package service

import (
	"errors"
	"fmt"

	"accounting-app/internal/models"
	"accounting-app/internal/repository"

	"gorm.io/gorm"
)

// Service layer errors
var (
	ErrAccountNotFound           = errors.New("account not found")
	ErrAccountInUse              = errors.New("account is in use and cannot be deleted")
	ErrInsufficientBalance       = errors.New("insufficient balance for this operation")
	ErrSameAccountTransfer       = errors.New("cannot transfer to the same account")
	ErrInvalidTransferAmount     = errors.New("transfer amount must be positive")
	ErrNegativeBalanceNotAllowed = errors.New("negative balance not allowed for non-credit accounts")
)

// AccountInput represents the input data for creating or updating an account
type AccountInput struct {
	UserID           uint               `json:"user_id"`
	Name             string             `json:"name" binding:"required"`
	Type             models.AccountType `json:"type" binding:"required"`
	Balance          float64            `json:"balance"`
	Currency         models.Currency    `json:"currency"`
	Icon             string             `json:"icon"`
	BillingDate      *int               `json:"billing_date,omitempty"`
	PaymentDate      *int               `json:"payment_date,omitempty"`
	WarningThreshold *float64           `json:"warning_threshold,omitempty"`
	AccountCode      string             `json:"account_code,omitempty"`
}

// TransferInput represents the input data for a transfer operation
type TransferInput struct {
	UserID        uint    `json:"user_id"`
	FromAccountID uint    `json:"from_account_id" binding:"required"`
	ToAccountID   uint    `json:"to_account_id" binding:"required"`
	Amount        float64 `json:"amount" binding:"required,gt=0"`
	Note          string  `json:"note"`
}

// AssetOverview represents the asset overview response
type AssetOverview struct {
	TotalAssets      float64 `json:"total_assets"`
	TotalLiabilities float64 `json:"total_liabilities"`
	NetWorth         float64 `json:"net_worth"`
}

// AccountService handles business logic for accounts
type AccountService struct {
	repo *repository.AccountRepository
	db   *gorm.DB
}

// NewAccountService creates a new AccountService instance
func NewAccountService(repo *repository.AccountRepository, db *gorm.DB) *AccountService {
	return &AccountService{
		repo: repo,
		db:   db,
	}
}

// CreateAccount creates a new account with business logic validation
func (s *AccountService) CreateAccount(userID uint, input AccountInput) (*models.Account, error) {
	// Set default currency if not provided
	if input.Currency == "" {
		input.Currency = models.CurrencyCNY
	}

	// Determine if this is a credit account type
	isCredit := models.IsCreditAccountType(input.Type)

	// Validate balance for non-credit accounts
	if !isCredit && input.Balance < 0 {
		return nil, ErrNegativeBalanceNotAllowed
	}

	// Create the account model
	account := &models.Account{
		UserID:           userID,
		Name:             input.Name,
		Type:             input.Type,
		Balance:          input.Balance,
		Currency:         input.Currency,
		Icon:             input.Icon,
		BillingDate:      input.BillingDate,
		PaymentDate:      input.PaymentDate,
		WarningThreshold: input.WarningThreshold,
		AccountCode:      input.AccountCode,
		IsCredit:         isCredit,
	}

	// Save to database
	if err := s.repo.Create(account); err != nil {
		return nil, fmt.Errorf("failed to create account: %w", err)
	}

	return account, nil
}

// GetAccount retrieves an account by ID and verifies ownership
func (s *AccountService) GetAccount(userID, id uint) (*models.Account, error) {
	account, err := s.repo.GetByID(userID, id)
	if err != nil {
		if errors.Is(err, repository.ErrAccountNotFound) {
			return nil, ErrAccountNotFound
		}
		return nil, fmt.Errorf("failed to get account: %w", err)
	}
	// Redundant check removed as repo filters by userID
	return account, nil
}

// GetAllAccounts retrieves all accounts for a specific user
func (s *AccountService) GetAllAccounts(userID uint) ([]models.Account, error) {
	accounts, err := s.repo.GetAll(userID)
	if err != nil {
		return nil, fmt.Errorf("failed to get accounts: %w", err)
	}
	return accounts, nil
}

// UpdateAccount updates an existing account after verifying ownership
func (s *AccountService) UpdateAccount(userID, id uint, input AccountInput) (*models.Account, error) {
	// Get existing account
	account, err := s.repo.GetByID(userID, id)
	if err != nil {
		if errors.Is(err, repository.ErrAccountNotFound) {
			return nil, ErrAccountNotFound
		}
		return nil, fmt.Errorf("failed to get account: %w", err)
	}
	// Redundant check removed
	// account.UserID match ensured by repo.GetByID(userID, id)

	// Determine if this is a credit account type
	isCredit := models.IsCreditAccountType(input.Type)

	// Validate balance for non-credit accounts
	if !isCredit && input.Balance < 0 {
		return nil, ErrNegativeBalanceNotAllowed
	}

	// Update fields
	account.Name = input.Name
	account.Type = input.Type
	account.Balance = input.Balance
	if input.Currency != "" {
		account.Currency = input.Currency
	}
	account.Icon = input.Icon
	account.BillingDate = input.BillingDate
	account.PaymentDate = input.PaymentDate
	account.WarningThreshold = input.WarningThreshold
	account.AccountCode = input.AccountCode
	account.IsCredit = isCredit

	// Save to database
	if err := s.repo.Update(account); err != nil {
		return nil, fmt.Errorf("failed to update account: %w", err)
	}

	return account, nil
}

// DeleteAccount deletes an account by ID after verifying ownership
func (s *AccountService) DeleteAccount(userID, id uint) error {
	_, err := s.repo.GetByID(userID, id)
	if err != nil {
		if errors.Is(err, repository.ErrAccountNotFound) {
			return ErrAccountNotFound
		}
		return fmt.Errorf("failed to check account existence: %w", err)
	}
	// Redundant check removed

	err = s.repo.Delete(userID, id)
	if err != nil {
		if errors.Is(err, repository.ErrAccountNotFound) {
			return ErrAccountNotFound
		}
		if errors.Is(err, repository.ErrAccountInUse) {
			return ErrAccountInUse
		}
		return fmt.Errorf("failed to delete account: %w", err)
	}
	return nil
}

// Transfer performs an atomic transfer between two accounts
// This operation is wrapped in a database transaction to ensure consistency
func (s *AccountService) Transfer(userID, fromAccountID, toAccountID uint, amount float64, note string) error {
	// Validate transfer parameters
	if fromAccountID == toAccountID {
		return ErrSameAccountTransfer
	}
	if amount <= 0 {
		return ErrInvalidTransferAmount
	}

	// Execute transfer within a transaction
	return s.db.Transaction(func(tx *gorm.DB) error {
		// Create a temporary repository for this transaction
		txRepo := repository.NewAccountRepository(tx)

		// Get source account
		fromAccount, err := txRepo.GetByID(userID, fromAccountID)
		if err != nil {
			if errors.Is(err, repository.ErrAccountNotFound) {
				return fmt.Errorf("source account not found: %w", ErrAccountNotFound)
			}
			return fmt.Errorf("failed to get source account: %w", err)
		}
		// Redundant check removed

		// Get destination account
		toAccount, err := txRepo.GetByID(userID, toAccountID)
		if err != nil {
			if errors.Is(err, repository.ErrAccountNotFound) {
				return fmt.Errorf("destination account not found: %w", ErrAccountNotFound)
			}
			return fmt.Errorf("failed to get destination account: %w", err)
		}
		// Redundant check removed

		// Calculate new balances
		newFromBalance := fromAccount.Balance - amount
		newToBalance := toAccount.Balance + amount

		// Check if source account can have negative balance
		if !fromAccount.IsCredit && newFromBalance < 0 {
			return ErrInsufficientBalance
		}

		// Update source account balance
		if err := txRepo.UpdateBalance(userID, fromAccountID, newFromBalance); err != nil {
			return fmt.Errorf("failed to update source account balance: %w", err)
		}

		// Update destination account balance
		if err := txRepo.UpdateBalance(userID, toAccountID, newToBalance); err != nil {
			return fmt.Errorf("failed to update destination account balance: %w", err)
		}

		return nil
	})
}

// GetAssetOverview calculates and returns the asset overview
// Total Assets = sum of all positive balances
// Total Liabilities = absolute value of sum of all negative balances
// Net Worth = Total Assets - Total Liabilities
func (s *AccountService) GetAssetOverview(userID uint) (*AssetOverview, error) {
	assets, liabilities, err := s.repo.GetTotalBalance(userID)
	if err != nil {
		return nil, fmt.Errorf("failed to calculate asset overview: %w", err)
	}

	return &AssetOverview{
		TotalAssets:      assets,
		TotalLiabilities: liabilities,
		NetWorth:         assets - liabilities,
	}, nil
}

// UpdateBalance updates the balance of an account
// This method validates that non-credit accounts cannot have negative balance
func (s *AccountService) UpdateBalance(userID uint, id uint, newBalance float64) error {
	// Get the account to check if it's a credit account
	account, err := s.repo.GetByID(userID, id)
	if err != nil {
		if errors.Is(err, repository.ErrAccountNotFound) {
			return ErrAccountNotFound
		}
		return fmt.Errorf("failed to get account: %w", err)
	}

	// Validate balance for non-credit accounts
	if !account.IsCredit && newBalance < 0 {
		return ErrNegativeBalanceNotAllowed
	}

	// Update the balance
	if err := s.repo.UpdateBalance(userID, id, newBalance); err != nil {
		return fmt.Errorf("failed to update balance: %w", err)
	}

	return nil
}

// CanHaveNegativeBalance checks if an account can have a negative balance
func (s *AccountService) CanHaveNegativeBalance(userID uint, id uint) (bool, error) {
	account, err := s.repo.GetByID(userID, id)
	if err != nil {
		if errors.Is(err, repository.ErrAccountNotFound) {
			return false, ErrAccountNotFound
		}
		return false, fmt.Errorf("failed to get account: %w", err)
	}
	return account.IsCredit, nil
}

// ValidateBalanceChange validates if a balance change is allowed for an account
// Returns nil if the change is valid, or an error if not
func (s *AccountService) ValidateBalanceChange(userID uint, id uint, balanceChange float64) error {
	account, err := s.repo.GetByID(userID, id)
	if err != nil {
		if errors.Is(err, repository.ErrAccountNotFound) {
			return ErrAccountNotFound
		}
		return fmt.Errorf("failed to get account: %w", err)
	}

	newBalance := account.Balance + balanceChange
	if !account.IsCredit && newBalance < 0 {
		return ErrInsufficientBalance
	}

	return nil
}

// GetCreditAccounts retrieves all credit-type accounts
func (s *AccountService) GetCreditAccounts(userID uint) ([]models.Account, error) {
	accounts, err := s.repo.GetCreditAccounts(userID)
	if err != nil {
		return nil, fmt.Errorf("failed to get credit accounts: %w", err)
	}
	return accounts, nil
}

// GetAccountsByType retrieves all accounts of a specific type
func (s *AccountService) GetAccountsByType(userID uint, accountType models.AccountType) ([]models.Account, error) {
	accounts, err := s.repo.GetByType(userID, accountType)
	if err != nil {
		return nil, fmt.Errorf("failed to get accounts by type: %w", err)
	}
	return accounts, nil
}

// GetAccountsByCurrency retrieves all accounts with a specific currency
func (s *AccountService) GetAccountsByCurrency(userID uint, currency models.Currency) ([]models.Account, error) {
	accounts, err := s.repo.GetByCurrency(userID, currency)
	if err != nil {
		return nil, fmt.Errorf("failed to get accounts by currency: %w", err)
	}
	return accounts, nil
}

// ReorderAccountsInput represents the input for reordering accounts
type ReorderAccountsInput struct {
	AccountIDs []uint `json:"account_ids" binding:"required"`
}

// ReorderAccounts updates the sort order of accounts based on the provided order
// Feature: accounting-feature-upgrade
// Validates: Requirements 1.3, 1.4
func (s *AccountService) ReorderAccounts(userID uint, accountIDs []uint) error {
	// Validate that all account IDs exist and belong to the user
	for _, id := range accountIDs {
		_, err := s.repo.GetByID(userID, id)
		if err != nil {
			if errors.Is(err, repository.ErrAccountNotFound) {
				return ErrAccountNotFound
			}
			return fmt.Errorf("failed to check account existence: %w", err)
		}
	}

	// Update sort order for each account within a transaction
	return s.db.Transaction(func(tx *gorm.DB) error {
		txRepo := repository.NewAccountRepository(tx)
		for i, id := range accountIDs {
			if err := txRepo.UpdateSortOrder(userID, id, i); err != nil {
				return fmt.Errorf("failed to update sort order for account %d: %w", id, err)
			}
		}
		return nil
	})
}