/**
 * The Dashboard's URL space (ADR 0057): what each Dashboard path means.
 *
 * Framework-free and free of browser globals, because two programs read it. The shell parses the
 * address bar with it, and `apps/api` serves the Dashboard page at the platform pages it lists, so a
 * refreshed or bookmarked deep link reaches the shell instead of a `404`.
 *
 * Module pages and platform pages share one top-level namespace:
 *
 * ```text
 * /                         Overview
 * /settings                 Settings
 * /settings/<section>       a platform Settings section: general, branding, displays, system
 * /settings/discover/<view> finding, installing, and managing modules: browse, installed, updates
 * /settings/<module-id>     a module's settings (and later its own deeper settings pages)
 * /account                  your account
 * /account/<section>        a personal settings page: appearance
 * /<module-id>[/…]          a module's page, and later its own sub-pages
 * ```
 *
 * The server serves the page at `/<module-id>` only for a module that is actually installed with a
 * Dashboard UI, and Core refuses any module whose id is a platform-owned name (`RESERVED_MODULE_IDS`
 * in `@ekklesia/core/modules`), so a module page can never shadow a platform page and an arbitrary
 * root path never becomes one.
 */

/**
 * The Dashboard's own top-level pages besides the Overview. Reserved whole, including paths no page
 * uses yet, so adding a Settings or Account page needs no server or proxy change. Each is also a
 * reserved module id in Core; `apps/api` tests hold the two lists together.
 */
export const DASHBOARD_PLATFORM_PAGES = Object.freeze(["settings", "account"] as const);

/**
 * The platform's own Settings sections, in their navigation order (ADR 0061). Every other name under
 * `/settings/` is a module's settings, at `/settings/<module-id>`. Module lifecycle — finding,
 * installing, turning on and off, updating — is Discover, at `/settings/discover/…`; configuring a
 * module is its own `/settings/<module-id>`. Each section name is also a reserved module id in Core,
 * so no module can take one, and `apps/api` tests hold the two lists together.
 */
export const SETTINGS_SECTIONS = Object.freeze([
  "general",
  "branding",
  "displays",
  "discover",
  "system",
] as const);

export type SettingsSection = (typeof SETTINGS_SECTIONS)[number];

/** The sections that are one page each. Discover is a group of views instead. */
export type SettingsPageSection = Exclude<SettingsSection, "discover">;

/**
 * The account area's personal settings pages, in navigation order (ADR 0062). Personal, never
 * church administration: that is Settings. They live beneath `/account/`, so none can collide with a
 * module id and none is reserved.
 */
export const ACCOUNT_SECTIONS = Object.freeze(["appearance"] as const);

export type AccountSection = (typeof ACCOUNT_SECTIONS)[number];

/**
 * Discover's views, in navigation order: Browse (find and install modules), Installed (see and
 * manage installed modules), and Updates (module and platform updates). Browse is where Discover
 * opens. They live beneath `/settings/discover/`, so none can collide with a module id.
 */
export const DISCOVER_VIEWS = Object.freeze(["browse", "installed", "updates"] as const);

export type DiscoverView = (typeof DISCOVER_VIEWS)[number];

/** What a Dashboard path means. A path the shell does not understand is `not-found`, never a guess. */
export type DashboardRoute =
  | { readonly kind: "overview" }
  | { readonly kind: "module"; readonly moduleId: string }
  | { readonly kind: "settings" }
  | { readonly kind: "settings-section"; readonly section: SettingsPageSection }
  | { readonly kind: "discover"; readonly view: DiscoverView }
  | { readonly kind: "module-settings"; readonly moduleId: string }
  | { readonly kind: "account" }
  | { readonly kind: "account-section"; readonly section: AccountSection }
  | { readonly kind: "not-found" };

/** A route the shell can link to. `not-found` is something the shell arrives at, never a target. */
export type DashboardDestination = Exclude<DashboardRoute, { readonly kind: "not-found" }>;

const platformPages: ReadonlySet<string> = new Set(DASHBOARD_PLATFORM_PAGES);
const settingsSections: ReadonlySet<string> = new Set(SETTINGS_SECTIONS);

const discoverViews: ReadonlySet<string> = new Set(DISCOVER_VIEWS);
const accountSections: ReadonlySet<string> = new Set(ACCOUNT_SECTIONS);

function isSettingsPageSection(value: string): value is SettingsPageSection {
  return value !== "discover" && settingsSections.has(value);
}

function isDiscoverView(value: string): value is DiscoverView {
  return discoverViews.has(value);
}

function isAccountSection(value: string): value is AccountSection {
  return accountSections.has(value);
}

function decodeSegment(segment: string): string | undefined {
  try {
    const decoded = decodeURIComponent(segment);
    return decoded.length > 0 ? decoded : undefined;
  } catch {
    return undefined;
  }
}

/**
 * Reads a path into a route.
 *
 * A single top-level segment that is not a platform page names a module's page. The id is only
 * decoded here, never validated against a pattern: the shell compares it with the navigation the
 * server resolved, and an id that matches no entry is an unavailable module, not a malformed URL.
 *
 * Everything below `/<module-id>/` belongs to that module's own pages (`/signage/timelines`), which
 * need a contract that does not exist yet, so it is `not-found` for now rather than silently showing
 * the module's first page. Settings and Account pages are likewise `not-found` until they exist. One
 * trailing slash is tolerated, as a person typing a URL expects; the shell then replaces it with the
 * canonical spelling.
 */
export function parseDashboardPath(pathname: string): DashboardRoute {
  if (pathname === "/") {
    return { kind: "overview" };
  }
  const segments = pathname.split("/");
  if (segments[0] !== "") {
    return { kind: "not-found" };
  }
  const meaningful = segments.at(-1) === "" ? segments.slice(1, -1) : segments.slice(1);
  const first = meaningful[0] === undefined ? undefined : decodeSegment(meaningful[0]);
  if (first === "settings") {
    return parseSettingsPath(meaningful.slice(1));
  }
  if (first === "account") {
    return parseAccountPath(meaningful.slice(1));
  }
  if (first === undefined || platformPages.has(first)) {
    return { kind: "not-found" };
  }
  if (meaningful.length === 1) {
    return { kind: "module", moduleId: first };
  }
  return { kind: "not-found" };
}

/**
 * Reads what follows `/settings`. One more segment is a platform section or, failing that, a
 * module's settings. `discover` is entered at Browse: `/settings/discover` reads as Browse, whose
 * canonical spelling is `/settings/discover/browse`, so the shell respells it in place and there is
 * one Browse address rather than two. Anything deeper is reserved for later Settings pages and is
 * `not-found` until they exist, rather than silently showing their parent.
 */
function parseSettingsPath(rest: readonly string[]): DashboardRoute {
  if (rest.length === 0) {
    return { kind: "settings" };
  }
  const name = rest[0] === undefined ? undefined : decodeSegment(rest[0]);
  if (name === undefined) {
    return { kind: "not-found" };
  }
  if (name === "discover") {
    if (rest.length === 1) {
      return { kind: "discover", view: "browse" };
    }
    const view = rest[1] === undefined ? undefined : decodeSegment(rest[1]);
    return rest.length === 2 && view !== undefined && isDiscoverView(view)
      ? { kind: "discover", view }
      : { kind: "not-found" };
  }
  if (rest.length !== 1) {
    return { kind: "not-found" };
  }
  return isSettingsPageSection(name)
    ? { kind: "settings-section", section: name }
    : { kind: "module-settings", moduleId: name };
}

/** Reads what follows `/account`: the account page itself, or one of its personal settings pages.
 * Anything else is `not-found` until it exists. */
function parseAccountPath(rest: readonly string[]): DashboardRoute {
  if (rest.length === 0) {
    return { kind: "account" };
  }
  const name = rest[0] === undefined ? undefined : decodeSegment(rest[0]);
  return rest.length === 1 && name !== undefined && isAccountSection(name)
    ? { kind: "account-section", section: name }
    : { kind: "not-found" };
}

/** The one canonical spelling of a route. A module id is percent-encoded, so no id can escape its
 * own path segment. */
export function dashboardPath(route: DashboardDestination): string {
  switch (route.kind) {
    case "overview":
      return "/";
    case "module":
      return `/${encodeURIComponent(route.moduleId)}`;
    case "settings":
      return "/settings";
    case "settings-section":
      return `/settings/${route.section}`;
    case "discover":
      return `/settings/discover/${route.view}`;
    case "module-settings":
      return `/settings/${encodeURIComponent(route.moduleId)}`;
    case "account":
      return "/account";
    case "account-section":
      return `/account/${route.section}`;
  }
}