/**
 * Builds and parses the site's page paths, by the one rule they all follow:
 * the path is a hierarchy, `@name` is a user (`@name@host` one whose account
 * lives on another instance), and any other word under the
 * root is a dump category or an app page, `/playlists` holding the playlist
 * categories. Under a user each segment is a fixed tab name, and under `dumps`
 * or `playlists` a slug. A view of a page is one more segment, and a default
 * view is the bare path. A category narrowing a list on another page, a
 * profile's or a search's, is a query, `?category=`, not a segment.
 *
 * The client links every page through the builders (`src/utils/shared/urls.ts`
 * wraps the dump and playlist ones), the emails prefix them with the
 * frontend's URL, and the server's matchers (the Open Graph tags, the
 * previews and backlinks of links to this site) read paths with
 * {@link parsePagePath}. Migration 0021 rewrote the v3 links in stored text
 * with the builders.
 *
 * @module
 */

import type { CategoryKind } from "./category.ts";
import { DIGEST_CADENCES, type DigestCadence } from "./settings.ts";
import { HANDLE_RE, splitHandle } from "./validation.ts";

/**
 * The feed tabs, in the tab bar's order. Each is a path segment: `/hot` for
 * every dump, `/music/hot` for a category. They are reserved at the root, so
 * no category can take one.
 */
export const FEED_TABS = ["hot", "new", "journal", "follow"] as const;
/** One of the {@link FEED_TABS}. */
export type FeedTab = (typeof FEED_TABS)[number];

/**
 * The tabs of a profile, each a segment after `/@name`. The bare `/@name` is
 * the overview.
 */
export const PROFILE_TABS = [
  "dumps",
  "upvoted",
  "playlists",
  "follow",
  "invitees",
  "manage",
] as const;
/** One of the {@link PROFILE_TABS}. */
export type ProfileTab = (typeof PROFILE_TABS)[number];

/**
 * The views of a `follow` tab, of what is followed: the users, at the bare
 * tab, or the playlists. A profile's lists them, `/@name/follow` and
 * `/@name/follow/playlists`, and a feed's shows their dumps, `/follow` and
 * `/follow/playlists`.
 */
export const FOLLOW_VIEWS = ["users", "playlists"] as const;
/** One of the {@link FOLLOW_VIEWS}. */
export type FollowView = (typeof FOLLOW_VIEWS)[number];

/**
 * The tabs of the search page: the dumps at `/search`, the others at
 * `/search/<tab>`.
 */
export const SEARCH_TABS = ["dumps", "users", "playlists"] as const;
/** One of the {@link SEARCH_TABS}. */
export type SearchTab = (typeof SEARCH_TABS)[number];

/**
 * The views of the community page, one per digest cadence: the week at
 * `/community`, the month at `/community/monthly`.
 */
export const COMMUNITY_VIEWS: readonly DigestCadence[] = DIGEST_CADENCES;

/** Passed to {@link dumpPath} and {@link playlistPath}, usually as the item. */
export interface OwnedItem {
  ownerUsername: string;
  slug: string;
}

const seg = encodeURIComponent;

/**
 * Writes a handle as a path segment. A host's characters are all safe in a
 * path, so `/@alice@music.example` keeps its `@` rather than `%40`.
 */
function handleSeg(handle: string): string {
  const { name, host } = splitHandle(handle);
  return host ? `${seg(name)}@${host}` : seg(name);
}

/**
 * Returns the path of `username`'s profile, of one of its tabs, or of a view
 * of the `follow` tab. The default view, `users`, is the bare tab.
 */
export function userPath(
  username: string,
  tab?: ProfileTab,
  view?: FollowView,
): string {
  const base = `/@${handleSeg(username)}`;
  if (!tab) return base;
  return view && view !== FOLLOW_VIEWS[0]
    ? `${base}/${tab}/${view}`
    : `${base}/${tab}`;
}

/**
 * Returns the path of a dump's page, `/@owner/dumps/<slug>`, or of its
 * editor, one segment more.
 */
export function dumpPath(dump: OwnedItem, view?: "edit"): string {
  const path = `${userPath(dump.ownerUsername)}/dumps/${seg(dump.slug)}`;
  return view ? `${path}/${view}` : path;
}

/**
 * Returns the path of a playlist's page, `/@owner/playlists/<slug>`, or of
 * the page with its editor open, one segment more.
 */
export function playlistPath(playlist: OwnedItem, view?: "edit"): string {
  const path = `${userPath(playlist.ownerUsername)}/playlists/${
    seg(playlist.slug)
  }`;
  return view ? `${path}/${view}` : path;
}

/**
 * Returns the path of a feed: every dump's at the root, or a category's at
 * `/<slug>`, on `tab`, or on the viewer's default tab without one. The
 * `follow` tab takes a `view`, the users followed by default.
 */
export function feedPath(
  categorySlug?: string,
  tab?: FeedTab,
  view?: FollowView,
): string {
  const base = categorySlug ? `/${seg(categorySlug)}` : "";
  if (!tab) return base || "/";
  return tab === "follow" && view && view !== FOLLOW_VIEWS[0]
    ? `${base}/${tab}/${view}`
    : `${base}/${tab}`;
}

/**
 * Returns the path of the page of every public playlist, `/playlists`, or of a
 * playlist category's at `/playlists/<slug>`.
 */
export function playlistsPath(categorySlug?: string): string {
  return categorySlug ? `/playlists/${seg(categorySlug)}` : "/playlists";
}

/**
 * Returns the path of the page of the category of `kind` at `slug`: a dump
 * category's feed ({@link feedPath}), or a playlist category's playlists
 * ({@link playlistsPath}). The category chips link to it.
 */
export function categoryPath(kind: CategoryKind, slug: string): string {
  return kind === "dump" ? feedPath(slug) : playlistsPath(slug);
}

/**
 * Returns the path of the search page on `tab`, searching `query` when it is
 * given.
 */
export function searchPath(tab: SearchTab = "dumps", query?: string): string {
  const base = tab === SEARCH_TABS[0] ? "/search" : `/search/${tab}`;
  return query ? `${base}?q=${seg(query)}` : base;
}

/** Returns the path of the community page on `view`, the week by default. */
export function communityPath(view: DigestCadence = "weekly"): string {
  return view === COMMUNITY_VIEWS[0] ? "/community" : `/community/${view}`;
}

/**
 * Returned by {@link parsePagePath} for a path under a user. `rest` holds the
 * segments after `/@name`, a profile tab and its view.
 */
export type PagePath =
  | { kind: "user"; username: string; rest: string[] }
  | { kind: "dump"; username: string; slug: string; edit: boolean }
  | { kind: "playlist"; username: string; slug: string; edit: boolean };

/**
 * Returns what a page path under a user names: a profile (with its tab), a
 * dump or a playlist (or its editor). It returns null for any other path, a
 * feed or an app page, and for one under a user that no page answers. The
 * names are decoded but not looked up, so the user or the item may not exist.
 */
export function parsePagePath(pathname: string): PagePath | null {
  let segments: string[];
  try {
    segments = pathname.replace(/\/+$/, "").split("/").slice(1).map(
      decodeURIComponent,
    );
  } catch {
    return null;
  }
  const [handle, section, slug, ...more] = segments;
  if (!handle?.startsWith("@") || !HANDLE_RE.test(handle.slice(1))) {
    return null;
  }
  const username = handle.slice(1);

  if ((section === "dumps" || section === "playlists") && slug) {
    if (more.length > 1 || (more.length === 1 && more[0] !== "edit")) {
      return null;
    }
    const kind = section === "dumps" ? "dump" : "playlist";
    return { kind, username, slug, edit: more.length === 1 };
  }

  const rest = segments.slice(1);
  if (rest.length === 0) return { kind: "user", username, rest };
  if (!(PROFILE_TABS as readonly string[]).includes(rest[0])) return null;
  if (rest.length === 1) return { kind: "user", username, rest };
  if (
    rest.length === 2 && rest[0] === "follow" &&
    (FOLLOW_VIEWS as readonly string[]).includes(rest[1]) &&
    rest[1] !== FOLLOW_VIEWS[0]
  ) return { kind: "user", username, rest };
  return null;
}