package auth

import (
	"context"
	"crypto/rand"
	"crypto/subtle"
	"encoding/hex"
	"errors"
	"fmt"
	"strings"
	"time"

	"github.com/jackc/pgx/v5"
	"github.com/jackc/pgx/v5/pgxpool"
	"golang.org/x/crypto/argon2"

	"github.com/jacek/pamietnik/backend/internal/domain"
)

const sessionDuration = 24 * time.Hour

var ErrInvalidCredentials = errors.New("invalid username or password")
var ErrSessionNotFound = errors.New("session not found or expired")

type Store struct {
	pool *pgxpool.Pool
}

func NewStore(pool *pgxpool.Pool) *Store {
	return &Store{pool: pool}
}

// HashPassword returns an argon2id hash of the password.
func HashPassword(password string) (string, error) {
	salt := make([]byte, 16)
	if _, err := rand.Read(salt); err != nil {
		return "", fmt.Errorf("generate salt: %w", err)
	}
	hash := argon2.IDKey([]byte(password), salt, 1, 64*1024, 4, 32)
	return fmt.Sprintf("$argon2id$%x$%x", salt, hash), nil
}

// VerifyPassword checks password against stored hash.
// Format: $argon2id$<saltHex>$<hashHex>
func VerifyPassword(password, stored string) bool {
	parts := strings.Split(stored, "$")
	// ["", "argon2id", "<saltHex>", "<hashHex>"]
	if len(parts) != 4 || parts[1] != "argon2id" {
		return false
	}
	salt, err := hex.DecodeString(parts[2])
	if err != nil {
		return false
	}
	expected, err := hex.DecodeString(parts[3])
	if err != nil {
		return false
	}
	hash := argon2.IDKey([]byte(password), salt, 1, 64*1024, 4, 32)
	return subtle.ConstantTimeCompare(hash, expected) == 1
}

// Login verifies credentials and creates a session.
func (s *Store) Login(ctx context.Context, username, password string) (domain.Session, error) {
	var user domain.User
	err := s.pool.QueryRow(ctx,
		`SELECT user_id, username, password_hash FROM users WHERE username = $1`,
		username,
	).Scan(&user.UserID, &user.Username, &user.PasswordHash)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			return domain.Session{}, ErrInvalidCredentials
		}
		return domain.Session{}, err
	}

	if !VerifyPassword(password, user.PasswordHash) {
		return domain.Session{}, ErrInvalidCredentials
	}

	sessionID, err := newSessionID()
	if err != nil {
		return domain.Session{}, fmt.Errorf("create session: %w", err)
	}
	now := time.Now().UTC()
	sess := domain.Session{
		SessionID: sessionID,
		UserID:    user.UserID,
		CreatedAt: now,
		ExpiresAt: now.Add(sessionDuration),
	}

	_, err = s.pool.Exec(ctx,
		`INSERT INTO sessions (session_id, user_id, created_at, expires_at)
		 VALUES ($1, $2, $3, $4)`,
		sess.SessionID, sess.UserID, sess.CreatedAt, sess.ExpiresAt,
	)
	if err != nil {
		return domain.Session{}, err
	}
	return sess, nil
}

// GetSession validates a session and returns user_id.
func (s *Store) GetSession(ctx context.Context, sessionID string) (domain.Session, error) {
	var sess domain.Session
	err := s.pool.QueryRow(ctx,
		`SELECT session_id, user_id, created_at, expires_at
		 FROM sessions
		 WHERE session_id = $1 AND expires_at > NOW()`,
		sessionID,
	).Scan(&sess.SessionID, &sess.UserID, &sess.CreatedAt, &sess.ExpiresAt)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			return domain.Session{}, ErrSessionNotFound
		}
		return domain.Session{}, err
	}
	return sess, nil
}

// Logout deletes a session.
func (s *Store) Logout(ctx context.Context, sessionID string) error {
	_, err := s.pool.Exec(ctx, `DELETE FROM sessions WHERE session_id = $1`, sessionID)
	if err != nil {
		return fmt.Errorf("delete session: %w", err)
	}
	return nil
}

func newSessionID() (string, error) {
	b := make([]byte, 32)
	if _, err := rand.Read(b); err != nil {
		return "", fmt.Errorf("generate session id: %w", err)
	}
	return hex.EncodeToString(b), nil
}