const API_BASE_URL = process.env['NEXT_PUBLIC_API_URL'] || 'http://localhost:3001/api/v1';

export class ApiError extends Error {
  constructor(
    public status: number,
    message: string,
  ) {
    super(message);
    this.name = 'ApiError';
  }
}

type RequestOptions = Omit<RequestInit, 'body'> & {
  body?: unknown;
};

function getCsrfToken(): string | undefined {
  if (typeof document === 'undefined') return undefined;
  const match = document.cookie.match(/(?:^|;\s*)XSRF-TOKEN=([^;]*)/);
  return match?.[1] ? decodeURIComponent(match[1]) : undefined;
}

const SAFE_METHODS = new Set(['GET', 'HEAD', 'OPTIONS']);

// Endpoints for which a transparent refresh-and-retry on 401 must NOT run
// (refresh itself, login, logout, exchange-token, etc.).
const AUTH_ENDPOINTS = new Set([
  '/auth/refresh',
  '/auth/login',
  '/auth/logout',
  '/auth/register',
  '/auth/exchange-token',
  '/auth/forgot-password',
  '/auth/reset-password',
]);

/**
 * Coalesce concurrent refresh attempts: if ten requests race a 401 at the same
 * moment, we should fire `/auth/refresh` once, not ten times.
 */
let refreshInflight: Promise<boolean> | null = null;

async function tryRefresh(): Promise<boolean> {
  if (refreshInflight) return refreshInflight;
  refreshInflight = (async () => {
    try {
      const csrfToken = getCsrfToken();
      const res = await fetch(`${API_BASE_URL}/auth/refresh`, {
        method: 'POST',
        credentials: 'include',
        headers: {
          'Content-Type': 'application/json',
          ...(csrfToken ? { 'X-CSRF-Token': csrfToken } : {}),
        },
        body: JSON.stringify({}),
      });
      return res.ok;
    } catch {
      return false;
    } finally {
      // Allow a new refresh attempt after this one settles.
      setTimeout(() => {
        refreshInflight = null;
      }, 0);
    }
  })();
  return refreshInflight;
}

async function doFetch(endpoint: string, options: RequestOptions): Promise<Response> {
  const { body, headers, ...rest } = options;
  const method = options.method?.toUpperCase() ?? 'GET';

  const csrfHeaders: HeadersInit = {};
  if (!SAFE_METHODS.has(method)) {
    const csrfToken = getCsrfToken();
    if (csrfToken) {
      csrfHeaders['X-CSRF-Token'] = csrfToken;
    }
  }

  return fetch(`${API_BASE_URL}${endpoint}`, {
    ...rest,
    credentials: 'include',
    headers: {
      'Content-Type': 'application/json',
      ...csrfHeaders,
      ...headers,
    },
    body: body ? JSON.stringify(body) : undefined,
  });
}

async function request<T>(endpoint: string, options: RequestOptions = {}): Promise<T> {
  let res = await doFetch(endpoint, options);

  // If the access_token expired silently, try one refresh+retry before giving
  // up. Skip for auth endpoints themselves to avoid loops.
  if (res.status === 401 && !AUTH_ENDPOINTS.has(endpoint)) {
    const refreshed = await tryRefresh();
    if (refreshed) {
      res = await doFetch(endpoint, options);
    }
  }

  if (!res.ok) {
    const error = await res.json().catch(() => ({ message: res.statusText }));
    throw new ApiError(res.status, error.message || 'Request failed');
  }

  return res.json();
}

export const apiClient = {
  get: <T>(endpoint: string, headers?: HeadersInit) =>
    request<T>(endpoint, { method: 'GET', headers }),

  post: <T>(endpoint: string, body?: unknown, headers?: HeadersInit) =>
    request<T>(endpoint, { method: 'POST', body, headers }),

  patch: <T>(endpoint: string, body?: unknown, headers?: HeadersInit) =>
    request<T>(endpoint, { method: 'PATCH', body, headers }),

  delete: <T>(endpoint: string, headers?: HeadersInit) =>
    request<T>(endpoint, { method: 'DELETE', headers }),
};
