package db

import (
	"crypto/rand"
	"crypto/sha256"
	"database/sql"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"strings"
	"time"

	"qbank/internal/models"
)

const timeLayout = "2006-01-02 15:04:05"

func parseTime(s string) time.Time {
	t, _ := time.Parse(timeLayout, s)
	return t
}

func parseNullTime(s sql.NullString) sql.NullTime {
	if !s.Valid {
		return sql.NullTime{}
	}
	t, err := time.Parse(timeLayout, s.String)
	if err != nil {
		return sql.NullTime{}
	}
	return sql.NullTime{Time: t, Valid: true}
}

// QuestionID computes the canonical ID for a question from its text.
func QuestionID(text string) string {
	h := sha256.Sum256([]byte(text))
	return fmt.Sprintf("%x", h[:8])
}

type SortOrder int

const (
	SortAlpha    SortOrder = iota // alphabetical by question text
	SortWeakest                   // lowest accuracy first (requires UserID)
	SortMostSeen                  // most-seen first (requires UserID)
)

type ListFilter struct {
	Source string
	Search string
	Sort   SortOrder
	UserID int64
}

type Repo struct {
	db *sql.DB
}

func New(db *sql.DB) *Repo {
	return &Repo{db: db}
}

func (r *Repo) CreateUser(name, passwordHash string) (int64, error) {
	res, err := r.db.Exec("INSERT INTO users (name, password_hash) VALUES (?, ?)", name, passwordHash)
	if err != nil {
		return 0, err
	}
	return res.LastInsertId()
}

func (r *Repo) GetUserByName(name string) (*models.User, error) {
	u := &models.User{}
	var createdAt string
	err := r.db.QueryRow(
		"SELECT id, name, password_hash, created_at FROM users WHERE name = ?", name,
	).Scan(&u.ID, &u.Name, &u.PasswordHash, &createdAt)
	if err != nil {
		return nil, err
	}
	u.CreatedAt = parseTime(createdAt)
	return u, nil
}

// InsertQuestion inserts q and its answers in a transaction. Duplicate questions
// (same text hash) are silently ignored; their answers are not re-inserted.
func (r *Repo) InsertQuestion(q *models.Question, answers []*models.Answer) error {
	q.ID = QuestionID(q.Text)
	tx, err := r.db.Begin()
	if err != nil {
		return err
	}
	defer tx.Rollback()

	res, err := tx.Exec(
		"INSERT OR IGNORE INTO questions (id, text, source) VALUES (?, ?, ?)",
		q.ID, q.Text, q.Source,
	)
	if err != nil {
		return err
	}
	if n, _ := res.RowsAffected(); n == 0 {
		return tx.Commit() // already exists
	}

	for i, a := range answers {
		a.QuestionID = q.ID
		a.Position = i
		res, err := tx.Exec(
			"INSERT INTO answers (question_id, text, is_correct, position) VALUES (?, ?, ?, ?)",
			a.QuestionID, a.Text, a.IsCorrect, a.Position,
		)
		if err != nil {
			return err
		}
		a.ID, _ = res.LastInsertId()
	}
	return tx.Commit()
}

func (r *Repo) GetQuestion(id string) (*models.Question, []*models.Answer, error) {
	q := &models.Question{}
	var createdAt string
	err := r.db.QueryRow(
		"SELECT id, text, source, created_at FROM questions WHERE id = ?", id,
	).Scan(&q.ID, &q.Text, &q.Source, &createdAt)
	if err != nil {
		return nil, nil, err
	}
	q.CreatedAt = parseTime(createdAt)

	rows, err := r.db.Query(
		"SELECT id, question_id, text, is_correct, position FROM answers WHERE question_id = ? ORDER BY position",
		id,
	)
	if err != nil {
		return nil, nil, err
	}
	defer rows.Close()

	var answers []*models.Answer
	for rows.Next() {
		a := &models.Answer{}
		if err := rows.Scan(&a.ID, &a.QuestionID, &a.Text, &a.IsCorrect, &a.Position); err != nil {
			return nil, nil, err
		}
		answers = append(answers, a)
	}
	return q, answers, rows.Err()
}

func (r *Repo) ListQuestions(f ListFilter) ([]*models.Question, error) {
	var where []string
	var args []any

	if f.Source != "" {
		where = append(where, "source = ?")
		args = append(args, f.Source)
	}
	if f.Search != "" {
		where = append(where, "text LIKE ?")
		args = append(args, "%"+f.Search+"%")
	}

	query := "SELECT id, text, source, created_at FROM questions"
	if len(where) > 0 {
		query += " WHERE " + strings.Join(where, " AND ")
	}
	query += " ORDER BY text COLLATE NOCASE ASC"

	rows, err := r.db.Query(query, args...)
	if err != nil {
		return nil, err
	}
	defer rows.Close()

	var qs []*models.Question
	for rows.Next() {
		q := &models.Question{}
		var createdAt string
		if err := rows.Scan(&q.ID, &q.Text, &q.Source, &createdAt); err != nil {
			return nil, err
		}
		q.CreatedAt = parseTime(createdAt)
		qs = append(qs, q)
	}
	return qs, rows.Err()
}

func (r *Repo) ListSources() ([]string, error) {
	rows, err := r.db.Query(
		"SELECT DISTINCT source FROM questions WHERE source != '' ORDER BY source",
	)
	if err != nil {
		return nil, err
	}
	defer rows.Close()

	var sources []string
	for rows.Next() {
		var s string
		if err := rows.Scan(&s); err != nil {
			return nil, err
		}
		sources = append(sources, s)
	}
	return sources, rows.Err()
}

func (r *Repo) CountQuestions() (int, error) {
	var n int
	return n, r.db.QueryRow("SELECT COUNT(*) FROM questions").Scan(&n)
}

func (r *Repo) CountAnswers() (int, error) {
	var n int
	return n, r.db.QueryRow("SELECT COUNT(*) FROM answers").Scan(&n)
}

func (r *Repo) CreateTest(userID int64, questionIDs []string) (int64, error) {
	ids, err := json.Marshal(questionIDs)
	if err != nil {
		return 0, err
	}
	res, err := r.db.Exec(
		"INSERT INTO tests (user_id, n_questions, question_ids) VALUES (?, ?, ?)",
		userID, len(questionIDs), string(ids),
	)
	if err != nil {
		return 0, err
	}
	return res.LastInsertId()
}

func (r *Repo) RecordAnswer(testID int64, questionID string, selectedAnswerID *int64, isCorrect bool) error {
	_, err := r.db.Exec(`
		INSERT INTO test_answers (test_id, question_id, selected_answer_id, is_correct, answered_at)
		VALUES (?, ?, ?, ?, CURRENT_TIMESTAMP)
		ON CONFLICT (test_id, question_id) DO UPDATE SET
			selected_answer_id = excluded.selected_answer_id,
			is_correct         = excluded.is_correct,
			answered_at        = excluded.answered_at`,
		testID, questionID, selectedAnswerID, isCorrect,
	)
	return err
}

func (r *Repo) FinishTest(id int64) error {
	_, err := r.db.Exec("UPDATE tests SET completed_at = CURRENT_TIMESTAMP WHERE id = ?", id)
	return err
}

func (r *Repo) GetTest(id int64) (*models.Test, error) {
	t := &models.Test{}
	var createdAt string
	var completedAt sql.NullString
	var ids string

	err := r.db.QueryRow(
		"SELECT id, user_id, created_at, completed_at, n_questions, question_ids FROM tests WHERE id = ?", id,
	).Scan(&t.ID, &t.UserID, &createdAt, &completedAt, &t.NQuestions, &ids)
	if err != nil {
		return nil, err
	}
	t.CreatedAt = parseTime(createdAt)
	t.CompletedAt = parseNullTime(completedAt)
	if err := json.Unmarshal([]byte(ids), &t.QuestionIDs); err != nil {
		return nil, err
	}
	return t, nil
}

func (r *Repo) ListTestsForUser(userID int64) ([]*models.Test, error) {
	rows, err := r.db.Query(`
		SELECT id, user_id, created_at, completed_at, n_questions, question_ids
		FROM tests WHERE user_id = ? ORDER BY created_at DESC`, userID,
	)
	if err != nil {
		return nil, err
	}
	defer rows.Close()

	var tests []*models.Test
	for rows.Next() {
		t := &models.Test{}
		var createdAt string
		var completedAt sql.NullString
		var ids string
		if err := rows.Scan(&t.ID, &t.UserID, &createdAt, &completedAt, &t.NQuestions, &ids); err != nil {
			return nil, err
		}
		t.CreatedAt = parseTime(createdAt)
		t.CompletedAt = parseNullTime(completedAt)
		if err := json.Unmarshal([]byte(ids), &t.QuestionIDs); err != nil {
			return nil, err
		}
		tests = append(tests, t)
	}
	return tests, rows.Err()
}

func (r *Repo) GetTestAnswers(testID int64) ([]*models.TestAnswer, error) {
	rows, err := r.db.Query(`
		SELECT test_id, question_id, selected_answer_id, is_correct, answered_at
		FROM test_answers WHERE test_id = ?`, testID,
	)
	if err != nil {
		return nil, err
	}
	defer rows.Close()

	var answers []*models.TestAnswer
	for rows.Next() {
		ta := &models.TestAnswer{}
		var answeredAt sql.NullString
		if err := rows.Scan(&ta.TestID, &ta.QuestionID, &ta.SelectedAnswerID, &ta.IsCorrect, &answeredAt); err != nil {
			return nil, err
		}
		ta.AnsweredAt = parseNullTime(answeredAt)
		answers = append(answers, ta)
	}
	return answers, rows.Err()
}

func (r *Repo) UpsertStat(userID int64, questionID string, gotItRight bool) error {
	correct := 0
	if gotItRight {
		correct = 1
	}
	_, err := r.db.Exec(`
		INSERT INTO user_question_stats (user_id, question_id, times_seen, times_correct, last_seen_at)
		VALUES (?, ?, 1, ?, CURRENT_TIMESTAMP)
		ON CONFLICT (user_id, question_id) DO UPDATE SET
			times_seen    = times_seen + 1,
			times_correct = times_correct + excluded.times_correct,
			last_seen_at  = CURRENT_TIMESTAMP`,
		userID, questionID, correct,
	)
	return err
}

func (r *Repo) GetStatsForUser(userID int64, questionIDs []string) (map[string]*models.UserQuestionStat, error) {
	result := make(map[string]*models.UserQuestionStat, len(questionIDs))
	if len(questionIDs) == 0 {
		return result, nil
	}

	placeholders := make([]string, len(questionIDs))
	args := make([]any, 0, len(questionIDs)+1)
	args = append(args, userID)
	for i, id := range questionIDs {
		placeholders[i] = "?"
		args = append(args, id)
	}

	rows, err := r.db.Query(fmt.Sprintf(`
		SELECT user_id, question_id, times_seen, times_correct, last_seen_at
		FROM user_question_stats
		WHERE user_id = ? AND question_id IN (%s)`,
		strings.Join(placeholders, ",")),
		args...,
	)
	if err != nil {
		return nil, err
	}
	defer rows.Close()

	for rows.Next() {
		s := &models.UserQuestionStat{}
		var lastSeen sql.NullString
		if err := rows.Scan(&s.UserID, &s.QuestionID, &s.TimesSeen, &s.TimesCorrect, &lastSeen); err != nil {
			return nil, err
		}
		s.LastSeenAt = parseNullTime(lastSeen)
		result[s.QuestionID] = s
	}
	return result, rows.Err()
}

// ── Draft (import review) ────────────────────────────────────────────────────

func newDraftID() string {
	b := make([]byte, 16)
	rand.Read(b)
	return hex.EncodeToString(b)
}

func (r *Repo) CreateDraft(userID int64, source string, questions []models.DraftQuestion) (string, error) {
	data, err := json.Marshal(questions)
	if err != nil {
		return "", err
	}
	id := newDraftID()
	_, err = r.db.Exec(
		"INSERT INTO import_drafts (id, user_id, source, questions) VALUES (?, ?, ?, ?)",
		id, userID, source, string(data),
	)
	if err != nil {
		return "", err
	}
	return id, nil
}

func (r *Repo) GetDraftForUser(id string, userID int64) (*models.Draft, error) {
	d := &models.Draft{}
	var questionsJSON, createdAt string
	err := r.db.QueryRow(
		"SELECT id, user_id, source, questions, created_at FROM import_drafts WHERE id = ? AND user_id = ?",
		id, userID,
	).Scan(&d.ID, &d.UserID, &d.Source, &questionsJSON, &createdAt)
	if err != nil {
		return nil, err
	}
	d.CreatedAt = parseTime(createdAt)
	if err := json.Unmarshal([]byte(questionsJSON), &d.Questions); err != nil {
		return nil, err
	}
	return d, nil
}

func (r *Repo) DeleteDraft(id string) error {
	_, err := r.db.Exec("DELETE FROM import_drafts WHERE id = ?", id)
	return err
}