"use strict";

/***********************************************************************************************************************************************************************
 * File Name: units.controller.js  (admin)
 * Type of Program: Controller
 * Description: Standalone Unit library — Units live independently of Courses.
 *
 *   /admin/units                     → library CRUD (list / create / update / archive / restore / permanent delete)
 *   /admin/units/:unitId/lessons     → attach / detach / reorder standalone Lessons on this Unit
 *   /admin/units/:unitId/quiz        → the Unit's quiz (travels with the Unit into every course it's attached to)
 *
 *   Membership in a course is a course_units row (managed from the course builder);
 *   archiving here removes the Unit from every course view at once.
 *
 * Author: Kenneth Obsequio (@lash0000)
 * Date Created: Jul. 7, 2026 (junction revamp — Units/Lessons run independently)
 ***********************************************************************************************************************************************************************/

const { Op } = require("sequelize");
const sequelize = require("../../config/db.config");
const R = require("../../utils/response.util");
const { paginate } = require("../../utils/paginate.util");
const { archiveOne, archiveMany } = require("../../utils/courses/archive.util");
const { restoreOne, restoreMany } = require("../../utils/courses/restore.util");
const { permanentDeleteOne, permanentDeleteMany } = require("../../utils/courses/permanentDelete.util");
const { getFieldValues } = require("../../utils/fieldValues.util");
const { flattenLessons, nextOrderIndex, reorderJunction } = require("../../utils/courses/hierarchy.util");
const { recomputeUnitDuration, recomputeCourseDuration } = require("../../utils/duration.util");
const { syncObjectivesCreate } = require("../../utils/courses/objectives.util");
const logActivity = require("../../utils/logActivity.util");

// ── Models ────────────────────────────────────────────────────────────────────

const {
  Course, Unit, Lesson, LessonPage,
  CourseUnit, UnitLesson, LessonObjective,
  UnitQuiz, QuizQuestion, QuizOption,
  UnitReadingProgress, LessonReadingProgress,
} = require("../../models/courses/courses.associations");
const CompletionRequirement = require("../../models/courses/completion_requirement.mdl");
const { VALID_ENTITY_TYPES } = require("../../utils/courses/completion_requirements.registry");

const mdl_Users = require("../../models/users/users.mdl");

const notDeleted  = { deletedAt: null };
const onlyDeleted = { deletedAt: { [Op.not]: null } };

// Refresh duration of every course this unit is attached to (post-archive/restore).
async function recomputeParentCourseDurations(unitId) {
  const links = await CourseUnit.findAll({ where: { unit_id: unitId }, attributes: ["course_id"] });
  for (const courseId of new Set(links.map((l) => String(l.course_id)))) {
    await recomputeCourseDuration(courseId);
  }
}

const UNIT_LIST_COMPUTED = [
  {
    key: "quiz_id",
    label: "Quiz ID",
    type: "text",
    literal: `(SELECT quiz_id FROM unit_quizzes WHERE unit_id = "Unit"."unit_id" AND "deletedAt" IS NULL LIMIT 1)`,
  },
  {
    key: "lesson_count",
    label: "Lessons",
    type: "number",
    literal: `(
      SELECT CAST(COUNT(*) AS INTEGER)
      FROM unit_lessons ul
      JOIN lessons l ON l.lesson_id = ul.lesson_id AND l."deletedAt" IS NULL
      WHERE ul.unit_id = "Unit"."unit_id"
    )`,
  },
  {
    // Not its own column — consumed by the Title cell on the frontend to
    // show a "Course" badge when a unit is already attached to at least one Course.
    key: "course_count",
    label: "Affiliated",
    type: "number",
    order: 2, // 1: Title, 2: Affiliated, 3: Subscription, 4: Duration — see units.mdl.js
    hidden: true,
    literal: `(
      SELECT CAST(COUNT(*) AS INTEGER)
      FROM course_units cu
      JOIN courses c ON c.course_id = cu.course_id AND c."deletedAt" IS NULL
      WHERE cu.unit_id = "Unit"."unit_id"
    )`,
  },
  {
    key: "course_status",
    label: "Course Status",
    type: "text",
    order: 3, // 1: Title, 2: Affiliated, 3: Course Status, 4: Subscription, 5: Duration — see units.mdl.js
    hidden: true,
    filterable: false,
    literal: `(
      CASE
        WHEN NOT EXISTS (
          SELECT 1 FROM course_units cu
          JOIN courses c ON c.course_id = cu.course_id AND c."deletedAt" IS NULL
          WHERE cu.unit_id = "Unit"."unit_id"
        ) THEN 'standalone'
        WHEN EXISTS (
          SELECT 1 FROM course_units cu
          JOIN courses c ON c.course_id = cu.course_id AND c."deletedAt" IS NULL
          WHERE cu.unit_id = "Unit"."unit_id" AND c.status = 'published'
        ) THEN 'published'
        ELSE 'draft'
      END
    )`,
  },
  {
    // Not its own column — consumed by the frontend's Subscription cell so a
    // unit gated only through an affiliated course (own `subscription` is
    // NULL) still shows a tier instead of "-". Distinct tiers across every
    // affiliated course, comma-joined (a unit can sit in several courses at
    // different tiers).
    key: "course_subscription",
    label: "Course Subscription",
    type: "text",
    hidden: true,
    filterable: false,
    literal: `(
      SELECT STRING_AGG(sub.subscription, ', ')
      FROM (
        SELECT DISTINCT c.subscription
        FROM course_units cu
        JOIN courses c ON c.course_id = cu.course_id AND c."deletedAt" IS NULL
        WHERE cu.unit_id = "Unit"."unit_id"
      ) sub
    )`,
  },
];

// ══════════════════════════════════════════════════════════════════════════════
// UNIT LIBRARY
// ══════════════════════════════════════════════════════════════════════════════

exports.getUnits = async (req, res) => {
  try {
    const result = await paginate(Unit, req, {
      auditOptions: { mdl_Users, parentAlias: "Unit" },
      context: "list",
      computedAttributes: UNIT_LIST_COMPUTED,
      findOptions: {
        where: { ...notDeleted },
        order: [["createdAt", "DESC"]],
      },
    });
    return R.success(res, "Units retrieved.", result);
  } catch (err) {
    console.error("[UNIT LIB][GET ALL]", err);
    return R.error(res, "Could not retrieve units.", 500);
  }
};

// Lightweight list for attach pickers: { unit_id, uuid, title, lesson_count, course_count, course_title }
exports.getUnitsFlat = async (req, res) => {
  try {
    const rows = await sequelize.query(`
      SELECT
        u.unit_id, u.uuid, u.title, u.description, u.duration_seconds,
        (SELECT CAST(COUNT(*) AS INTEGER) FROM unit_lessons ul
           JOIN lessons l ON l.lesson_id = ul.lesson_id AND l."deletedAt" IS NULL
          WHERE ul.unit_id = u.unit_id)                       AS lesson_count,
        (SELECT CAST(COUNT(*) AS INTEGER) FROM course_units cu
           JOIN courses c ON c.course_id = cu.course_id AND c."deletedAt" IS NULL
          WHERE cu.unit_id = u.unit_id)                       AS course_count,
        (SELECT c.title FROM course_units cu
           JOIN courses c ON c.course_id = cu.course_id AND c."deletedAt" IS NULL
          WHERE cu.unit_id = u.unit_id
          LIMIT 1)                                            AS course_title
      FROM units u
      WHERE u."deletedAt" IS NULL
      ORDER BY u.title ASC
    `, { type: sequelize.QueryTypes.SELECT });
    return R.success(res, "Units retrieved.", rows);
  } catch (err) {
    console.error("[UNIT LIB][GET FLAT]", err);
    return R.error(res, "Could not retrieve units.", 500);
  }
};

// Bundle picker (Tier Plan wizard) — mirrors getCoursesBySubscription in
// controllers/admin/courses.controller.js.
exports.getUnitsBySubscription = async (req, res) => {
  try {
    const { slug } = req.query;
    if (!slug) return R.error(res, 'slug query param is required.', 400);

    const rows = await Unit.findAll({
      where: { ...notDeleted, subscription: slug },
      attributes: ['unit_id', 'title', 'description', 'subscription'],
      order: [['title', 'ASC']],
    });

    // A unit may belong to any number of other plans (Tier Plans v2, silent
    // duplication across bundles is intentional) — no conflict to report here.
    const data = rows.map((u) => u.toJSON());

    return R.success(res, 'Units retrieved.', data);
  } catch (err) {
    console.error('[UNIT LIB][BY SUBSCRIPTION]', err);
    return R.error(res, 'Could not retrieve units.', 500);
  }
};

exports.getUnit = async (req, res) => {
  try {
    const { unitId } = req.params;

    const unit = await Unit.findOne({
      where: { unit_id: unitId, ...notDeleted },
      include: [
        { model: Lesson,   as: "lessons", where: notDeleted, required: false, through: { attributes: ["order_index"] } },
        { model: UnitQuiz, as: "quiz",    required: false },
        { model: Course,   as: "courses", where: notDeleted, required: false, attributes: ["course_id", "uuid", "title", "subscription"], through: { attributes: ["order_index"] } },
      ],
    });

    if (!unit) return R.error(res, "Unit not found.", 404);

    const plain = unit.toJSON();
    plain.lessons = flattenLessons(plain.lessons);
    return R.success(res, "Unit retrieved.", { data: plain });
  } catch (err) {
    console.error("[UNIT LIB][GET ONE]", err);
    return R.error(res, "Could not retrieve unit.", 500);
  }
};

exports.createUnit = async (req, res) => {
  const t = await sequelize.transaction();
  try {
    const { title, description, subscription, course_id, order, createdBy } = req.body;
    if (!title) return R.error(res, "Title is required.", 400);

    const unit = await Unit.create({
      title,
      subscription:      subscription || null,
      description:      description ?? null,
      duration_seconds: 0,
      createdBy:        createdBy ?? req.user?.user_id ?? null,
    }, { transaction: t });

    // Optional immediate attach — lets the course builder create-and-attach in one call
    if (course_id) {
      const course = await Course.findOne({ where: { course_id, ...notDeleted }, transaction: t });
      if (!course) {
        await t.rollback();
        return R.error(res, "Course not found.", 404);
      }
      const order_index = order ?? await nextOrderIndex(CourseUnit, { course_id }, t);
      await CourseUnit.create({
        course_id,
        unit_id: unit.unit_id,
        order_index,
        createdBy: createdBy ?? req.user?.user_id ?? null,
      }, { transaction: t });
    }

    await t.commit();
    logActivity(req.user?.user_id, "create_unit", { entityType: "unit", entityId: unit.unit_id, details: { title: unit.title, attached_course_id: course_id ?? null } });
    return R.success(res, "Unit created.", { data: unit }, 201);
  } catch (err) {
    await t.rollback();
    console.error("[UNIT LIB][CREATE]", err);
    return R.error(res, "Could not create unit.", 500);
  }
};

// One consolidated call: creates the unit, its lessons (with objectives + page
// blocks), and attaches each lesson to the unit — all in a single transaction.
// Body: { title, description, subscription,
//         lessons: [{ title, description, objectives?: string[], blocks?: [] }],
//         createdBy }
const CREATE_UNIT_FULL_LIMITS = { maxLessons: 50, maxObjectives: 20, maxBlocks: 100 };

exports.createUnitFull = async (req, res) => {
  const t = await sequelize.transaction();
  try {
    const { title, description, subscription, lessons = [], createdBy } = req.body;

    if (!title) return R.error(res, "Title is required.", 400);
    if (!Array.isArray(lessons)) return R.error(res, "Lessons must be a list.", 400);
    if (lessons.length > CREATE_UNIT_FULL_LIMITS.maxLessons) {
      return R.error(res, `A unit can have at most ${CREATE_UNIT_FULL_LIMITS.maxLessons} lessons.`, 400);
    }

    for (const lessonInput of lessons) {
      if (!lessonInput?.title) return R.error(res, "Each lesson needs a title.", 400);
      if (lessonInput.objectives !== undefined && !Array.isArray(lessonInput.objectives)) {
        return R.error(res, "Lesson objectives must be a list.", 400);
      }
      if (lessonInput.objectives?.length > CREATE_UNIT_FULL_LIMITS.maxObjectives) {
        return R.error(res, `A lesson can have at most ${CREATE_UNIT_FULL_LIMITS.maxObjectives} objectives.`, 400);
      }
      if (lessonInput.blocks !== undefined && !Array.isArray(lessonInput.blocks)) {
        return R.error(res, "Lesson page blocks must be a list.", 400);
      }
      if (lessonInput.blocks?.length > CREATE_UNIT_FULL_LIMITS.maxBlocks) {
        return R.error(res, `A lesson page can have at most ${CREATE_UNIT_FULL_LIMITS.maxBlocks} blocks.`, 400);
      }
      if (lessonInput.requirements !== undefined) {
        if (!Array.isArray(lessonInput.requirements)) return R.error(res, "Lesson requirements must be a list.", 400);
        const seenTypes = new Set();
        for (const r of lessonInput.requirements) {
          const allowedEntityTypes = VALID_ENTITY_TYPES[r.type];
          if (!allowedEntityTypes) return R.error(res, `Unknown requirement type "${r.type}".`, 400);
          if (!allowedEntityTypes.includes("lesson")) return R.error(res, `"${r.type}" cannot be configured on a lesson.`, 400);
          if (seenTypes.has(r.type)) return R.error(res, `Duplicate "${r.type}" requirement — only one per lesson is allowed.`, 400);
          seenTypes.add(r.type);
        }
      }
    }

    const by = createdBy ?? req.user?.user_id ?? null;

    const unit = await Unit.create({
      title,
      subscription:      subscription || null,
      description:       description ?? null,
      duration_seconds:  0,
      createdBy:         by,
    }, { transaction: t });

    for (let i = 0; i < lessons.length; i++) {
      const lessonInput = lessons[i];

      const lesson = await Lesson.create({
        title:             lessonInput.title,
        description:       lessonInput.description ?? null,
        duration_seconds:  0,
        createdBy:         by,
      }, { transaction: t });

      await LessonPage.create({
        lesson_id: lesson.lesson_id,
        blocks:    lessonInput.blocks ?? [],
        createdBy: by,
      }, { transaction: t });

      await syncObjectivesCreate(LessonObjective, "lesson_id", lesson.lesson_id, lessonInput.objectives ?? [], t);

      if (lessonInput.requirements?.length) {
        const rows = lessonInput.requirements.map((r, ri) => ({
          entity_type:  "lesson",
          entity_id:    lesson.lesson_id,
          type:         r.type,
          order:        r.order ?? ri,
          min_percent:  r.type === "watch_percent"   ? (r.min_percent ?? 100) : null,
          button_label: r.type === "manual_complete" ? (r.button_label || null) : null,
          is_required:  r.is_required ?? true,
          createdBy:    by,
          updatedBy:    by,
        }));
        await CompletionRequirement.bulkCreate(rows, { transaction: t });
      }

      await UnitLesson.create({
        unit_id:     unit.unit_id,
        lesson_id:   lesson.lesson_id,
        order_index: i,
        createdBy:   by,
      }, { transaction: t });
    }

    await t.commit();

    if (lessons.length) {
      try { await recomputeUnitDuration(unit.unit_id); }
      catch (durErr) { console.error("[UNIT LIB][CREATE FULL][DURATION]", durErr); }
    }

    logActivity(req.user?.user_id, "create_unit", { entityType: "unit", entityId: unit.unit_id, details: { title: unit.title, lessons: lessons.length } });
    return R.success(res, "Unit created.", { data: unit }, 201);
  } catch (err) {
    await t.rollback();
    console.error("[UNIT LIB][CREATE FULL]", err);
    return R.error(res, "Could not create unit.", 500);
  }
};

exports.updateUnit = async (req, res) => {
  try {
    const { unitId } = req.params;

    const unit = await Unit.findOne({ where: { unit_id: unitId, ...notDeleted } });
    if (!unit) return R.error(res, "Unit not found.", 404);

    const { title, description, subscription, updatedBy } = req.body;

    if (title        !== undefined) unit.title        = title;
    if (description  !== undefined) unit.description  = description;
    if (subscription !== undefined) unit.subscription = subscription || null;
    unit.updatedBy = updatedBy ?? req.user?.user_id ?? null;

    await unit.save();
    logActivity(req.user?.user_id, "update_unit", { entityType: "unit", entityId: Number(unitId) });
    return R.success(res, "Unit updated.", { data: unit });
  } catch (err) {
    console.error("[UNIT LIB][UPDATE]", err);
    return R.error(res, "Could not update unit.", 500);
  }
};

exports.getUnitArchiveImpact = async (req, res) => {
  try {
    const { unitId } = req.params;

    const [completionCount, progressCount, courseCount] = await Promise.all([
      UnitReadingProgress.count({ where: { unit_id: unitId, status: "completed" } }),
      LessonReadingProgress.count({ where: { unit_id: unitId }, distinct: true, col: "user_id" }),
      CourseUnit.count({ where: { unit_id: unitId } }),
    ]);

    return R.success(res, "Impact retrieved.", { completionCount, progressCount, courseCount });
  } catch (err) {
    console.error("[UNIT LIB][ARCHIVE IMPACT]", err);
    return R.error(res, "Could not retrieve impact.", 500);
  }
};

exports.archiveUnit = async (req, res) => {
  const t = await sequelize.transaction();
  try {
    const { unitId } = req.params;
    const record = await archiveOne(Unit, { unit_id: unitId, ...notDeleted }, req.user.user_id, t);
    if (!record) return R.error(res, "Unit not found.", 404);
    await t.commit();
    try { await recomputeParentCourseDurations(unitId); } catch (durErr) { console.error("[UNIT LIB][ARCHIVE][DURATION]", durErr); }
    logActivity(req.user.user_id, "archive_unit", { entityType: "unit", entityId: Number(unitId) });
    return R.success(res, "Unit archived.");
  } catch (err) {
    await t.rollback();
    console.error("[UNIT LIB][ARCHIVE]", err);
    return R.error(res, "Could not archive unit.", 500);
  }
};

exports.bulkArchiveUnits = async (req, res) => {
  const t = await sequelize.transaction();
  try {
    const { ids = [] } = req.body;
    if (!ids.length) return R.error(res, "No IDs provided.", 400);

    const units = await Unit.findAll({ where: { unit_id: ids, ...notDeleted } });
    const validIds = units.map((u) => u.unit_id);

    const count = await archiveMany(Unit, "unit_id", validIds, req.user.user_id, t);
    await t.commit();
    for (const id of validIds) {
      try { await recomputeParentCourseDurations(id); } catch (durErr) { console.error("[UNIT LIB][BULK ARCHIVE][DURATION]", durErr); }
    }
    logActivity(req.user.user_id, "bulk_archive_units", { entityType: "unit", details: { ids: validIds, count } });
    return R.success(res, `${count} unit${count !== 1 ? "s" : ""} archived.`);
  } catch (err) {
    await t.rollback();
    console.error("[UNIT LIB][BULK ARCHIVE]", err);
    return R.error(res, "Could not archive units.", 500);
  }
};

exports.getArchivedUnits = async (req, res) => {
  try {
    const result = await paginate(Unit, req, {
      auditOptions: { mdl_Users, parentAlias: "Unit" },
      context: "archived",
      findOptions: {
        where: { ...onlyDeleted },
        paranoid: false,
        order: [["deletedAt", "DESC"]],
      },
    });
    return R.success(res, "Archived units retrieved.", result);
  } catch (err) {
    console.error("[UNIT LIB][GET ARCHIVES]", err);
    return R.error(res, "Could not retrieve archived units.", 500);
  }
};

exports.getArchivedUnit = async (req, res) => {
  try {
    const { unitId } = req.params;
    const unit = await Unit.findOne({
      where: { unit_id: unitId, ...onlyDeleted },
      paranoid: false,
    });
    if (!unit) return R.error(res, "Archived unit not found.", 404);
    return R.success(res, "Archived unit retrieved.", { data: unit.toJSON() });
  } catch (err) {
    console.error("[UNIT LIB][GET ARCHIVE ONE]", err);
    return R.error(res, "Could not retrieve archived unit.", 500);
  }
};

exports.restoreUnit = async (req, res) => {
  const t = await sequelize.transaction();
  try {
    const { unitId } = req.params;
    const record = await restoreOne(Unit, { unit_id: unitId, ...onlyDeleted }, req.user.user_id, t);
    if (!record) return R.error(res, "Archived unit not found.", 404);
    await t.commit();
    try { await recomputeParentCourseDurations(unitId); } catch (durErr) { console.error("[UNIT LIB][RESTORE][DURATION]", durErr); }
    logActivity(req.user.user_id, "restore_unit", { entityType: "unit", entityId: Number(unitId) });
    return R.success(res, "Unit restored.", { data: record });
  } catch (err) {
    await t.rollback();
    console.error("[UNIT LIB][RESTORE]", err);
    return R.error(res, "Could not restore unit.", 500);
  }
};

exports.bulkRestoreUnits = async (req, res) => {
  const t = await sequelize.transaction();
  try {
    const { ids = [] } = req.body;
    if (!ids.length) return R.error(res, "No IDs provided.", 400);

    const units = await Unit.findAll({ where: { unit_id: ids, ...onlyDeleted }, paranoid: false });
    const validIds = units.map((u) => u.unit_id);

    const count = await restoreMany(Unit, "unit_id", validIds, req.user.user_id, t);
    await t.commit();
    for (const id of validIds) {
      try { await recomputeParentCourseDurations(id); } catch (durErr) { console.error("[UNIT LIB][BULK RESTORE][DURATION]", durErr); }
    }
    logActivity(req.user.user_id, "bulk_restore_units", { entityType: "unit", details: { ids: validIds, count } });
    return R.success(res, `${count} unit${count !== 1 ? "s" : ""} restored.`);
  } catch (err) {
    await t.rollback();
    console.error("[UNIT LIB][BULK RESTORE]", err);
    return R.error(res, "Could not restore units.", 500);
  }
};

exports.getUnitPermanentDeleteImpact = async (req, res) => {
  try {
    const { unitId } = req.params;

    const [lessonCount, courseCount] = await Promise.all([
      UnitLesson.count({ where: { unit_id: unitId } }),
      CourseUnit.count({ where: { unit_id: unitId } }),
    ]);

    return R.success(res, "Impact retrieved.", { lessonCount, courseCount });
  } catch (err) {
    console.error("[UNIT LIB][PERMANENT DELETE IMPACT]", err);
    return R.error(res, "Could not retrieve impact.", 500);
  }
};

exports.permanentlyDeleteUnit = async (req, res) => {
  const t = await sequelize.transaction();
  try {
    const { unitId } = req.params;
    const record = await permanentDeleteOne(Unit, { unit_id: unitId }, t);
    if (record === null) return R.error(res, "Unit not found.", 404);
    if (record === false) return R.error(res, "Unit must be archived before it can be permanently deleted.", 400);

    // Junction rows don't cascade from a paranoid destroy — clean them explicitly
    await CourseUnit.destroy({ where: { unit_id: unitId }, transaction: t });
    await UnitLesson.destroy({ where: { unit_id: unitId }, transaction: t });

    await t.commit();
    logActivity(req.user.user_id, "permanently_delete_unit", { entityType: "unit", entityId: Number(unitId) });
    return R.success(res, "Unit permanently deleted.");
  } catch (err) {
    await t.rollback();
    console.error("[UNIT LIB][PERMANENT DELETE]", err);
    return R.error(res, "Could not permanently delete unit.", 500);
  }
};

exports.bulkPermanentlyDeleteUnits = async (req, res) => {
  const t = await sequelize.transaction();
  try {
    const { ids = [] } = req.body;
    if (!ids.length) return R.error(res, "No IDs provided.", 400);

    const units = await Unit.findAll({ where: { unit_id: ids }, paranoid: false });
    const validIds = units.map((u) => u.unit_id);

    const count = await permanentDeleteMany(Unit, "unit_id", validIds, t);
    await CourseUnit.destroy({ where: { unit_id: validIds }, transaction: t });
    await UnitLesson.destroy({ where: { unit_id: validIds }, transaction: t });

    await t.commit();
    logActivity(req.user.user_id, "bulk_permanently_delete_units", { entityType: "unit", details: { ids: validIds, count } });
    return R.success(res, `${count} unit${count !== 1 ? "s" : ""} permanently deleted.`);
  } catch (err) {
    await t.rollback();
    console.error("[UNIT LIB][BULK PERMANENT DELETE]", err);
    return R.error(res, "Could not permanently delete units.", 500);
  }
};

exports.getUnitFieldValues = getFieldValues(Unit, "UNIT");

// ══════════════════════════════════════════════════════════════════════════════
// UNIT ⇄ LESSON MEMBERSHIP (attach / detach / reorder)
// ══════════════════════════════════════════════════════════════════════════════

// POST /admin/units/:unitId/lessons  { lesson_ids: [..] } — append existing lessons
exports.attachLessons = async (req, res) => {
  const t = await sequelize.transaction();
  try {
    const { unitId } = req.params;
    const { lesson_ids = [] } = req.body;
    if (!lesson_ids.length) return R.error(res, "lesson_ids is required.", 400);

    const unit = await Unit.findOne({ where: { unit_id: unitId, ...notDeleted }, transaction: t });
    if (!unit) return R.error(res, "Unit not found.", 404);

    const lessons = await Lesson.findAll({ where: { lesson_id: lesson_ids, ...notDeleted }, transaction: t });
    if (lessons.length !== lesson_ids.length) {
      await t.rollback();
      return R.error(res, "One or more lessons were not found.", 404);
    }

    const existing = await UnitLesson.findAll({ where: { unit_id: unitId, lesson_id: lesson_ids }, transaction: t });
    const existingSet = new Set(existing.map((r) => String(r.lesson_id)));
    const toAttach = lesson_ids.filter((id) => !existingSet.has(String(id)));

    let order = await nextOrderIndex(UnitLesson, { unit_id: unitId }, t);
    await UnitLesson.bulkCreate(
      toAttach.map((lesson_id) => ({
        unit_id: unitId,
        lesson_id,
        order_index: order++,
        createdBy: req.user?.user_id ?? null,
      })),
      { transaction: t }
    );

    await t.commit();
    try { await recomputeUnitDuration(unitId); await recomputeParentCourseDurations(unitId); } catch (durErr) { console.error("[UNIT LIB][ATTACH LESSONS][DURATION]", durErr); }
    logActivity(req.user?.user_id, "attach_lessons", { entityType: "unit", entityId: Number(unitId), details: { lesson_ids: toAttach } });
    return R.success(res, `${toAttach.length} lesson${toAttach.length !== 1 ? "s" : ""} attached.`, { attached: toAttach, skipped: lesson_ids.filter((id) => existingSet.has(String(id))) });
  } catch (err) {
    await t.rollback();
    console.error("[UNIT LIB][ATTACH LESSONS]", err);
    return R.error(res, "Could not attach lessons.", 500);
  }
};

// DELETE /admin/units/:unitId/lessons/:lessonId — detach (lesson survives in the library)
exports.detachLesson = async (req, res) => {
  try {
    const { unitId, lessonId } = req.params;

    const removed = await UnitLesson.destroy({ where: { unit_id: unitId, lesson_id: lessonId } });
    if (!removed) return R.error(res, "Lesson is not attached to this unit.", 404);

    try { await recomputeUnitDuration(unitId); await recomputeParentCourseDurations(unitId); } catch (durErr) { console.error("[UNIT LIB][DETACH LESSON][DURATION]", durErr); }
    logActivity(req.user?.user_id, "detach_lesson", { entityType: "unit", entityId: Number(unitId), details: { lesson_id: Number(lessonId) } });
    return R.success(res, "Lesson detached.");
  } catch (err) {
    console.error("[UNIT LIB][DETACH LESSON]", err);
    return R.error(res, "Could not detach lesson.", 500);
  }
};

// PUT /admin/units/:unitId/lessons/order  { lesson_ids: [orderedIds] }
exports.reorderLessons = async (req, res) => {
  const t = await sequelize.transaction();
  try {
    const { unitId } = req.params;
    const { lesson_ids = [] } = req.body;
    if (!lesson_ids.length) return R.error(res, "lesson_ids is required.", 400);

    const unit = await Unit.findOne({ where: { unit_id: unitId, ...notDeleted }, transaction: t });
    if (!unit) return R.error(res, "Unit not found.", 404);

    await reorderJunction(UnitLesson, "unit_id", unitId, "lesson_id", lesson_ids, t);
    await t.commit();
    logActivity(req.user?.user_id, "reorder_lessons", { entityType: "unit", entityId: Number(unitId), details: { lesson_ids } });
    return R.success(res, "Lesson order updated.");
  } catch (err) {
    await t.rollback();
    console.error("[UNIT LIB][REORDER LESSONS]", err);
    return R.error(res, "Could not reorder lessons.", 500);
  }
};

// ══════════════════════════════════════════════════════════════════════════════
// UNIT QUIZ  (1:1 with the Unit — travels with it into every attached course)
// ══════════════════════════════════════════════════════════════════════════════

exports.getQuiz = async (req, res) => {
  try {
    const { unitId } = req.params;

    const unit = await Unit.findOne({ where: { unit_id: unitId, ...notDeleted } });
    if (!unit) return R.error(res, "Unit not found.", 404);

    const quiz = await UnitQuiz.findOne({
      where: { unit_id: unitId, ...notDeleted },
      include: [{
        model: QuizQuestion, as: "questions",
        where: notDeleted, required: false,
        include: [{ model: QuizOption, as: "options", order: [["order_index", "ASC"]] }],
      }],
    });

    if (!quiz) return R.error(res, "Quiz not found.", 404);
    return R.success(res, "Quiz retrieved.", { data: quiz });
  } catch (err) {
    console.error("[UNIT LIB][QUIZ][GET]", err);
    return R.error(res, "Could not retrieve quiz.", 500);
  }
};

exports.createQuiz = async (req, res) => {
  try {
    const { unitId } = req.params;
    const { title, is_required, passing_score, max_questions, shuffle_questions, createdBy } = req.body;

    const unit = await Unit.findOne({ where: { unit_id: unitId, ...notDeleted } });
    if (!unit) return R.error(res, "Unit not found.", 404);

    const existing = await UnitQuiz.findOne({ where: { unit_id: unitId, ...notDeleted } });
    if (existing) return R.error(res, "Quiz already exists for this unit.", 409);

    const quiz = await UnitQuiz.create({
      unit_id:           unitId,
      title:             title ?? null,
      is_required:       is_required ?? false,
      passing_score:     passing_score ?? 70,
      max_questions:     max_questions ?? null,
      shuffle_questions: shuffle_questions ?? false,
      createdBy:         createdBy ?? req.user?.user_id ?? null,
    });

    logActivity(req.user?.user_id, "create_quiz", { entityType: "quiz", entityId: quiz.quiz_id });
    return R.success(res, "Quiz created.", { data: quiz }, 201);
  } catch (err) {
    console.error("[UNIT LIB][QUIZ][CREATE]", err);
    return R.error(res, "Could not create quiz.", 500);
  }
};

exports.updateQuiz = async (req, res) => {
  try {
    const { unitId, quizId } = req.params;
    const { title, is_required, passing_score, max_questions, shuffle_questions, updatedBy } = req.body;

    const quiz = await UnitQuiz.findOne({ where: { quiz_id: quizId, unit_id: unitId, ...notDeleted } });
    if (!quiz) return R.error(res, "Quiz not found.", 404);

    if (title !== undefined)             quiz.title = title;
    if (is_required !== undefined)       quiz.is_required = is_required;
    if (passing_score !== undefined)     quiz.passing_score = passing_score;
    if (max_questions !== undefined)     quiz.max_questions = max_questions;
    if (shuffle_questions !== undefined) quiz.shuffle_questions = shuffle_questions;
    quiz.updatedBy = updatedBy ?? req.user?.user_id ?? null;

    await quiz.save();
    logActivity(req.user?.user_id, "update_quiz", { entityType: "quiz", entityId: Number(quizId) });
    return R.success(res, "Quiz updated.", { data: quiz });
  } catch (err) {
    console.error("[UNIT LIB][QUIZ][UPDATE]", err);
    return R.error(res, "Could not update quiz.", 500);
  }
};

exports.deleteQuiz = async (req, res) => {
  const t = await sequelize.transaction();
  try {
    const { unitId, quizId } = req.params;
    const record = await archiveOne(UnitQuiz, { quiz_id: quizId, unit_id: unitId, ...notDeleted }, req.user?.user_id, t);
    if (!record) return R.error(res, "Quiz not found.", 404);
    await t.commit();
    logActivity(req.user?.user_id, "archive_quiz", { entityType: "quiz", entityId: Number(quizId) });
    return R.success(res, "Quiz archived.");
  } catch (err) {
    await t.rollback();
    console.error("[UNIT LIB][QUIZ][ARCHIVE]", err);
    return R.error(res, "Could not archive quiz.", 500);
  }
};

exports.getArchivedQuiz = async (req, res) => {
  try {
    const { unitId } = req.params;

    const unit = await Unit.findOne({ where: { unit_id: unitId }, paranoid: false });
    if (!unit) return R.error(res, "Unit not found.", 404);

    const quiz = await UnitQuiz.findOne({
      where: { unit_id: unitId, ...onlyDeleted },
      paranoid: false,
    });
    if (!quiz) return R.error(res, "Archived quiz not found.", 404);
    return R.success(res, "Archived quiz retrieved.", { data: quiz.toJSON() });
  } catch (err) {
    console.error("[UNIT LIB][QUIZ][GET ARCHIVE]", err);
    return R.error(res, "Could not retrieve archived quiz.", 500);
  }
};

exports.restoreQuiz = async (req, res) => {
  const t = await sequelize.transaction();
  try {
    const { unitId, quizId } = req.params;
    const record = await restoreOne(UnitQuiz, { quiz_id: quizId, unit_id: unitId, ...onlyDeleted }, req.user?.user_id, t);
    if (!record) return R.error(res, "Archived quiz not found.", 404);
    await t.commit();
    logActivity(req.user?.user_id, "restore_quiz", { entityType: "quiz", entityId: Number(quizId) });
    return R.success(res, "Quiz restored.", { data: record });
  } catch (err) {
    await t.rollback();
    console.error("[UNIT LIB][QUIZ][RESTORE]", err);
    return R.error(res, "Could not restore quiz.", 500);
  }
};