id, project_id, filename, full_text, type, created_at) VALUES (?, ?, ?, ?, ?, ?)",
            (transcript_id, project_id, filename, full_text, transcript_type, now)
        )
        conn.commit()
        conn.close()
    
    def get_transcript(self, transcript_id: str) -> Optional[dict]:
        """获取转录记录"""
        conn = self.get_conn()
        row = conn.execute("SELECT * FROM transcripts WHERE id = ?", (transcript_id,)).fetchone()
        conn.close()
        if row:
            return dict(row)
        return None
    
    def list_project_transcripts(self, project_id: str) -> List[dict]:
        """列出项目的所有转录"""
        conn = self.get_conn()
        rows = conn.execute(
            "SELECT * FROM transcripts WHERE project_id = ? ORDER BY created_at DESC",
            (project_id,)
        ).fetchall()
        conn.close()
        return [dict(r) for r in rows]
    
    # Relation operations
    def create_relation(self, project_id: str, source_entity_id: str, target_entity_id: str,
                       relation_type: str = "related", evidence: str = "", transcript_id: str = ""):
        """创建实体关系"""
        conn = self.get_conn()
        relation_id = str(uuid.uuid4())[:8]
        now = datetime.now().isoformat()
        conn.execute(
            """INSERT INTO entity_relations 
               (id, project_id, source_entity_id, target_entity_id, relation_type, evidence, transcript_id, created_at)
               VALUES (?, ?, ?, ?, ?, ?, ?, ?)""",
            (relation_id, project_id, source_entity_id, target_entity_id, relation_type, evidence, transcript_id, now)
        )
        conn.commit()
        conn.close()
        return relation_id
    
    def get_entity_relations(self, entity_id: str) -> List[dict]:
        """获取实体的所有关系"""
        conn = self.get_conn()
        rows = conn.execute(
            """SELECT * FROM entity_relations 
               WHERE source_entity_id = ? OR target_entity_id = ?
               ORDER BY created_at DESC""",
            (entity_id, entity_id)
        ).fetchall()
        conn.close()
        return [dict(r) for r in rows]
    
    def list_project_relations(self, project_id: str) -> List[dict]:
        """列出项目的所有关系"""
        conn = self.get_conn()
        rows = conn.execute(
            "SELECT * FROM entity_relations WHERE project_id = ? ORDER BY created_at DESC",
            (project_id,)
        ).fetchall()
        conn.close()
        return [dict(r) for r in rows]
    
    def update_entity(self, entity_id: str, **kwargs) -> Entity:
        """更新实体信息"""
        conn = self.get_conn()
        
        # 构建更新字段
        allowed_fields = ['name', 'type', 'definition', 'canonical_name']
        updates = []
        values = []
        
        for field in allowed_fields:
            if field in kwargs:
                updates.append(f"{field} = ?")
                values.append(kwargs[field])
        
        # 处理别名
        if 'aliases' in kwargs:
            updates.append("aliases = ?")
            values.append(json.dumps(kwargs['aliases']))
        
        if not updates:
            conn.close()
            return self.get_entity(entity_id)
        
        updates.append("updated_at = ?")
        values.append(datetime.now().isoformat())
        values.append(entity_id)
        
        query = f"UPDATE entities SET {', '.join(updates)} WHERE id = ?"
        conn.execute(query, values)
        conn.commit()
        conn.close()
        
        return self.get_entity(entity_id)
    
    def delete_entity(self, entity_id: str):
        """删除实体及其关联数据"""
        conn = self.get_conn()
        
        # 删除提及记录
        conn.execute("DELETE FROM entity_mentions WHERE entity_id = ?", (entity_id,))
        
        # 删除关系
        conn.execute("DELETE FROM entity_relations WHERE source_entity_id = ? OR target_entity_id = ?", 
                     (entity_id, entity_id))
        
        # 删除实体
        conn.execute("DELETE FROM entities WHERE id = ?", (entity_id,))
        
        conn.commit()
        conn.close()
    
    def delete_relation(self, relation_id: str):
        """删除关系"""
        conn = self.get_conn()
        conn.execute("DELETE FROM entity_relations WHERE id = ?", (relation_id,))
        conn.commit()
        conn.close()
    
    def update_relation(self, relation_id: str, **kwargs) -> dict:
        """更新关系"""
        conn = self.get_conn()
        
        allowed_fields = ['relation_type', 'evidence']
        updates = []
        values = []
        
        for field in allowed_fields:
            if field in kwargs:
                updates.append(f"{field} = ?")
                values.append(kwargs[field])
        
        if updates:
            query = f"UPDATE entity_relations SET {', '.join(updates)} WHERE id = ?"
            values.append(relation_id)
            conn.execute(query, values)
            conn.commit()
        
        row = conn.execute("SELECT * FROM entity_relations WHERE id = ?", (relation_id,)).fetchone()
        conn.close()
        
        return dict(row) if row else None
    
    def update_transcript(self, transcript_id: str, full_text: str) -> dict:
        """更新转录文本"""
        conn = self.get_conn()
        now = datetime.now().isoformat()
        
        conn.execute(
            "UPDATE transcripts SET full_text = ?, updated_at = ? WHERE id = ?",
            (full_text, now, transcript_id)
        )
        conn.commit()
        
        row = conn.execute("SELECT * FROM transcripts WHERE id = ?", (transcript_id,)).fetchone()
        conn.close()
        
        return dict(row) if row else None
    
    # Phase 3: Glossary operations
    def add_glossary_term(self, project_id: str, term: str, pronunciation: str = "") -> str:
        """添加术语到术语表"""
        conn = self.get_conn()
        
        # 检查是否已存在
        existing = conn.execute(
            "SELECT * FROM glossary WHERE project_id = ? AND term = ?",
            (project_id, term)
        ).fetchone()
        
        if existing:
            # 更新频率
            conn.execute(
                "UPDATE glossary SET frequency = frequency + 1 WHERE id = ?",
                (existing['id'],)
            )
            conn.commit()
            conn.close()
            return existing['id']
        
        term_id = str(uuid.uuid4())[:8]
        conn.execute(
            "INSERT INTO glossary (id, project_id, term, pronunciation, frequency) VALUES (?, ?, ?, ?, ?)",
            (term_id, project_id, term, pronunciation, 1)
        )
        conn.commit()
        conn.close()
        return term_id
    
    def list_glossary(self, project_id: str) -> List[dict]:
        """列出项目术语表"""
        conn = self.get_conn()
        rows = conn.execute(
            "SELECT * FROM glossary WHERE project_id = ? ORDER BY frequency DESC",
            (project_id,)
        ).fetchall()
        conn.close()
        return [dict(r) for r in rows]
    
    def delete_glossary_term(self, term_id: str):
        """删除术语"""
        conn = self.get_conn()
        conn.execute("DELETE FROM glossary WHERE id = ?", (term_id,))
        conn.commit()
        conn.close()
    
    # Phase 3: Get all entities for embedding
    def get_all_entities_for_embedding(self, project_id: str) -> List[Entity]:
        """获取所有实体用于 embedding 计算"""
        return self.list_project_entities(project_id)
    
    # Phase 4: Agent & Provenance methods
    def get_relation_with_details(self, relation_id: str) -> Optional[dict]:
        """获取关系详情，包含源文档信息"""
        conn = self.get_conn()
        row = conn.execute(
            """SELECT r.*, 
                s.name as source_name, t.name as target_name,
                tr.filename as transcript_filename, tr.full_text as transcript_text
               FROM entity_relations r
               JOIN entities s ON r.source_entity_id = s.id
               JOIN entities t ON r.target_entity_id = t.id
               LEFT JOIN transcripts tr ON r.transcript_id = tr.id
               WHERE r.id = ?""",
            (relation_id,)
        ).fetchone()
        conn.close()
        if row:
            return dict(row)
        return None
    
    def get_entity_with_mentions(self, entity_id: str) -> Optional[dict]:
        """获取实体详情及所有提及位置"""
        conn = self.get_conn()
        
        # 获取实体信息
        entity_row = conn.execute(
            "SELECT * FROM entities WHERE id = ?", (entity_id,)
        ).fetchone()
        
        if not entity_row:
            conn.close()
            return None
        
        entity = dict(entity_row)
        entity['aliases'] = json.loads(entity['aliases']) if entity['aliases'] else []
        
        # 获取提及位置
        mentions = conn.execute(
            """SELECT m.*, t.filename, t.created_at as transcript_date
               FROM entity_mentions m
               JOIN transcripts t ON m.transcript_id = t.id
               WHERE m.entity_id = ?
               ORDER BY t.created_at, m.start_pos""",
            (entity_id,)
        ).fetchall()
        
        entity['mentions'] = [dict(m) for m in mentions]
        entity['mention_count'] = len(mentions)
        
        # 获取相关关系
        relations = conn.execute(
            """SELECT r.*, 
                s.name as source_name, t.name as target_name
               FROM entity_relations r
               JOIN entities s ON r.source_entity_id = s.id
               JOIN entities t ON r.target_entity_id = t.id
               WHERE r.source_entity_id = ? OR r.target_entity_id = ?
               ORDER BY r.created_at DESC""",
            (entity_id, entity_id)
        ).fetchall()
        
        entity['relations'] = [dict(r) for r in relations]
        
        conn.close()
        return entity
    
    def search_entities(self, project_id: str, query: str) -> List[Entity]:
        """搜索实体"""
        conn = self.get_conn()
        rows = conn.execute(
            """SELECT * FROM entities 
               WHERE project_id = ? AND 
               (name LIKE ? OR definition LIKE ? OR aliases LIKE ?)
               ORDER BY name""",
            (project_id, f'%{query}%', f'%{query}%', f'%{query}%')
        ).fetchall()
        conn.close()
        
        entities = []
        for row in rows:
            data = dict(row)
            data['aliases'] = json.loads(data['aliases']) if data['aliases'] else []
            entities.append(Entity(**data))
        return entities
    
    def get_project_summary(self, project_id: str) -> dict:
        """获取项目摘要信息，用于 RAG 上下文"""
        conn = self.get_conn()
        
        # 项目基本信息
        project = conn.execute(
            "SELECT * FROM projects WHERE id = ?", (project_id,)
        ).fetchone()
        
        # 统计信息
        entity_count = conn.execute(
            "SELECT COUNT(*) as count FROM entities WHERE project_id = ?",
            (project_id,)
        ).fetchone()['count']
        
        transcript_count = conn.execute(
            "SELECT COUNT(*) as count FROM transcripts WHERE project_id = ?",
            (project_id,)
        ).fetchone()['count']
        
        relation_count = conn.execute(
            "SELECT COUNT(*) as count FROM entity_relations WHERE project_id = ?",
            (project_id,)
        ).fetchone()['count']
        
        # 获取最近的转录文本片段
        recent_transcripts = conn.execute(
            """SELECT filename, full_text, created_at 
               FROM transcripts 
               WHERE project_id = ? 
               ORDER BY created_at DESC 
               LIMIT 5""",
            (project_id,)
        ).fetchall()
        
        # 获取高频实体
        top_entities = conn.execute(
            """SELECT e.name, e.type, e.definition, COUNT(m.id) as mention_count
               FROM entities e
               LEFT JOIN entity_mentions m ON e.id = m.entity_id
               WHERE e.project_id = ?
               GROUP BY e.id
               ORDER BY mention_count DESC
               LIMIT 10""",
            (project_id,)
        ).fetchall()
        
        conn.close()
        
        return {
            'project': dict(project) if project else {},
            'statistics': {
                'entity_count': entity_count,
                'transcript_count': transcript_count,
                'relation_count': relation_count
            },
            'recent_transcripts': [dict(t) for t in recent_transcripts],
            'top_entities': [dict(e) for e in top_entities]
        }

    # Phase 5: Timeline operations
    def get_project_timeline(self, project_id: str, entity_id: str = None, start_date: str = None, end_date: str = None) -> List[dict]:
        """获取项目时间线数据 - 按时间顺序的实体提及和事件"""
        conn = self.get_conn()
        
        # 构建查询条件
        conditions = ["t.project_id = ?"]
        params = [project_id]
        
        if entity_id:
            conditions.append("m.entity_id = ?")
            params.append(entity_id)
        
        if start_date:
            conditions.append("t.created_at >= ?")
            params.append(start_date)
        
        if end_date:
            conditions.append("t.created_at <= ?")
            params.append(end_date)
        
        where_clause = " AND ".join(conditions)
        
        # 获取实体提及时间线
        mentions = conn.execute(
            f"""SELECT m.*, e.name as entity_name, e.type as entity_type, e.definition,
                       t.filename, t.created_at as event_date, t.type as source_type
               FROM entity_mentions m
               JOIN entities e ON m.entity_id = e.id
               JOIN transcripts t ON m.transcript_id = t.id
               WHERE {where_clause}
               ORDER BY t.created_at, m.start_pos""",
            params
        ).fetchall()
        
        # 获取关系创建时间线
        relation_conditions = ["r.project_id = ?"]
        relation_params = [project_id]
        
        if start_date:
            relation_conditions.append("r.created_at >= ?")
            relation_params.append(start_date)
        
        if end_date:
            relation_conditions.append("r.created_at <= ?")
            relation_params.append(end_date)
        
        relation_where = " AND ".join(relation_conditions)
        
        relations = conn.execute(
            f"""SELECT r.*, 
                       s.name as source_name, t.name as target_name,
                       tr.filename, r.created_at as event_date
               FROM entity_relations r
               JOIN entities s ON r.source_entity_id = s.id
               JOIN entities t ON r.target_entity_id = t.id
               LEFT JOIN transcripts tr ON r.transcript_id = tr.id
               WHERE {relation_where}
               ORDER BY r.created_at""",
            relation_params
        ).fetchall()
        
        conn.close()
        
        # 合并并格式化时间线事件
        timeline_events = []
        
        for m in mentions:
            timeline_events.append({
                'id': m['id'],
                'type': 'mention',
                'event_date': m['event_date'],
                'entity_id': m['entity_id'],
                'entity_name': m['entity_name'],
                'entity_type': m['entity_type'],
                'entity_definition': m['definition'],
                'text_snippet': m['text_snippet'],
                'confidence': m['confidence'],
                'source': {
                    'transcript_id': m['transcript_id'],
                    'filename': m['filename'],
                    'type': m['source_type']
                }
            })
        
        for r in relations:
            timeline_events.append({
                'id': r['id'],
                'type': 'relation',
                'event_date': r['event_date'],
                'relation_type': r['relation_type'],
                'source_entity': r['source_name'],
                'target_entity': r['target_name'],
                'evidence': r['evidence'],
                'source': {
                    'transcript_id': r.get('transcript_id'),
                    'filename': r['filename']
                }
            })
        
        # 按时间排序
        timeline_events.sort(key=lambda x: x['event_date'])
        
        return timeline_events
    
    def get_entity_timeline_summary(self, project_id: str) -> dict:
        """获取项目实体时间线摘要统计"""
        conn = self.get_conn()
        
        # 按日期统计提及数量
        daily_stats = conn.execute(
            """SELECT DATE(t.created_at) as date, COUNT(*) as count
               FROM entity_mentions m
               JOIN transcripts t ON m.transcript_id = t.id
               WHERE t.project_id = ?
               GROUP BY DATE(t.created_at)
               ORDER BY date""",
            (project_id,)
        ).fetchall()
        
        # 按实体统计提及数量
        entity_stats = conn.execute(
            """SELECT e.name, e.type, COUNT(m.id) as mention_count,
                       MIN(t.created_at) as first_mentioned,
                       MAX(t.created_at) as last_mentioned
               FROM entities e
               LEFT JOIN entity_mentions m ON e.id = m.entity_id
               LEFT JOIN transcripts t ON m.transcript_id = t.id
               WHERE e.project_id = ?
               GROUP BY e.id
               ORDER BY mention_count DESC
               LIMIT 20""",
            (project_id,)
        ).fetchall()
        
        # 获取活跃时间段
        active_periods = conn.execute(
            """SELECT 
                   DATE(t.created_at) as date,
                   COUNT(DISTINCT m.entity_id) as active_entities,
                   COUNT(m.id) as total_mentions
               FROM transcripts t
               LEFT JOIN entity_mentions m ON t.id = m.transcript_id
               WHERE t.project_id = ?
               GROUP BY DATE(t.created_at)
               ORDER BY date""",
            (project_id,)
        ).fetchall()
        
        conn.close()
        
        return {
            'daily_activity': [dict(d) for d in daily_stats],
            'top_entities': [dict(e) for e in entity_stats],
            'active_periods': [dict(a) for a in active_periods]
        }
    
    def get_transcript_context(self, transcript_id: str, position: int, context_chars: int = 200) -> str:
        """获取转录文本的上下文"""
        conn = self.get_conn()
        row = conn.execute(
            "SELECT full_text FROM transcripts WHERE id = ?",
            (transcript_id,)
        ).fetchone()
        conn.close()
        
        if not row:
            return ""
        
        text = row['full_text']
        start = max(0, position - context_chars)
        end = min(len(text), position + context_chars)
        return text[start:end]


# Singleton instance
_db_manager = None


def get_db_manager() -> DatabaseManager:
    global _db_manager
    if _db_manager is None:
        _db_manager = DatabaseManager()
    return _db_manager