import net from "node:net";

import { pickPrimaryTailnetIPv4 } from "../infra/tailnet.js";

export function isLoopbackAddress(ip: string | undefined): boolean {
  if (!ip) return false;
  if (ip === "127.0.0.1") return true;
  if (ip.startsWith("127.")) return true;
  if (ip === "::1") return true;
  if (ip.startsWith("::ffff:127.")) return true;
  return false;
}

/**
 * Resolves gateway bind host with fallback strategy.
 *
 * Modes:
 * - loopback: 127.0.0.1 (rarely fails, but handled gracefully)
 * - lan: always 0.0.0.0 (no fallback)
 * - auto: Tailnet IPv4 if available, else 0.0.0.0
 * - custom: User-specified IP, fallback to 0.0.0.0 if unavailable
 *
 * @returns The bind address to use (never null)
 */
export async function resolveGatewayBindHost(
  bind: import("../config/config.js").BridgeBindMode | undefined,
  customHost?: string,
): Promise<string> {
  const mode = bind ?? "loopback";

  if (mode === "loopback") {
    // 127.0.0.1 rarely fails, but handle gracefully
    if (await canBindTo("127.0.0.1")) return "127.0.0.1";
    return "0.0.0.0"; // extreme fallback
  }

  if (mode === "lan") {
    return "0.0.0.0";
  }

  if (mode === "custom") {
    const host = customHost?.trim();
    if (!host) return "0.0.0.0"; // invalid config → fall back to all

    if (isValidIPv4(host) && (await canBindTo(host))) return host;
    // Custom IP failed → fall back to LAN
    return "0.0.0.0";
  }

  if (mode === "auto") {
    const tailnetIP = pickPrimaryTailnetIPv4();
    if (tailnetIP && (await canBindTo(tailnetIP))) return tailnetIP;
    return "0.0.0.0";
  }

  return "0.0.0.0";
}

/**
 * Test if we can bind to a specific host address.
 * Creates a temporary server, attempts to bind, then closes it.
 *
 * @param host - The host address to test
 * @returns True if we can successfully bind to this address
 */
async function canBindTo(host: string): Promise<boolean> {
  return new Promise((resolve) => {
    const testServer = net.createServer();
    testServer.once("error", () => {
      resolve(false);
    });
    testServer.once("listening", () => {
      testServer.close();
      resolve(true);
    });
    // Use port 0 to let OS pick an available port for testing
    testServer.listen(0, host);
  });
}

/**
 * Validate if a string is a valid IPv4 address.
 *
 * @param host - The string to validate
 * @returns True if valid IPv4 format
 */
function isValidIPv4(host: string): boolean {
  const parts = host.split(".");
  if (parts.length !== 4) return false;
  return parts.every((part) => {
    const n = parseInt(part, 10);
    return !Number.isNaN(n) && n >= 0 && n <= 255 && part === String(n);
  });
}

export function isLoopbackHost(host: string): boolean {
  return isLoopbackAddress(host);
}