// Package auth provides authentication services.
package auth

import (
	"context"
	"crypto/rand"
	"encoding/base64"
	"errors"
	"fmt"
	"log/slog"
	"net/http"
	"strings"
	"time"

	"github.com/gorilla/sessions"
	"go.uber.org/fx"
	"golang.org/x/crypto/argon2"

	"git.eeqj.de/sneak/upaas/internal/config"
	"git.eeqj.de/sneak/upaas/internal/database"
	"git.eeqj.de/sneak/upaas/internal/logger"
	"git.eeqj.de/sneak/upaas/internal/models"
)

const (
	sessionName   = "upaas_session"
	sessionUserID = "user_id"
)

// Argon2 parameters.
const (
	argonTime    = 1
	argonMemory  = 64 * 1024
	argonThreads = 4
	argonKeyLen  = 32
	saltLen      = 16
)

// Session duration constants.
const (
	sessionMaxAgeDays    = 7
	sessionMaxAgeSeconds = 86400 * sessionMaxAgeDays
)

var (
	// ErrInvalidCredentials is returned when username/password is incorrect.
	ErrInvalidCredentials = errors.New("invalid credentials")
	// ErrUserExists is returned when trying to create a user that already exists.
	ErrUserExists = errors.New("user already exists")
)

// ServiceParams contains dependencies for Service.
type ServiceParams struct {
	fx.In

	Logger   *logger.Logger
	Config   *config.Config
	Database *database.Database
}

// Service provides authentication functionality.
type Service struct {
	log    *slog.Logger
	db     *database.Database
	store  *sessions.CookieStore
	params *ServiceParams
}

// New creates a new auth Service.
func New(_ fx.Lifecycle, params ServiceParams) (*Service, error) {
	store := sessions.NewCookieStore([]byte(params.Config.SessionSecret))
	store.Options = &sessions.Options{
		Path:     "/",
		MaxAge:   sessionMaxAgeSeconds,
		HttpOnly: true,
		SameSite: http.SameSiteLaxMode,
	}

	return &Service{
		log:    params.Logger.Get(),
		db:     params.Database,
		store:  store,
		params: &params,
	}, nil
}

// HashPassword hashes a password using Argon2id.
func (svc *Service) HashPassword(password string) (string, error) {
	salt := make([]byte, saltLen)

	_, err := rand.Read(salt)
	if err != nil {
		return "", fmt.Errorf("failed to generate salt: %w", err)
	}

	hash := argon2.IDKey(
		[]byte(password),
		salt,
		argonTime,
		argonMemory,
		argonThreads,
		argonKeyLen,
	)

	// Encode as base64: salt$hash
	saltB64 := base64.StdEncoding.EncodeToString(salt)
	hashB64 := base64.StdEncoding.EncodeToString(hash)

	return saltB64 + "$" + hashB64, nil
}

// VerifyPassword verifies a password against a hash.
func (svc *Service) VerifyPassword(hashedPassword, password string) bool {
	// Parse salt$hash format using strings.Cut (more reliable than fmt.Sscanf)
	saltB64, hashB64, found := strings.Cut(hashedPassword, "$")
	if !found || saltB64 == "" || hashB64 == "" {
		return false
	}

	salt, err := base64.StdEncoding.DecodeString(saltB64)
	if err != nil {
		return false
	}

	expectedHash, err := base64.StdEncoding.DecodeString(hashB64)
	if err != nil {
		return false
	}

	// Compute hash with same parameters
	computedHash := argon2.IDKey(
		[]byte(password),
		salt,
		argonTime,
		argonMemory,
		argonThreads,
		argonKeyLen,
	)

	// Constant-time comparison
	if len(computedHash) != len(expectedHash) {
		return false
	}

	var result byte

	for idx := range computedHash {
		result |= computedHash[idx] ^ expectedHash[idx]
	}

	return result == 0
}

// IsSetupRequired checks if initial setup is needed (no users exist).
func (svc *Service) IsSetupRequired(ctx context.Context) (bool, error) {
	exists, err := models.UserExists(ctx, svc.db)
	if err != nil {
		return false, fmt.Errorf("failed to check if user exists: %w", err)
	}

	return !exists, nil
}

// CreateUser creates the initial admin user.
func (svc *Service) CreateUser(
	ctx context.Context,
	username, password string,
) (*models.User, error) {
	// Check if user already exists
	exists, err := models.UserExists(ctx, svc.db)
	if err != nil {
		return nil, fmt.Errorf("failed to check if user exists: %w", err)
	}

	if exists {
		return nil, ErrUserExists
	}

	// Hash password
	hash, err := svc.HashPassword(password)
	if err != nil {
		return nil, fmt.Errorf("failed to hash password: %w", err)
	}

	// Create user
	user := models.NewUser(svc.db)
	user.Username = username
	user.PasswordHash = hash

	err = user.Save(ctx)
	if err != nil {
		return nil, fmt.Errorf("failed to save user: %w", err)
	}

	svc.log.Info("user created", "username", username)

	return user, nil
}

// Authenticate validates credentials and returns the user.
func (svc *Service) Authenticate(
	ctx context.Context,
	username, password string,
) (*models.User, error) {
	user, err := models.FindUserByUsername(ctx, svc.db, username)
	if err != nil {
		return nil, fmt.Errorf("failed to find user: %w", err)
	}

	if user == nil {
		return nil, ErrInvalidCredentials
	}

	if !svc.VerifyPassword(user.PasswordHash, password) {
		return nil, ErrInvalidCredentials
	}

	return user, nil
}

// CreateSession creates a session for the user.
func (svc *Service) CreateSession(
	respWriter http.ResponseWriter,
	request *http.Request,
	user *models.User,
) error {
	session, err := svc.store.Get(request, sessionName)
	if err != nil {
		return fmt.Errorf("failed to get session: %w", err)
	}

	session.Values[sessionUserID] = user.ID

	saveErr := session.Save(request, respWriter)
	if saveErr != nil {
		return fmt.Errorf("failed to save session: %w", saveErr)
	}

	return nil
}

// GetCurrentUser returns the currently logged-in user, or nil if not logged in.
//
//nolint:nilerr // Session errors are not propagated - they indicate no user
func (svc *Service) GetCurrentUser(
	ctx context.Context,
	request *http.Request,
) (*models.User, error) {
	session, sessionErr := svc.store.Get(request, sessionName)
	if sessionErr != nil {
		// Session error means no user - this is not an error condition
		return nil, nil //nolint:nilnil // Expected behavior for no session
	}

	userID, ok := session.Values[sessionUserID].(int64)
	if !ok {
		return nil, nil //nolint:nilnil // No user ID in session is valid
	}

	user, err := models.FindUser(ctx, svc.db, userID)
	if err != nil {
		return nil, fmt.Errorf("failed to find user: %w", err)
	}

	return user, nil
}

// DestroySession destroys the current session.
func (svc *Service) DestroySession(
	respWriter http.ResponseWriter,
	request *http.Request,
) error {
	session, err := svc.store.Get(request, sessionName)
	if err != nil {
		return fmt.Errorf("failed to get session: %w", err)
	}

	session.Options.MaxAge = -1 * int(time.Second)

	saveErr := session.Save(request, respWriter)
	if saveErr != nil {
		return fmt.Errorf("failed to save session: %w", saveErr)
	}

	return nil
}