package models

import (
	"context"
	"database/sql"
	"errors"
	"fmt"
	"time"
)

// ErrUserNotFound is a sentinel error returned by any lookup method below
// when no matching row exists. Callers use errors.Is(err, ErrUserNotFound)
// to distinguish "not found" (an expected, normal outcome - e.g. checking
// if an email is already registered) from a real database failure.
var ErrUserNotFound = errors.New("user not found")

// UserRepository is the ONLY place in the entire application that writes
// SQL queries for the users table. Every handler that needs to read or
// write user data goes through here instead of touching *sql.DB directly.
//
// Why bother with this layer instead of just calling db.Query in handlers?
//   - If you ever swap MySQL for Postgres (or add a cache in front), you
//     change this one file - no handler code needs to know or care.
//   - It gives handlers a small, purpose-built vocabulary (Create,
//     FindByEmail, FindByID, SetGoogleID) instead of raw SQL strings
//     scattered across the codebase.
type UserRepository struct {
	db *sql.DB
}

// NewUserRepository is the constructor. Go doesn't have classes/constructors
// built into the language - "NewXxx returns a *Xxx" is just a naming
// convention the whole ecosystem follows.
func NewUserRepository(db *sql.DB) *UserRepository {
	return &UserRepository{db: db}
}

// Create inserts a new user row. It takes a *User (pointer) specifically so
// it can write the newly generated auto-increment ID back into the
// caller's struct after the insert succeeds - the caller passes in a User
// with ID == 0, and walks away with u.ID populated.
func (r *UserRepository) Create(ctx context.Context, u *User) error {
	res, err := r.db.ExecContext(ctx,
		"INSERT INTO users (email, password_hash, google_id, created_at) VALUES (?, ?, ?, ?)",
		u.Email, u.PasswordHash, u.GoogleID, time.Now(),
	)
	if err != nil {
		return fmt.Errorf("create user: %w", err)
	}

	id, err := res.LastInsertId()
	if err != nil {
		return fmt.Errorf("get last insert id: %w", err)
	}
	u.ID = int(id)
	return nil
}

// FindByEmail looks up a user by their email address. This is what the
// login flow uses: the user submits an email, we look up the matching row,
// then compare the submitted password against the stored PasswordHash.
func (r *UserRepository) FindByEmail(ctx context.Context, email string) (*User, error) {
	var u User
	err := r.db.QueryRowContext(ctx,
		"SELECT id, email, password_hash, google_id, created_at FROM users WHERE email = ?", email,
	).Scan(&u.ID, &u.Email, &u.PasswordHash, &u.GoogleID, &u.CreatedAt)

	if errors.Is(err, sql.ErrNoRows) {
		// sql.ErrNoRows is the driver's own sentinel for "query matched
		// zero rows". We translate it into OUR sentinel (ErrUserNotFound)
		// so callers never need to know or care that the underlying
		// storage is SQL at all.
		return nil, ErrUserNotFound
	}
	if err != nil {
		return nil, fmt.Errorf("find user by email: %w", err)
	}
	return &u, nil
}

// FindByID looks up a user by their numeric ID. This is used after a
// session is validated: the session only stores the user's ID, so we look
// the rest of the user up fresh on every authenticated request (see
// middleware.RequireAuth).
func (r *UserRepository) FindByID(ctx context.Context, id int) (*User, error) {
	var u User
	err := r.db.QueryRowContext(ctx,
		"SELECT id, email, password_hash, google_id, created_at FROM users WHERE id = ?", id,
	).Scan(&u.ID, &u.Email, &u.PasswordHash, &u.GoogleID, &u.CreatedAt)

	if errors.Is(err, sql.ErrNoRows) {
		return nil, ErrUserNotFound
	}
	if err != nil {
		return nil, fmt.Errorf("find user by id: %w", err)
	}
	return &u, nil
}

// SetGoogleID links a Google account to an existing user row - used the
// first time a user who originally registered with a password logs in via
// "Sign in with Google" using the same email address.
func (r *UserRepository) SetGoogleID(ctx context.Context, userID int, googleID string) error {
	_, err := r.db.ExecContext(ctx,
		"UPDATE users SET google_id = ? WHERE id = ?", googleID, userID,
	)
	if err != nil {
		return fmt.Errorf("set google id: %w", err)
	}
	return nil
}