import { readdirSync } from "node:fs";
import path from "node:path";

export interface RouterContext {
  agentId: string;
}

export interface RouterModule {
  resolve(ctx: RouterContext): string | Promise<string>;
}

export interface LoadedRouter {
  name: string;
  filePath: string;
  module: RouterModule;
}

const _G = globalThis as Record<string, unknown>;
const ROUTERS_KEY = "_prismFacetRouters";
const LOAD_COUNTER_KEY = "_prismFacetLoadCounter";

function getRouterMap(): Map<string, LoadedRouter> {
  if (!(_G[ROUTERS_KEY] instanceof Map)) {
    _G[ROUTERS_KEY] = new Map<string, LoadedRouter>();
  }
  return _G[ROUTERS_KEY] as Map<string, LoadedRouter>;
}

function routerNameFromFile(filename: string): string {
  return filename.replace(/\.(ts|js|mjs)$/, "");
}

export async function loadRouters(
  routersDir: string,
  log: { info(msg: string): void; warn(msg: string): void }
): Promise<void> {
  const map = getRouterMap();
  // Don't clear() — that would wipe externally-registered routers added
  // by other plugins via __prismFacet.addRouter. Instead, track which
  // routers came from the filesystem this run and remove only those
  // that disappeared. External routers (filePath sentinel) are never
  // touched here.
  const FILE_SENTINEL_PREFIX = "<external:";
  const fileRoutersBefore = new Set<string>();
  for (const [name, r] of map.entries()) {
    if (!r.filePath.startsWith(FILE_SENTINEL_PREFIX)) fileRoutersBefore.add(name);
  }

  if (typeof _G[LOAD_COUNTER_KEY] !== "number") _G[LOAD_COUNTER_KEY] = 0;
  (_G[LOAD_COUNTER_KEY] as number)++;
  const counter = _G[LOAD_COUNTER_KEY] as number;

  let files: string[];
  try {
    files = readdirSync(routersDir).filter(
      (f) => /\.(ts|js|mjs)$/.test(f) && !f.startsWith(".")
    );
  } catch {
    log.warn(`[prism-facet] routers directory not found: ${routersDir}`);
    // No file-based routers this run — drop any leftover file routers.
    for (const name of fileRoutersBefore) map.delete(name);
    return;
  }

  const seenThisRun = new Set<string>();
  for (const file of files) {
    const name = routerNameFromFile(file);
    const filePath = path.resolve(routersDir, file);
    try {
      const mod = (await import(`${filePath}?v=${counter}`)) as RouterModule;
      if (typeof mod.resolve !== "function") {
        log.warn(`[prism-facet] router ${name}: no resolve() export, skipping`);
        continue;
      }
      map.set(name, { name, filePath, module: mod });
      seenThisRun.add(name);
      log.info(`[prism-facet] router loaded: ${name}`);
    } catch (err) {
      log.warn(`[prism-facet] router ${name}: failed to load — ${String(err)}`);
    }
  }
  // Drop file-based routers that disappeared between loads. Untouched
  // routers stay.
  for (const stale of fileRoutersBefore) {
    if (!seenThisRun.has(stale)) map.delete(stale);
  }
}

export function getRouters(): LoadedRouter[] {
  return Array.from(getRouterMap().values());
}

export function getRouterNames(): string[] {
  return Array.from(getRouterMap().keys());
}

/**
 * Cross-plugin API: register a router programmatically. Other plugins
 * (e.g. ClawPrompts) call this via globalThis.__prismFacet.addRouter
 * to publish a router without dropping a .ts file in PrismFacet's
 * routersDir. Replaces any existing router of the same name.
 */
export function addExternalRouter(
  name: string,
  resolveFn: (ctx: RouterContext) => string | Promise<string>,
): void {
  const map = getRouterMap();
  map.set(name, {
    name,
    filePath: `<external: registered via __prismFacet.addRouter>`,
    module: { resolve: resolveFn },
  });
}