/**
 * Stores the categories and which dumps and playlists belong to them, a set
 * of categories for each {@link CategoryKind}. The category routes manage and
 * list them, the dump and playlist services file each dump or playlist they
 * write under its categories, and every service that lists dumps or playlists
 * fills in their `categoryIds` and filters them by category with
 * {@link categoryJoin}.
 *
 * @module
 */

import { APIException } from "../lib/api-exception.ts";
import type {
  Category,
  CategoryKind,
  CreateCategoryRequest,
  UpdateCategoryRequest,
} from "../../shared/category.ts";
import type { Dump } from "../../shared/dump.ts";
import { categoryRowToApi, isCategoryRow } from "../model/category.ts";
import { db, transaction } from "../db/connection.ts";
import {
  DUMP_SELECT_COLUMNS_ALIASED as SELECT_COLS_ALIASED,
  dumpRowToApi,
  isDumpRow,
} from "../model/dump.ts";
import { type Page, pageOf } from "../lib/pagination.ts";
import { isReservedSlug } from "../lib/reserved-slugs.ts";
import { validateSlug } from "../lib/slugify.ts";
import { VALIDATION } from "../../shared/validation.ts";
import { parseRows } from "../model/rows.ts";

/**
 * Names, for each kind, the table that files things under its categories and
 * the column there of the thing filed.
 */
const LINKS: Record<CategoryKind, { table: string; column: string }> = {
  dump: { table: "dump_categories", column: "dump_id" },
  playlist: { table: "playlist_categories", column: "playlist_id" },
};

/**
 * Throws a 400 unless `slug` is a valid slug for a category of `kind`: a slug
 * within a category's length, and for a dump category none of the top-level
 * paths other pages hold (reserved-slugs.ts), since its feed lives at the URL
 * root. A playlist category's lives under `/playlists/`.
 */
function validateCategorySlug(slug: string, kind: CategoryKind): void {
  validateSlug(slug, VALIDATION.CATEGORY_SLUG_MAX);
  if (kind === "dump" && isReservedSlug(slug)) {
    throw new APIException(400, `Slug "${slug}" is reserved`);
  }
}

function requireName(name: string): string {
  const trimmed = name.trim();
  if (!trimmed) {
    throw new APIException(400, "Name is required");
  }
  if (trimmed.length > VALIDATION.CATEGORY_NAME_MAX) {
    throw new APIException(
      400,
      `Name must be at most ${VALIDATION.CATEGORY_NAME_MAX} characters`,
    );
  }
  return trimmed;
}

/**
 * Rethrows a UNIQUE violation from SQLite as a 409 naming the slug or name
 * that is taken, and any other error as is. SQLite's message names the
 * column, so this matches on it.
 */
function uniqueConflict(err: unknown, slug: string, name: string): never {
  const msg = err instanceof Error ? err.message : String(err);
  if (msg.includes("categories.name")) {
    throw new APIException(409, `Name "${name}" is already in use`);
  }
  if (msg.includes("UNIQUE") || msg.includes("unique")) {
    throw new APIException(409, `Slug "${slug}" is already in use`);
  }
  throw err as Error;
}

/**
 * Returns category `id`, for {@link updateCategory}.
 *
 * @throws {APIException} 404 when there is no such category.
 */
export function getCategoryById(id: string): Category {
  const row = db.prepare(`SELECT * FROM categories WHERE id = ?;`).get(id);
  if (!row || !isCategoryRow(row)) {
    throw new APIException(404, "Category not found");
  }
  return categoryRowToApi(row);
}

/**
 * Returns the category of `kind` at `slug`, for {@link listDumpsByCategory}
 * and the list of every public playlist.
 *
 * @throws {APIException} 404 when there is no such category.
 */
export function getCategoryBySlug(kind: CategoryKind, slug: string): Category {
  const row = db.prepare(
    `SELECT * FROM categories WHERE kind = ? AND slug = ?;`,
  ).get(kind, slug);
  if (!row || !isCategoryRow(row)) {
    throw new APIException(404, "Category not found");
  }
  return categoryRowToApi(row);
}

/**
 * Returns the id of the category of `kind` at `slug`, or undefined when there
 * is none.
 */
export function getCategoryIdBySlug(
  kind: CategoryKind,
  slug: string,
): string | undefined {
  const row = db.prepare(
    `SELECT id FROM categories WHERE kind = ? AND slug = ?;`,
  ).get(kind, slug);
  return row && typeof (row as { id?: unknown }).id === "string"
    ? (row as { id: string }).id
    : undefined;
}

/**
 * Returns the id of the category of `kind` whose slug the `name` parameter of
 * `params`, `?category=` by default, gives, undefined without one, or
 * "missing" when it names none, so that the list comes back empty rather than
 * unfiltered. The follows routes filter the index's "followed" tab with it,
 * the users routes a profile's lists, and the search route its results.
 */
export function resolveCategoryFilter(
  kind: CategoryKind,
  params: URLSearchParams,
  name = "category",
): string | "missing" | undefined {
  const slug = params.get(name);
  if (!slug) return undefined;
  return getCategoryIdBySlug(kind, slug) ?? "missing";
}

/**
 * Returns the join that keeps only the rows of `alias`, a dump or playlist as
 * `kind` says, filed under `categoryId`, with its parameters, or nothing
 * without one. A query puts the join after its `FROM` and the parameters
 * before its own.
 */
export function categoryJoin(
  kind: CategoryKind,
  alias: string,
  categoryId?: string,
): [string, string[]] {
  if (!categoryId) return ["", []];
  const { table, column } = LINKS[kind];
  return [
    `INNER JOIN ${table} cat ON cat.${column} = ${alias}.id AND cat.category_id = ?`,
    [categoryId],
  ];
}

/**
 * Returns every category, of both kinds, by kind, position then slug, for
 * `GET /api/categories`.
 */
export function listCategories(): Category[] {
  const rows = db.prepare(
    `SELECT * FROM categories ORDER BY kind ASC, position ASC, slug ASC;`,
  ).all();
  return parseRows(rows, isCategoryRow, categoryRowToApi);
}

/**
 * Creates a category, for `POST /api/categories`. The name is stored trimmed,
 * and the position defaults to 0.
 *
 * @throws {APIException} 400 when the slug or name is invalid, 409 when
 * either is taken within its kind.
 */
export function createCategory(req: CreateCategoryRequest): Category {
  validateCategorySlug(req.slug, req.kind);
  const name = requireName(req.name);

  const id = crypto.randomUUID();
  const createdAt = new Date();
  try {
    db.prepare(
      `INSERT INTO categories (id, kind, slug, name, position, created_at)
       VALUES (?, ?, ?, ?, ?, ?);`,
    ).run(
      id,
      req.kind,
      req.slug,
      name,
      req.position ?? 0,
      createdAt.toISOString(),
    );
  } catch (err) {
    uniqueConflict(err, req.slug, name);
  }

  return {
    id,
    kind: req.kind,
    slug: req.slug,
    name,
    position: req.position ?? 0,
    createdAt,
  };
}

/**
 * Applies `req` to category `id`, for `PUT /api/categories/:id`, and returns
 * the category as it now is. Fields `req` leaves out keep their value.
 *
 * @throws {APIException} 404 when there is no such category, 400 when the new
 * slug or name is invalid, 409 when either is taken within its kind.
 */
export function updateCategory(
  id: string,
  req: UpdateCategoryRequest,
): Category {
  const existing = getCategoryById(id);

  const newSlug = req.slug ?? existing.slug;
  if (req.slug !== undefined && req.slug !== existing.slug) {
    validateCategorySlug(req.slug, existing.kind);
  }
  const newName = req.name !== undefined
    ? requireName(req.name)
    : existing.name;
  const newPosition = req.position ?? existing.position;
  const now = new Date();

  try {
    db.prepare(
      `UPDATE categories SET slug = ?, name = ?, position = ?, updated_at = ? WHERE id = ?;`,
    ).run(
      newSlug,
      newName,
      newPosition,
      now.toISOString(),
      id,
    );
  } catch (err) {
    uniqueConflict(err, newSlug, newName);
  }

  return {
    ...existing,
    slug: newSlug,
    name: newName,
    position: newPosition,
    updatedAt: now,
  };
}

/**
 * Deletes category `id`, for `DELETE /api/categories/:id`. Its dumps or
 * playlists stay, and lose it from their `categoryIds`.
 *
 * @throws {APIException} 404 when there is no such category.
 */
export function deleteCategory(id: string): void {
  const result = db.prepare(`DELETE FROM categories WHERE id = ?;`).run(id);
  if (result.changes === 0) {
    throw new APIException(404, "Category not found");
  }
}

/**
 * Checks that every id in `categoryIds` names a category of `kind`, so that a
 * dump is never filed under a playlist category, nor the other way round. The
 * dump and playlist services run it before they write a row or store an
 * uploaded file, so a bad id fails the request before anything is written.
 *
 * @throws {APIException} 400 when an id names no category of `kind`.
 */
export function assertCategoriesExist(
  kind: CategoryKind,
  categoryIds: string[],
): void {
  const unique = [...new Set(categoryIds)];

  if (unique.length > 0) {
    const placeholders = unique.map(() => "?").join(", ");
    const found = db.prepare(
      `SELECT COUNT(*) as count FROM categories
       WHERE kind = ? AND id IN (${placeholders});`,
    ).get(kind, ...unique) as { count: number } | undefined;
    if ((found?.count ?? 0) !== unique.length) {
      throw new APIException(400, "One or more categories do not exist");
    }
  }
}

/**
 * Replaces the categories of the dump or playlist `id`, as `kind` says, with
 * `categoryIds`, duplicates dropped. An empty list takes it out of every
 * category. Called inside the dump or playlist service's transaction, it
 * joins that one.
 *
 * @throws {APIException} 400 when an id names no category of `kind`.
 */
export function setCategories(
  kind: CategoryKind,
  id: string,
  categoryIds: string[],
): void {
  assertCategoriesExist(kind, categoryIds);
  const unique = [...new Set(categoryIds)];
  const { table, column } = LINKS[kind];

  transaction(() => {
    db.prepare(`DELETE FROM ${table} WHERE ${column} = ?;`).run(id);
    const insert = db.prepare(
      `INSERT INTO ${table} (${column}, category_id) VALUES (?, ?);`,
    );
    for (const categoryId of unique) {
      insert.run(id, categoryId);
    }
  });
}

/**
 * Sets `categoryIds` on each of `items`, dumps or playlists as `kind` says, in
 * place, in one query. Their rows don't carry them, so every service that
 * returns dumps or playlists calls this first.
 */
export function attachCategoryIds(
  kind: CategoryKind,
  items: { id: string; categoryIds?: string[] }[],
): void {
  if (items.length === 0) return;
  const { table, column } = LINKS[kind];
  const ids = items.map((d) => d.id);
  const placeholders = ids.map(() => "?").join(", ");
  const rows = db.prepare(
    `SELECT ${column} as id, category_id FROM ${table} WHERE ${column} IN (${placeholders});`,
  ).all(...ids) as Array<{ id: string; category_id: string }>;

  const byItem = new Map<string, string[]>();
  for (const { id, category_id } of rows) {
    const list = byItem.get(id) ?? [];
    list.push(category_id);
    byItem.set(id, list);
  }
  for (const item of items) {
    item.categoryIds = byItem.get(item.id) ?? [];
  }
}

/**
 * Returns a page of the dumps in the dump category at `slug`, newest first,
 * for `GET /api/dumps?category=`. Private dumps are left out, except
 * `requestingUserId`'s own.
 *
 * @throws {APIException} 404 when there is no such category.
 */
export function listDumpsByCategory(
  slug: string,
  page: number,
  limit: number,
  requestingUserId?: string,
): Page<Dump> {
  const category = getCategoryBySlug("dump", slug);
  const offset = (page - 1) * limit;

  const privacyClause = requestingUserId
    ? "(d.is_private = 0 OR d.user_id = ?)"
    : "d.is_private = 0";
  const privacyParams = requestingUserId ? [requestingUserId] : [];

  const [catJoin, catParams] = categoryJoin("dump", "d", category.id);
  const rows = db.prepare(
    `SELECT ${SELECT_COLS_ALIASED}
     FROM dumps d ${catJoin}
     WHERE ${privacyClause}
     ORDER BY d.created_at DESC LIMIT ? OFFSET ?;`,
  ).all(...catParams, ...privacyParams, limit + 1, offset);

  const items = parseRows(rows, isDumpRow, dumpRowToApi);
  attachCategoryIds("dump", items);
  return pageOf(items, limit);
}