const fs = require("node:fs");
const path = require("node:path");

const YMODEM = Object.freeze({
  SOH: 0x01,
  STX: 0x02,
  EOT: 0x04,
  ACK: 0x06,
  NAK: 0x15,
  CAN: 0x18,
  CRC16: 0x43,
  BACKSPACE: 0x08,
  PACKET_SIZE_128: 128,
  PACKET_SIZE_1024: 1024,
});

const DEFAULT_TIMEOUT_MS = 60_000;
const DEFAULT_RETRY_LIMIT = 10;
const YMODEM_CANCEL_SEQUENCE = Buffer.from([
  YMODEM.CAN,
  YMODEM.CAN,
  YMODEM.CAN,
  YMODEM.CAN,
  YMODEM.CAN,
  YMODEM.BACKSPACE,
  YMODEM.BACKSPACE,
  YMODEM.BACKSPACE,
  YMODEM.BACKSPACE,
  YMODEM.BACKSPACE,
]);

class YmodemTransferError extends Error {
  constructor(message, code = "YMODEM_TRANSFER_ERROR") {
    super(message);
    this.name = "YmodemTransferError";
    this.code = code;
  }
}

function crc16Xmodem(buffer, start = 0, length = buffer.length - start) {
  let crc = 0;
  for (let i = start; i < start + length; i += 1) {
    let code = ((crc >>> 8) ^ buffer[i]) & 0xff;
    code ^= code >>> 4;
    crc = ((crc << 8) ^ (code << 12) ^ (code << 5) ^ code) & 0xffff;
  }
  return crc;
}

function createPacket(header, blockNumber, payload) {
  const size = payload.length;
  const packet = Buffer.alloc(3 + size + 2, 0x00);
  packet[0] = header;
  packet[1] = blockNumber & 0xff;
  packet[2] = 0xff - packet[1];
  payload.copy(packet, 3);
  packet.writeUInt16BE(crc16Xmodem(packet, 3, size), packet.length - 2);
  return packet;
}

function createYmodemFileInfoPacket({ filename, size, mtime = 0, mode = 0o100644 }) {
  const baseName = path.basename(filename || "file");
  const metadata = `${baseName}\0${size} ${Math.trunc(mtime).toString(8)} ${mode.toString(8)}`;
  const metadataBytes = Buffer.from(metadata, "utf8");
  const packetSize = metadataBytes.length >= YMODEM.PACKET_SIZE_128
    ? YMODEM.PACKET_SIZE_1024
    : YMODEM.PACKET_SIZE_128;
  const payload = Buffer.alloc(packetSize, 0x00);
  metadataBytes.copy(payload, 0, 0, Math.min(metadataBytes.length, packetSize));
  return createPacket(
    packetSize === YMODEM.PACKET_SIZE_1024 ? YMODEM.STX : YMODEM.SOH,
    0,
    payload,
  );
}

function createYmodemDataPackets(buffer) {
  if (!Buffer.isBuffer(buffer)) {
    buffer = Buffer.from(buffer);
  }
  const packets = [];
  let blockNumber = 1;
  for (let offset = 0; offset < buffer.length; offset += YMODEM.PACKET_SIZE_1024) {
    const payload = Buffer.alloc(YMODEM.PACKET_SIZE_1024, 0x1a);
    buffer.copy(payload, 0, offset, Math.min(offset + YMODEM.PACKET_SIZE_1024, buffer.length));
    packets.push(createPacket(YMODEM.STX, blockNumber, payload));
    blockNumber = (blockNumber + 1) & 0xff;
  }
  return packets;
}

function createYmodemEndSessionPacket() {
  return createPacket(YMODEM.SOH, 0, Buffer.alloc(YMODEM.PACKET_SIZE_128, 0x00));
}

function sanitizeYmodemFilename(filename) {
  const normalized = String(filename || "").replace(/\\/g, "/");
  const baseName = path.basename(normalized).replace(/[<>:"/\\|?*\x00-\x1f]/g, "_").trim();
  if (!baseName || baseName === "." || baseName === "..") {
    return "ymodem-received.bin";
  }
  return baseName;
}

function parseYmodemFileInfoPayload(payload) {
  const separatorIndex = payload.indexOf(0x00);
  const rawName = (separatorIndex >= 0 ? payload.subarray(0, separatorIndex) : payload).toString("utf8");
  if (!rawName) return null;

  const metadata = separatorIndex >= 0
    ? payload.subarray(separatorIndex + 1).toString("ascii").replace(/\0.*$/u, "").trim()
    : "";
  const [sizeText] = metadata.split(/\s+/u);
  if (!/^\d+$/u.test(sizeText || "")) {
    throw new YmodemTransferError("YMODEM file header has an invalid size", "YMODEM_INVALID_SIZE");
  }
  const parsedSize = Number.parseInt(sizeText, 10);

  return {
    fileName: sanitizeYmodemFilename(rawName),
    totalBytes: Number.isFinite(parsedSize) && parsedSize >= 0 ? parsedSize : 0,
  };
}

async function resolveUniqueDestinationPath(destinationDir, fileName) {
  const parsed = path.parse(fileName);
  for (let attempt = 0; attempt < 10_000; attempt += 1) {
    const candidateName = attempt === 0
      ? fileName
      : `${parsed.name} (${attempt})${parsed.ext}`;
    const candidatePath = path.join(destinationDir, candidateName);
    try {
      await fs.promises.access(candidatePath);
    } catch (error) {
      if (error?.code === "ENOENT") return candidatePath;
      throw error;
    }
  }
  throw new YmodemTransferError("Could not choose a destination file name", "YMODEM_DESTINATION_EXISTS");
}

function writeAndDrain(serialPort, buffer) {
  return new Promise((resolve, reject) => {
    let settled = false;
    const done = (err) => {
      if (settled) return;
      if (err) {
        settled = true;
        reject(err);
        return;
      }
      if (typeof serialPort.drain !== "function") {
        settled = true;
        resolve();
        return;
      }
      serialPort.drain((drainErr) => {
        if (settled) return;
        settled = true;
        drainErr ? reject(drainErr) : resolve();
      });
    };

    try {
      serialPort.write(buffer, done);
    } catch (error) {
      reject(error);
    }
  });
}

function createSerialByteReader(serialPort, abortSignal) {
  const queue = [];
  const waiters = [];

  const rejectAll = (error) => {
    while (waiters.length) {
      const waiter = waiters.shift();
      clearTimeout(waiter.timer);
      waiter.reject(error);
    }
  };

  const onData = (chunk) => {
    for (const byte of Buffer.from(chunk)) {
      if (waiters.length) {
        const waiter = waiters.shift();
        clearTimeout(waiter.timer);
        waiter.resolve(byte);
      } else {
        queue.push(byte);
      }
    }
  };

  const onAbort = () => {
    rejectAll(new YmodemTransferError("YMODEM transfer cancelled", "YMODEM_CANCELLED"));
  };
  const onError = (error) => {
    rejectAll(error instanceof Error ? error : new Error(String(error)));
  };
  const onClose = () => {
    rejectAll(new YmodemTransferError("Serial port closed during YMODEM transfer", "YMODEM_SERIAL_CLOSED"));
  };

  serialPort.on("data", onData);
  serialPort.on?.("error", onError);
  serialPort.on?.("close", onClose);
  abortSignal?.addEventListener?.("abort", onAbort, { once: true });

  return {
    unreadByte(byte) {
      queue.unshift(byte);
    },
    readByte(timeoutMs) {
      if (abortSignal?.aborted) {
        return Promise.reject(new YmodemTransferError("YMODEM transfer cancelled", "YMODEM_CANCELLED"));
      }
      if (queue.length) {
        return Promise.resolve(queue.shift());
      }
      return new Promise((resolve, reject) => {
        const timer = setTimeout(() => {
          const index = waiters.findIndex((waiter) => waiter.timer === timer);
          if (index >= 0) waiters.splice(index, 1);
          reject(new YmodemTransferError("Timed out waiting for YMODEM receiver", "YMODEM_TIMEOUT"));
        }, timeoutMs);
        waiters.push({ resolve, reject, timer });
      });
    },
    cleanup() {
      serialPort.off("data", onData);
      serialPort.off?.("error", onError);
      serialPort.off?.("close", onClose);
      abortSignal?.removeEventListener?.("abort", onAbort);
      rejectAll(new YmodemTransferError("YMODEM reader closed", "YMODEM_CLOSED"));
    },
  };
}

function sendYmodemCancel(serialPort) {
  if (!serialPort || typeof serialPort.write !== "function") return Promise.resolve();
  return writeAndDrain(serialPort, YMODEM_CANCEL_SEQUENCE).catch(() => {});
}

async function readExpected(reader, expected, timeoutMs, label) {
  for (;;) {
    const byte = await reader.readByte(timeoutMs);
    if (byte === YMODEM.CAN) {
      const next = await reader.readByte(1_000).catch(() => null);
      if (next === YMODEM.CAN) {
        throw new YmodemTransferError("YMODEM transfer cancelled by receiver", "YMODEM_REMOTE_CANCELLED");
      }
      if (next !== null) {
        reader.unreadByte?.(next);
      }
      continue;
    }
    if (expected.includes(byte)) return byte;
    if (byte === YMODEM.CRC16 && expected.includes(YMODEM.CRC16)) return byte;
    if (byte === YMODEM.NAK && expected.includes(YMODEM.NAK)) return byte;
    if (byte === YMODEM.ACK && expected.includes(YMODEM.ACK)) return byte;
    if (byte === YMODEM.CRC16 || byte === YMODEM.ACK || byte === YMODEM.NAK) continue;
    throw new YmodemTransferError(`Unexpected YMODEM byte while waiting for ${label}: 0x${byte.toString(16)}`);
  }
}

async function sendPacketWithRetry({ serialPort, reader, packet, timeoutMs, retryLimit, onProgress }) {
  for (let attempt = 0; attempt < retryLimit; attempt += 1) {
    await writeAndDrain(serialPort, packet);
    onProgress?.();
    const response = await readExpected(reader, [YMODEM.ACK, YMODEM.NAK], timeoutMs, "packet ACK");
    if (response === YMODEM.ACK) return;
  }
  throw new YmodemTransferError("YMODEM receiver rejected the packet too many times", "YMODEM_RETRY_LIMIT");
}

async function readYmodemFrame(reader, timeoutMs) {
  const header = await reader.readByte(timeoutMs);
  if (header === YMODEM.CAN) {
    const next = await reader.readByte(1_000).catch(() => null);
    if (next === YMODEM.CAN) {
      throw new YmodemTransferError("YMODEM transfer cancelled by sender", "YMODEM_REMOTE_CANCELLED");
    }
    if (next !== null) {
      reader.unreadByte?.(next);
    }
    return readYmodemFrame(reader, timeoutMs);
  }
  if (header === YMODEM.EOT) {
    return { type: "eot" };
  }
  if (header !== YMODEM.SOH && header !== YMODEM.STX) {
    throw new YmodemTransferError(`Unexpected YMODEM packet header: 0x${header.toString(16)}`);
  }

  const payloadSize = header === YMODEM.SOH ? YMODEM.PACKET_SIZE_128 : YMODEM.PACKET_SIZE_1024;
  const blockNumber = await reader.readByte(timeoutMs);
  const blockComplement = await reader.readByte(timeoutMs);
  const payload = Buffer.alloc(payloadSize);
  for (let i = 0; i < payloadSize; i += 1) {
    payload[i] = await reader.readByte(timeoutMs);
  }
  const sentCrc = ((await reader.readByte(timeoutMs)) << 8) | await reader.readByte(timeoutMs);
  const expectedCrc = crc16Xmodem(payload);
  const valid = blockComplement === (0xff - blockNumber) && sentCrc === expectedCrc;

  return {
    type: "packet",
    blockNumber,
    payload,
    valid,
  };
}

async function readYmodemPacketWithRetry({
  serialPort,
  reader,
  expectedBlockNumber,
  requestByte,
  timeoutMs,
  retryLimit,
  label,
}) {
  for (let attempt = 0; attempt < retryLimit; attempt += 1) {
    if (requestByte !== undefined) {
      await writeAndDrain(serialPort, Buffer.from([requestByte]));
    }

    let frame;
    try {
      frame = await readYmodemFrame(reader, timeoutMs);
    } catch (error) {
      if (error?.code === "YMODEM_TIMEOUT" && attempt < retryLimit - 1) {
        continue;
      }
      throw error;
    }

    if (frame.type === "eot") {
      return frame;
    }
    if (frame.valid && frame.blockNumber === expectedBlockNumber) {
      return frame;
    }
    if (frame.valid && frame.blockNumber === ((expectedBlockNumber - 1) & 0xff)) {
      await writeAndDrain(serialPort, Buffer.from([YMODEM.ACK]));
      continue;
    }
    await writeAndDrain(serialPort, Buffer.from([YMODEM.NAK]));
  }

  throw new YmodemTransferError(`YMODEM sender did not provide a valid ${label}`, "YMODEM_RETRY_LIMIT");
}

async function receiveYmodemFileData({
  serialPort,
  reader,
  fileHandle,
  totalBytes,
  timeoutMs,
  retryLimit,
  onProgress,
}) {
  let expectedBlockNumber = 1;
  let writtenBytes = 0;
  let rejectedPackets = 0;

  for (;;) {
    let frame;
    try {
      frame = await readYmodemFrame(reader, timeoutMs);
    } catch (error) {
      if (error?.code === "YMODEM_TIMEOUT" && rejectedPackets < retryLimit) {
        rejectedPackets += 1;
        await writeAndDrain(serialPort, Buffer.from([YMODEM.NAK]));
        continue;
      }
      throw error;
    }

    if (frame.type === "eot") {
      await writeAndDrain(serialPort, Buffer.from([YMODEM.NAK]));
      const secondEot = await readYmodemFrame(reader, timeoutMs);
      if (secondEot.type !== "eot") {
        throw new YmodemTransferError("YMODEM sender did not confirm end of file", "YMODEM_EOT_EXPECTED");
      }
      await writeAndDrain(serialPort, Buffer.from([YMODEM.ACK]));
      if (writtenBytes !== totalBytes) {
        throw new YmodemTransferError(
          `YMODEM received incomplete file (${writtenBytes}/${totalBytes} bytes)`,
          "YMODEM_INCOMPLETE_FILE",
        );
      }
      return writtenBytes;
    }

    if (!frame.valid) {
      rejectedPackets += 1;
      if (rejectedPackets > retryLimit) {
        throw new YmodemTransferError("YMODEM sender sent too many invalid packets", "YMODEM_RETRY_LIMIT");
      }
      await writeAndDrain(serialPort, Buffer.from([YMODEM.NAK]));
      continue;
    }

    if (frame.blockNumber === ((expectedBlockNumber - 1) & 0xff)) {
      await writeAndDrain(serialPort, Buffer.from([YMODEM.ACK]));
      continue;
    }
    if (frame.blockNumber !== expectedBlockNumber) {
      rejectedPackets += 1;
      if (rejectedPackets > retryLimit) {
        throw new YmodemTransferError("YMODEM sender sent packets out of order", "YMODEM_RETRY_LIMIT");
      }
      await writeAndDrain(serialPort, Buffer.from([YMODEM.NAK]));
      continue;
    }

    const remainingBytes = Math.max(0, totalBytes - writtenBytes);
    const bytesToWrite = Math.min(frame.payload.length, remainingBytes);
    if (bytesToWrite > 0) {
      await fileHandle.write(frame.payload, 0, bytesToWrite);
      writtenBytes += bytesToWrite;
    }

    rejectedPackets = 0;
    expectedBlockNumber = (expectedBlockNumber + 1) & 0xff;
    await writeAndDrain(serialPort, Buffer.from([YMODEM.ACK]));
    onProgress?.({ transferredBytes: writtenBytes, totalBytes, stage: "data" });
  }
}

async function receiveYmodemFiles(serialPort, {
  destinationDir,
  timeoutMs = DEFAULT_TIMEOUT_MS,
  retryLimit = DEFAULT_RETRY_LIMIT,
  abortSignal,
  onProgress,
} = {}) {
  if (!serialPort) {
    throw new YmodemTransferError("Serial session is not available", "YMODEM_NO_SERIAL");
  }
  if (!destinationDir || typeof destinationDir !== "string") {
    throw new YmodemTransferError("No destination directory selected", "YMODEM_NO_DESTINATION");
  }

  const resolvedDestinationDir = path.resolve(destinationDir);
  let destinationStat;
  try {
    destinationStat = await fs.promises.stat(resolvedDestinationDir);
  } catch (error) {
    throw new YmodemTransferError("Selected destination is not a directory", "YMODEM_DESTINATION_NOT_DIRECTORY");
  }
  if (!destinationStat.isDirectory()) {
    throw new YmodemTransferError("Selected destination is not a directory", "YMODEM_DESTINATION_NOT_DIRECTORY");
  }

  const reader = createSerialByteReader(serialPort, abortSignal);
  const files = [];

  try {
    for (;;) {
      const headerFrame = await readYmodemPacketWithRetry({
        serialPort,
        reader,
        expectedBlockNumber: 0,
        requestByte: YMODEM.CRC16,
        timeoutMs,
        retryLimit,
        label: "file header",
      });

      if (headerFrame.type === "eot") {
        await writeAndDrain(serialPort, Buffer.from([YMODEM.ACK]));
        continue;
      }

      const fileInfo = parseYmodemFileInfoPayload(headerFrame.payload);
      await writeAndDrain(serialPort, Buffer.from([YMODEM.ACK]));
      if (!fileInfo) {
        break;
      }

      await writeAndDrain(serialPort, Buffer.from([YMODEM.CRC16]));
      onProgress?.({ transferredBytes: 0, totalBytes: fileInfo.totalBytes, stage: "header" });

      const filePath = await resolveUniqueDestinationPath(resolvedDestinationDir, fileInfo.fileName);
      let fileHandle;
      let closeNeeded = false;
      let createdFile = false;
      try {
        fileHandle = await fs.promises.open(filePath, "wx");
        closeNeeded = true;
        createdFile = true;
        const writtenBytes = await receiveYmodemFileData({
          serialPort,
          reader,
          fileHandle,
          totalBytes: fileInfo.totalBytes,
          timeoutMs,
          retryLimit,
          onProgress,
        });
        await fileHandle.close();
        closeNeeded = false;

        files.push({
          fileName: path.basename(filePath),
          filePath,
          totalBytes: fileInfo.totalBytes,
          writtenBytes,
        });
      } catch (error) {
        if (closeNeeded) {
          await fileHandle.close().catch(() => {});
        }
        if (createdFile) {
          await fs.promises.rm(filePath, { force: true }).catch(() => {});
        }
        throw error;
      }
    }

    const totalBytes = files.reduce((sum, file) => sum + file.totalBytes, 0);
    const writtenBytes = files.reduce((sum, file) => sum + file.writtenBytes, 0);
    return {
      files,
      fileCount: files.length,
      totalBytes,
      writtenBytes,
      fileName: files[0]?.fileName,
      filePath: files[0]?.filePath,
    };
  } finally {
    reader.cleanup();
  }
}

async function sendYmodemBuffer(serialPort, {
  filename,
  buffer,
  mtime = 0,
  timeoutMs = DEFAULT_TIMEOUT_MS,
  retryLimit = DEFAULT_RETRY_LIMIT,
  abortSignal,
  onProgress,
} = {}) {
  if (!serialPort) {
    throw new YmodemTransferError("Serial session is not available", "YMODEM_NO_SERIAL");
  }
  const fileBuffer = Buffer.isBuffer(buffer) ? buffer : Buffer.from(buffer || []);
  const dataPackets = createYmodemDataPackets(fileBuffer);
  const packets = [
    createYmodemFileInfoPacket({ filename, size: fileBuffer.length, mtime }),
    ...dataPackets,
  ];
  const reader = createSerialByteReader(serialPort, abortSignal);
  let packetsSent = 0;

  try {
    await readExpected(reader, [YMODEM.CRC16], timeoutMs, "initial receiver request");
    await sendPacketWithRetry({
      serialPort,
      reader,
      packet: packets[0],
      timeoutMs,
      retryLimit,
      onProgress: () => {
        packetsSent += 1;
        onProgress?.({ transferredBytes: 0, totalBytes: fileBuffer.length, stage: "header" });
      },
    });

    for (let attempt = 0; attempt < retryLimit; attempt += 1) {
      const response = await readExpected(reader, [YMODEM.CRC16, YMODEM.NAK], timeoutMs, "data receiver request");
      if (response === YMODEM.CRC16) break;
      await sendPacketWithRetry({
        serialPort,
        reader,
        packet: packets[0],
        timeoutMs,
        retryLimit,
        onProgress: () => {
          packetsSent += 1;
          onProgress?.({ transferredBytes: 0, totalBytes: fileBuffer.length, stage: "header" });
        },
      });
      if (attempt === retryLimit - 1) {
        throw new YmodemTransferError("YMODEM receiver did not request data after header", "YMODEM_RETRY_LIMIT");
      }
    }

    for (let i = 1; i < packets.length; i += 1) {
      await sendPacketWithRetry({
        serialPort,
        reader,
        packet: packets[i],
        timeoutMs,
        retryLimit,
        onProgress: () => {
          packetsSent += 1;
          onProgress?.({
            transferredBytes: Math.min(i * YMODEM.PACKET_SIZE_1024, fileBuffer.length),
            totalBytes: fileBuffer.length,
            stage: "data",
          });
        },
      });
    }

    await writeAndDrain(serialPort, Buffer.from([YMODEM.EOT]));
    const eotResponse = await readExpected(reader, [YMODEM.NAK, YMODEM.ACK], timeoutMs, "first EOT response");
    if (eotResponse === YMODEM.NAK) {
      await writeAndDrain(serialPort, Buffer.from([YMODEM.EOT]));
      await readExpected(reader, [YMODEM.ACK], timeoutMs, "second EOT ACK");
    }

    await readExpected(reader, [YMODEM.CRC16], timeoutMs, "end session request");
    await sendPacketWithRetry({
      serialPort,
      reader,
      packet: createYmodemEndSessionPacket(),
      timeoutMs,
      retryLimit,
    });

    onProgress?.({
      transferredBytes: fileBuffer.length,
      totalBytes: fileBuffer.length,
      stage: "complete",
    });

    return {
      fileName: path.basename(filename || "file"),
      totalBytes: fileBuffer.length,
      writtenBytes: fileBuffer.length,
      packetsSent,
    };
  } finally {
    reader.cleanup();
  }
}

async function sendYmodemFile(serialPort, filePath, options = {}) {
  const stat = await fs.promises.stat(filePath);
  if (!stat.isFile()) {
    throw new YmodemTransferError("Selected path is not a file", "YMODEM_NOT_FILE");
  }
  const buffer = await fs.promises.readFile(filePath);
  return sendYmodemBuffer(serialPort, {
    ...options,
    filename: filePath,
    buffer,
    mtime: Math.floor(stat.mtimeMs / 1000),
  });
}

module.exports = {
  YMODEM,
  YmodemTransferError,
  crc16Xmodem,
  createYmodemFileInfoPacket,
  createYmodemDataPackets,
  createYmodemEndSessionPacket,
  receiveYmodemFiles,
  sendYmodemCancel,
  sendYmodemBuffer,
  sendYmodemFile,
};