from typing import Optional, List, Dict, Union
from datetime import datetime
import hashlib
import os
from sqlalchemy.orm import Session
from sqlalchemy import desc
from models.migration import Migration

def calculate_script_checksum(script_path: str) -> str:
    """
    Calculate SHA-256 checksum of migration script file.
    
    Args:
        script_path: Path to the migration script file
        
    Returns:
        str: Hexadecimal checksum of the file
    """
    if not os.path.exists(script_path):
        raise FileNotFoundError(f"Migration script not found at: {script_path}")
        
    sha256_hash = hashlib.sha256()
    with open(script_path, "rb") as f:
        for byte_block in iter(lambda: f.read(4096), b""):
            sha256_hash.update(byte_block)
    return sha256_hash.hexdigest()

def get_latest_migration(db: Session) -> Optional[Migration]:
    """
    Get the most recently applied migration.
    
    Args:
        db: Database session
        
    Returns:
        Migration: Latest applied migration or None if no migrations exist
    """
    return db.query(Migration).filter(Migration.applied == True)\
             .order_by(desc(Migration.version)).first()

def validate_migration_version(version: str) -> bool:
    """
    Validate migration version format (e.g. v1.0.0).
    
    Args:
        version: Version string to validate
        
    Returns:
        bool: True if version format is valid
    """
    import re
    pattern = r'^v\d+\.\d+\.\d+$'
    return bool(re.match(pattern, version))

def mark_migration_complete(db: Session, migration: Migration, execution_time_ms: int) -> Migration:
    """
    Mark a migration as successfully applied.
    
    Args:
        db: Database session
        migration: Migration object to update
        execution_time_ms: Migration execution time in milliseconds
        
    Returns:
        Migration: Updated migration object
    """
    migration.applied = True
    migration.execution_time = datetime.utcnow()
    db.commit()
    db.refresh(migration)
    return migration

def verify_migration_integrity(db: Session, script_path: str) -> Dict[str, bool]:
    """
    Verify migration script integrity by comparing checksums.
    
    Args:
        db: Database session
        script_path: Path to migration script
        
    Returns:
        dict: Status of integrity check
    """
    current_checksum = calculate_script_checksum(script_path)
    migration = db.query(Migration).filter(Migration.script_path == script_path).first()
    
    if not migration:
        return {"valid": False, "error": "Migration not found in database"}
        
    if not migration.checksum:
        return {"valid": False, "error": "No checksum stored for migration"}
        
    return {
        "valid": current_checksum == migration.checksum,
        "stored_checksum": migration.checksum,
        "current_checksum": current_checksum
    }