/**
 * React hook over the `/api/servers` endpoints. Fetches the list on mount,
 * holds it in local state, and exposes addServer / updateServer / removeServer
 * mutators that round-trip through the backend (re-fetching after each write
 * to stay in sync with the on-disk truth).
 */

import { useCallback, useEffect, useState } from "react";
import { mcpConfigToServerEntries } from "../mcp/serverList.js";
import type {
  InspectorServerSettings,
  MCPConfig,
  MCPServerConfig,
  ServerEntry,
} from "../mcp/types.js";

export interface UseServersOptions {
  /** Base URL of the remote server (typically `window.location.origin`). */
  baseUrl: string;
  /** Optional auth token for the `x-mcp-remote-auth` header. */
  authToken?: string;
  /** Fetch function to use (default: globalThis.fetch). Useful in tests. */
  fetchFn?: typeof fetch;
}

export interface UseServersResult {
  servers: ServerEntry[];
  loading: boolean;
  error: string | undefined;
  refresh: () => Promise<void>;
  addServer: (id: string, config: MCPServerConfig) => Promise<void>;
  updateServer: (
    originalId: string,
    newId: string,
    config: MCPServerConfig,
  ) => Promise<void>;
  /**
   * Patch only the `settings` node on an existing server entry, leaving the
   * transport config and id untouched. Routes through `PUT /api/servers/:id`
   * with the current `config` plus the new settings.
   */
  updateServerSettings: (
    id: string,
    settings: InspectorServerSettings,
  ) => Promise<void>;
  removeServer: (id: string) => Promise<void>;
}

function buildHeaders(
  authToken: string | undefined,
  includeJsonBody: boolean,
): Record<string, string> {
  const headers: Record<string, string> = {};
  if (includeJsonBody) headers["Content-Type"] = "application/json";
  if (authToken) headers["x-mcp-remote-auth"] = `Bearer ${authToken}`;
  return headers;
}

async function readErrorMessage(res: Response): Promise<string> {
  try {
    const body = (await res.json()) as { error?: unknown };
    if (typeof body?.error === "string") return body.error;
  } catch {
    /* ignore */
  }
  return `HTTP ${res.status}`;
}

export function useServers(opts: UseServersOptions): UseServersResult {
  const { baseUrl, authToken, fetchFn } = opts;
  const doFetch = fetchFn ?? globalThis.fetch;
  // Normalize once — saves us from re-string-munging on every mutator call.
  const base = baseUrl.replace(/\/$/, "");

  const [servers, setServers] = useState<ServerEntry[]>([]);
  const [loading, setLoading] = useState<boolean>(true);
  const [error, setError] = useState<string | undefined>(undefined);

  // Inner refresh that the public `refresh` and the SSE-triggered background
  // refresh both share. `background` skips the loading-state toggle so
  // consumers rendering a spinner or skeleton don't flash on every external
  // mcp.json edit — the existing list stays on screen while the re-fetch
  // resolves. `error` is still reset / set either way so a real error
  // surfaces even from a background refresh.
  const refreshInternal = useCallback(
    async (background: boolean): Promise<void> => {
      if (!background) setLoading(true);
      setError(undefined);
      try {
        const res = await doFetch(`${base}/api/servers`, {
          method: "GET",
          headers: buildHeaders(authToken, false),
        });
        if (!res.ok) {
          throw new Error(await readErrorMessage(res));
        }
        const body = (await res.json()) as MCPConfig;
        setServers(mcpConfigToServerEntries(body));
      } catch (err) {
        setError(err instanceof Error ? err.message : String(err));
      } finally {
        if (!background) setLoading(false);
      }
    },
    [base, authToken, doFetch],
  );

  const refresh = useCallback(
    (): Promise<void> => refreshInternal(false),
    [refreshInternal],
  );

  useEffect(() => {
    void refresh();
  }, [refresh]);

  // Subscribe to `/api/servers/events` so external edits to mcp.json
  // (the user editing the file by hand, or `mcp.json` written by another
  // tool) propagate without a manual page refresh. We deliberately use
  // fetch() + ReadableStream rather than EventSource because EventSource
  // can't send the existing `x-mcp-remote-auth: Bearer …` header — the
  // backend's auth contract is unchanged. The event payload itself is
  // ignored: any signal triggers a re-fetch, since the GET handler is the
  // sole source of truth for the list shape.
  useEffect(() => {
    const controller = new AbortController();
    void (async () => {
      try {
        const res = await doFetch(`${base}/api/servers/events`, {
          method: "GET",
          headers: buildHeaders(authToken, false),
          signal: controller.signal,
        });
        if (!res.ok || !res.body) return;
        const reader = res.body.getReader();
        const decoder = new TextDecoder();
        let buffer = "";
        // SSE frames are separated by a blank line (`\n\n`). We don't parse
        // the event type or data — `refreshInternal()` re-fetches the
        // canonical state regardless — so we just count frames and fire a
        // single background refresh per decode chunk. Two `change`
        // broadcasts landing in the same chunk become one re-fetch instead
        // of two concurrent ones whose setState order is unspecified.
        // Cross-chunk debounce is not added: `awaitWriteFinish`'s 100ms
        // stability threshold already serializes external edits at the
        // source, and chained fetches against the same GET endpoint are
        // idempotent enough that a rare back-to-back pair just costs one
        // extra round-trip.
        while (true) {
          const { done, value } = await reader.read();
          if (done) break;
          buffer += decoder.decode(value, { stream: true });
          let sawFrame = false;
          let frameEnd = buffer.indexOf("\n\n");
          while (frameEnd !== -1) {
            buffer = buffer.slice(frameEnd + 2);
            sawFrame = true;
            frameEnd = buffer.indexOf("\n\n");
          }
          if (sawFrame) void refreshInternal(true);
        }
      } catch {
        // AbortError on unmount, or a transient network blip. No reconnect:
        // a real failure leaves the hook in last-known-good state and the
        // user can hit refresh manually. The dev-tool reload story is fine.
      }
    })();
    return () => controller.abort();
  }, [base, authToken, doFetch, refreshInternal]);

  const addServer = useCallback(
    async (id: string, config: MCPServerConfig): Promise<void> => {
      const res = await doFetch(`${base}/api/servers`, {
        method: "POST",
        headers: buildHeaders(authToken, true),
        body: JSON.stringify({ id, config }),
      });
      if (!res.ok) {
        throw new Error(await readErrorMessage(res));
      }
      await refresh();
    },
    [base, authToken, doFetch, refresh],
  );

  const updateServer = useCallback(
    async (
      originalId: string,
      newId: string,
      config: MCPServerConfig,
    ): Promise<void> => {
      // `settings` is intentionally omitted from the body. The backend route
      // treats omission as "preserve the existing settings node on disk", so
      // a config-only save (e.g. ServerConfigModal) cannot silently wipe
      // persisted headers / metadata / OAuth credentials. To explicitly
      // clear settings, send `settings: null`.
      const res = await doFetch(
        `${base}/api/servers/${encodeURIComponent(originalId)}`,
        {
          method: "PUT",
          headers: buildHeaders(authToken, true),
          body: JSON.stringify({ id: newId, config }),
        },
      );
      if (!res.ok) {
        throw new Error(await readErrorMessage(res));
      }
      await refresh();
    },
    [base, authToken, doFetch, refresh],
  );

  const updateServerSettings = useCallback(
    async (id: string, settings: InspectorServerSettings): Promise<void> => {
      // Settings-only PUT — we deliberately omit `config` so the route
      // preserves the on-disk transport config inside its write lock.
      // Reading `existing.config` from in-memory `servers` here would pin a
      // stale snapshot at scheduling time and could silently revert a
      // separate concurrent edit (e.g. a future file-watcher refreshing
      // `servers` between debounce schedule and flush). The server is the
      // single source of truth for config.
      const res = await doFetch(
        `${base}/api/servers/${encodeURIComponent(id)}`,
        {
          method: "PUT",
          headers: buildHeaders(authToken, true),
          body: JSON.stringify({ id, settings }),
        },
      );
      if (!res.ok) {
        throw new Error(await readErrorMessage(res));
      }
      await refresh();
    },
    [base, authToken, doFetch, refresh],
  );

  const removeServer = useCallback(
    async (id: string): Promise<void> => {
      const res = await doFetch(
        `${base}/api/servers/${encodeURIComponent(id)}`,
        {
          method: "DELETE",
          headers: buildHeaders(authToken, false),
        },
      );
      if (!res.ok) {
        throw new Error(await readErrorMessage(res));
      }
      await refresh();
    },
    [base, authToken, doFetch, refresh],
  );

  return {
    servers,
    loading,
    error,
    refresh,
    addServer,
    updateServer,
    updateServerSettings,
    removeServer,
  };
}