// Minimal RFC 6455 WebSocket server built only on Node's built-in modules.
// The project rule is "no external JS libraries other than jQuery", so the
// transport is implemented here rather than pulled from npm. It supports the
// subset the game needs: text and binary messages, fragmentation, ping/pong,
// close, and large frames (state snapshots are hundreds of KiB).

import { createHash } from "node:crypto";
import { EventEmitter } from "node:events";

const GUID = "258EAFA5-E914-47DA-95CA-C5AB0DC85B11";

const OP_CONTINUATION = 0x0;
const OP_TEXT = 0x1;
const OP_BINARY = 0x2;
const OP_CLOSE = 0x8;
const OP_PING = 0x9;
const OP_PONG = 0xa;

export class WebSocketServer extends EventEmitter {
  constructor(httpServer, options = {}) {
    super();
    this.path = options.path || "/ws";
    this._nextId = 1;
    this._connections = new Map();
    httpServer.on("upgrade", (request, socket, head) => {
      const url = request.url || "";
      const path = url.split("?")[0];
      if (path !== this.path) {
        socket.destroy();
        return;
      }
      this._accept(request, socket, head);
    });
  }

  connectionCount() {
    return this._connections.size;
  }

  _accept(request, socket, head) {
    const wsKey = request.headers["sec-websocket-key"];
    if (!wsKey || (request.headers.upgrade || "").toLowerCase() !== "websocket") {
      socket.destroy();
      return;
    }
    const accept = createHash("sha1").update(wsKey + GUID).digest("base64");
    socket.write(
      "HTTP/1.1 101 Switching Protocols\r\n" +
        "Upgrade: websocket\r\n" +
        "Connection: Upgrade\r\n" +
        `Sec-WebSocket-Accept: ${accept}\r\n` +
        "\r\n"
    );
    socket.setNoDelay(true);

    const id = this._nextId++;
    const connection = new Connection(id, socket, head);
    // The peer's address and the page's Origin, so the server can tell a local
    // connection (which may use the testing tools) from a remote one.
    connection.remoteAddress = socket.remoteAddress || "";
    connection.origin = request.headers.origin || "";
    this._connections.set(id, connection);
    this.emit("connection", connection);
    connection.on("close", () => this._connections.delete(id));
  }
}

class Connection extends EventEmitter {
  constructor(id, socket, head) {
    super();
    this.id = id;
    this.socket = socket;
    this._buffer = Buffer.alloc(0);
    this._fragments = [];
    this._fragmentOpcode = null;
    this._closed = false;

    if (head && head.length > 0) this._buffer = Buffer.concat([this._buffer, head]);
    socket.on("data", (chunk) => {
      this._buffer = Buffer.concat([this._buffer, chunk]);
      this._parse();
    });
    socket.on("error", () => this._close());
    socket.on("close", () => this._close());
    if (this._buffer.length > 0) this._parse();
  }

  send(data) {
    if (this._closed) return;
    const payload = Buffer.isBuffer(data) ? data : Buffer.from(String(data), "utf8");
    const opcode = Buffer.isBuffer(data) ? OP_BINARY : OP_TEXT;
    try {
      this.socket.write(encodeFrame(opcode, payload));
    } catch {
      this._close();
    }
  }

  close(code = 1000, reason = "") {
    if (this._closed) return;
    const reasonBuffer = Buffer.from(reason, "utf8");
    const payload = Buffer.alloc(2 + reasonBuffer.length);
    payload.writeUInt16BE(code, 0);
    reasonBuffer.copy(payload, 2);
    try {
      // Flush the close frame before shutting the socket down; destroying
      // immediately can discard it.
      this.socket.write(encodeFrame(OP_CLOSE, payload));
      this.socket.end();
    } catch {
      this._close();
    }
  }

  _close() {
    if (this._closed) return;
    this._closed = true;
    try {
      this.socket.destroy();
    } catch {
      // ignore
    }
    this.emit("close");
  }

  _parse() {
    while (true) {
      const buffer = this._buffer;
      if (buffer.length < 2) return;
      const b0 = buffer[0];
      const b1 = buffer[1];
      const fin = (b0 & 0x80) !== 0;
      const opcode = b0 & 0x0f;
      const masked = (b1 & 0x80) !== 0;
      let length = b1 & 0x7f;
      let offset = 2;
      if (length === 126) {
        if (buffer.length < 4) return;
        length = buffer.readUInt16BE(2);
        offset = 4;
      } else if (length === 127) {
        if (buffer.length < 10) return;
        length = Number(buffer.readBigUInt64BE(2));
        offset = 10;
      }
      let maskKey = null;
      if (masked) {
        if (buffer.length < offset + 4) return;
        maskKey = buffer.subarray(offset, offset + 4);
        offset += 4;
      }
      if (buffer.length < offset + length) return;
      let payload = buffer.subarray(offset, offset + length);
      if (masked) {
        const out = Buffer.allocUnsafe(length);
        for (let i = 0; i < length; i++) out[i] = payload[i] ^ maskKey[i & 3];
        payload = out;
      }
      this._buffer = buffer.subarray(offset + length);
      this._handleFrame(fin, opcode, payload);
    }
  }

  _handleFrame(fin, opcode, payload) {
    if (opcode === OP_CLOSE) {
      this.close(1000);
      return;
    }
    if (opcode === OP_PING) {
      this.socket.write(encodeFrame(OP_PONG, payload));
      return;
    }
    if (opcode === OP_PONG) return;

    if (opcode === OP_CONTINUATION) {
      this._fragments.push(payload);
    } else {
      this._fragmentOpcode = opcode;
      this._fragments = [payload];
    }
    if (!fin) return;
    const full = Buffer.concat(this._fragments);
    const isBinary = this._fragmentOpcode === OP_BINARY;
    this._fragments = [];
    this._fragmentOpcode = null;
    this.emit("message", isBinary ? full : full.toString("utf8"));
  }
}

function encodeFrame(opcode, payload) {
  const length = payload.length;
  let header;
  if (length < 126) {
    header = Buffer.alloc(2);
    header[1] = length;
  } else if (length < 65536) {
    header = Buffer.alloc(4);
    header[1] = 126;
    header.writeUInt16BE(length, 2);
  } else {
    header = Buffer.alloc(10);
    header[1] = 127;
    header.writeBigUInt64BE(BigInt(length), 2);
  }
  header[0] = 0x80 | opcode;
  return Buffer.concat([header, payload]);
}