package service

import (
	"crypto/aes"
	"crypto/cipher"
	"crypto/rand"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"time"

	"gorm.io/gorm"
)

// Service layer errors for backup
var (
	ErrInvalidPassword     = errors.New("invalid password: must be at least 8 characters")
	ErrBackupFailed        = errors.New("backup operation failed")
	ErrInvalidBackupFile   = errors.New("invalid backup file")
	ErrChecksumMismatch    = errors.New("checksum verification failed")
	ErrDecryptionFailed    = errors.New("decryption failed")
)

// BackupService handles data backup and restore operations
type BackupService struct {
	db     *gorm.DB
	dbPath string // Optional: explicit database path
}

// NewBackupService creates a new BackupService instance
func NewBackupService(db *gorm.DB) *BackupService {
	return &BackupService{
		db:     db,
		dbPath: "", // Will be auto-detected
	}
}

// NewBackupServiceWithPath creates a new BackupService instance with explicit database path
func NewBackupServiceWithPath(db *gorm.DB, dbPath string) *BackupService {
	return &BackupService{
		db:     db,
		dbPath: dbPath,
	}
}

// BackupRequest represents the input for creating a backup
type BackupRequest struct {
	Password string `json:"password" binding:"required,min=8"`
	FilePath string `json:"file_path,omitempty"` // Optional custom path
}

// BackupResponse represents the response after creating a backup
type BackupResponse struct {
	FilePath string `json:"file_path"`
	Checksum string `json:"checksum"`
	Size     int64  `json:"size"`
	Created  string `json:"created"`
}

// RestoreRequest represents the input for restoring from a backup
type RestoreRequest struct {
	Password string `json:"password" binding:"required"`
	FilePath string `json:"file_path" binding:"required"`
	Checksum string `json:"checksum,omitempty"` // Optional checksum verification
}

// ExportBackup creates an encrypted backup of the database
// The backup file format:
// - First 32 bytes: SHA256 checksum of encrypted data
// - Next 12 bytes: AES-GCM nonce
// - Remaining bytes: Encrypted database content
func (s *BackupService) ExportBackup(req BackupRequest) (*BackupResponse, error) {
	// Validate password
	if len(req.Password) < 8 {
		return nil, ErrInvalidPassword
	}

	// Get database file path
	dbPath, err := s.getDatabasePath()
	if err != nil {
		return nil, fmt.Errorf("failed to get database path: %w", err)
	}

	// Read database file
	dbData, err := os.ReadFile(dbPath)
	if err != nil {
		return nil, fmt.Errorf("failed to read database file: %w", err)
	}

	// Encrypt the data
	encryptedData, err := s.encryptData(dbData, req.Password)
	if err != nil {
		return nil, fmt.Errorf("failed to encrypt data: %w", err)
	}

	// Calculate checksum of encrypted data
	checksum := s.calculateChecksum(encryptedData)

	// Prepare backup file with checksum prefix
	backupData := append([]byte(checksum), encryptedData...)

	// Determine output file path
	outputPath := req.FilePath
	if outputPath == "" {
		timestamp := time.Now().Format("20060102_150405")
		outputPath = filepath.Join(".", fmt.Sprintf("backup_%s.enc", timestamp))
	}

	// Write backup file
	if err := os.WriteFile(outputPath, backupData, 0600); err != nil {
		return nil, fmt.Errorf("failed to write backup file: %w", err)
	}

	// Get file info
	fileInfo, err := os.Stat(outputPath)
	if err != nil {
		return nil, fmt.Errorf("failed to get file info: %w", err)
	}

	return &BackupResponse{
		FilePath: outputPath,
		Checksum: checksum,
		Size:     fileInfo.Size(),
		Created:  time.Now().Format(time.RFC3339),
	}, nil
}

// ImportBackup restores the database from an encrypted backup file
func (s *BackupService) ImportBackup(req RestoreRequest) error {
	// Read backup file
	backupData, err := os.ReadFile(req.FilePath)
	if err != nil {
		return fmt.Errorf("failed to read backup file: %w", err)
	}

	// Backup file must have at least checksum (64 bytes hex = 32 bytes) + nonce (12 bytes) + some data
	if len(backupData) < 76 {
		return ErrInvalidBackupFile
	}

	// Extract checksum (first 64 bytes as hex string)
	storedChecksum := string(backupData[:64])
	encryptedData := backupData[64:]

	// Verify checksum
	calculatedChecksum := s.calculateChecksum(encryptedData)
	if storedChecksum != calculatedChecksum {
		return ErrChecksumMismatch
	}

	// If checksum provided in request, verify it matches
	if req.Checksum != "" && req.Checksum != storedChecksum {
		return ErrChecksumMismatch
	}

	// Decrypt the data
	decryptedData, err := s.decryptData(encryptedData, req.Password)
	if err != nil {
		return fmt.Errorf("failed to decrypt data: %w", err)
	}

	// Get database file path
	dbPath, err := s.getDatabasePath()
	if err != nil {
		return fmt.Errorf("failed to get database path: %w", err)
	}

	// Close existing database connections
	sqlDB, err := s.db.DB()
	if err != nil {
		return fmt.Errorf("failed to get database connection: %w", err)
	}
	if err := sqlDB.Close(); err != nil {
		return fmt.Errorf("failed to close database connection: %w", err)
	}

	// Create backup of current database before overwriting
	backupPath := dbPath + ".backup"
	if err := s.copyFile(dbPath, backupPath); err != nil {
		// If backup fails, log but continue (original file still exists)
		fmt.Printf("Warning: failed to create backup of current database: %v\n", err)
	}

	// Write decrypted data to database file
	if err := os.WriteFile(dbPath, decryptedData, 0600); err != nil {
		// Try to restore from backup if write fails
		if backupErr := s.copyFile(backupPath, dbPath); backupErr != nil {
			return fmt.Errorf("failed to write database file and restore backup: %w, %w", err, backupErr)
		}
		return fmt.Errorf("failed to write database file: %w", err)
	}

	// Remove temporary backup
	os.Remove(backupPath)

	return nil
}

// encryptData encrypts data using AES-256-GCM with the provided password
func (s *BackupService) encryptData(data []byte, password string) ([]byte, error) {
	// Derive a 32-byte key from password using SHA256
	key := sha256.Sum256([]byte(password))

	// Create AES cipher
	block, err := aes.NewCipher(key[:])
	if err != nil {
		return nil, fmt.Errorf("failed to create cipher: %w", err)
	}

	// Create GCM mode
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, fmt.Errorf("failed to create GCM: %w", err)
	}

	// Generate nonce
	nonce := make([]byte, gcm.NonceSize())
	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
		return nil, fmt.Errorf("failed to generate nonce: %w", err)
	}

	// Encrypt data (nonce is prepended to ciphertext)
	ciphertext := gcm.Seal(nonce, nonce, data, nil)

	return ciphertext, nil
}

// decryptData decrypts data using AES-256-GCM with the provided password
func (s *BackupService) decryptData(encryptedData []byte, password string) ([]byte, error) {
	// Derive a 32-byte key from password using SHA256
	key := sha256.Sum256([]byte(password))

	// Create AES cipher
	block, err := aes.NewCipher(key[:])
	if err != nil {
		return nil, fmt.Errorf("failed to create cipher: %w", err)
	}

	// Create GCM mode
	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, fmt.Errorf("failed to create GCM: %w", err)
	}

	// Check minimum size
	nonceSize := gcm.NonceSize()
	if len(encryptedData) < nonceSize {
		return nil, ErrInvalidBackupFile
	}

	// Extract nonce and ciphertext
	nonce := encryptedData[:nonceSize]
	ciphertext := encryptedData[nonceSize:]

	// Decrypt data
	plaintext, err := gcm.Open(nil, nonce, ciphertext, nil)
	if err != nil {
		return nil, ErrDecryptionFailed
	}

	return plaintext, nil
}

// calculateChecksum calculates SHA256 checksum of data and returns hex string
func (s *BackupService) calculateChecksum(data []byte) string {
	hash := sha256.Sum256(data)
	return hex.EncodeToString(hash[:])
}

// getDatabasePath returns the path to the SQLite database file
func (s *BackupService) getDatabasePath() (string, error) {
	// If explicit path is set, use it
	if s.dbPath != "" {
		if _, err := os.Stat(s.dbPath); err != nil {
			return "", fmt.Errorf("database file not found at %s: %w", s.dbPath, err)
		}
		return s.dbPath, nil
	}

	// Get the database connection
	sqlDB, err := s.db.DB()
	if err != nil {
		return "", err
	}

	// Ensure we have a valid connection
	if err := sqlDB.Ping(); err != nil {
		return "", fmt.Errorf("database connection not available: %w", err)
	}

	// For SQLite, try common database paths
	possiblePaths := []string{
		"data/accounting.db",
		"./accounting.db",
		"../data/accounting.db",
	}

	for _, dbPath := range possiblePaths {
		if _, err := os.Stat(dbPath); err == nil {
			return dbPath, nil
		}
	}

	// If no standard path found, return error
	return "", fmt.Errorf("database file not found in standard locations")
}

// copyFile copies a file from src to dst
func (s *BackupService) copyFile(src, dst string) error {
	sourceData, err := os.ReadFile(src)
	if err != nil {
		return err
	}
	return os.WriteFile(dst, sourceData, 0600)
}

// VerifyBackup verifies the integrity of a backup file without restoring it
func (s *BackupService) VerifyBackup(filePath string) (bool, string, error) {
	// Read backup file
	backupData, err := os.ReadFile(filePath)
	if err != nil {
		return false, "", fmt.Errorf("failed to read backup file: %w", err)
	}

	// Backup file must have at least checksum + nonce + some data
	if len(backupData) < 76 {
		return false, "", ErrInvalidBackupFile
	}

	// Extract checksum
	storedChecksum := string(backupData[:64])
	encryptedData := backupData[64:]

	// Verify checksum
	calculatedChecksum := s.calculateChecksum(encryptedData)
	isValid := storedChecksum == calculatedChecksum

	return isValid, storedChecksum, nil
}