道闸屏显数据下发（crc16校验）

const bin2base64 = (input) => {
  var base64_chars =
    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
  var ret =[];
  var i = 0;
  var j = 0;
  var char_array_3 = new Array(3);
  var char_array_4 = new Array(4);
  var in_len = input.length;
  var pos = 0;

  while (in_len--) {
    char_array_3[i++] = input[pos++];
    if (i == 3) {
      char_array_4[0] = (char_array_3[0] & 0xfc) >> 2;
      char_array_4[1] =
        ((char_array_3[0] & 0x03) << 4) + ((char_array_3[1] & 0xf0) >> 4);
      char_array_4[2] =
        ((char_array_3[1] & 0x0f) << 2) + ((char_array_3[2] & 0xc0) >> 6);
      char_array_4[3] = char_array_3[2] & 0x3f;

      for (i = 0; i < 4; i++) ret += base64_chars.charAt(char_array_4[i]);
      i = 0;
    }
  }

  if (i) {
    for (j = i; j < 3; j++) char_array_3[j] = 0;

    char_array_4[0] = (char_array_3[0] & 0xfc) >> 2;
    char_array_4[1] =
      ((char_array_3[0] & 0x03) << 4) + ((char_array_3[1] & 0xf0) >> 4);
    char_array_4[2] =
      ((char_array_3[1] & 0x0f) << 2) + ((char_array_3[2] & 0xc0) >> 6);
    char_array_4[3] = char_array_3[2] & 0x3f;

    for (j = 0; j < i + 1; j++) ret += base64_chars.charAt(char_array_4[j]);

    while (i++ < 3) ret += "=";
  }

  return ret;
}

const hex2Base64 = (hexStr) => {
  hexStr = hexStr.replace(/ /g, '')
  var binary = [];
  for (var i = 0; i < hexStr.length / 2; i++) {
    var h = hexStr.substr(i * 2, 2);
    binary[i] = parseInt(h, 16);
  }
  return bin2base64(binary);
}

const table = [
  0x0000, 0x8005, 0x800f, 0x000a, 0x801b, 0x001e, 0x0014, 0x8011,
  0x8033, 0x0036, 0x003c, 0x8039, 0x0028, 0x802d, 0x8027, 0x0022,
  0x8063, 0x0066, 0x006c, 0x8069, 0x0078, 0x807d, 0x8077, 0x0072,
  0x0050, 0x8055, 0x805f, 0x005a, 0x804b, 0x004e, 0x0044, 0x8041,
  0x80c3, 0x00c6, 0x00cc, 0x80c9, 0x00d8, 0x80dd, 0x80d7, 0x00d2,
  0x00f0, 0x80f5, 0x80ff, 0x00fa, 0x80eb, 0x00ee, 0x00e4, 0x80e1,
  0x00a0, 0x80a5, 0x80af, 0x00aa, 0x80bb, 0x00be, 0x00b4, 0x80b1,
  0x8093, 0x0096, 0x009c, 0x8099, 0x0088, 0x808d, 0x8087, 0x0082,
  0x8183, 0x0186, 0x018c, 0x8189, 0x0198, 0x819d, 0x8197, 0x0192,
  0x01b0, 0x81b5, 0x81bf, 0x01ba, 0x81ab, 0x01ae, 0x01a4, 0x81a1,
  0x01e0, 0x81e5, 0x81ef, 0x01ea, 0x81fb, 0x01fe, 0x01f4, 0x81f1,
  0x81d3, 0x01d6, 0x01dc, 0x81d9, 0x01c8, 0x81cd, 0x81c7, 0x01c2,
  0x0140, 0x8145, 0x814f, 0x014a, 0x815b, 0x015e, 0x0154, 0x8151,
  0x8173, 0x0176, 0x017c, 0x8179, 0x0168, 0x816d, 0x8167, 0x0162,
  0x8123, 0x0126, 0x012c, 0x8129, 0x0138, 0x813d, 0x8137, 0x0132,
  0x0110, 0x8115, 0x811f, 0x011a, 0x810b, 0x010e, 0x0104, 0x8101,
  0x8303, 0x0306, 0x030c, 0x8309, 0x0318, 0x831d, 0x8317, 0x0312,
  0x0330, 0x8335, 0x833f, 0x033a, 0x832b, 0x032e, 0x0324, 0x8321,
  0x0360, 0x8365, 0x836f, 0x036a, 0x837b, 0x037e, 0x0374, 0x8371,
  0x8353, 0x0356, 0x035c, 0x8359, 0x0348, 0x834d, 0x8347, 0x0342,
  0x03c0, 0x83c5, 0x83cf, 0x03ca, 0x83db, 0x03de, 0x03d4, 0x83d1,
  0x83f3, 0x03f6, 0x03fc, 0x83f9, 0x03e8, 0x83ed, 0x83e7, 0x03e2,
  0x83a3, 0x03a6, 0x03ac, 0x83a9, 0x03b8, 0x83bd, 0x83b7, 0x03b2,
  0x0390, 0x8395, 0x839f, 0x039a, 0x838b, 0x038e, 0x0384, 0x8381,
  0x0280, 0x8285, 0x828f, 0x028a, 0x829b, 0x029e, 0x0294, 0x8291,
  0x82b3, 0x02b6, 0x02bc, 0x82b9, 0x02a8, 0x82ad, 0x82a7, 0x02a2,
  0x82e3, 0x02e6, 0x02ec, 0x82e9, 0x02f8, 0x82fd, 0x82f7, 0x02f2,
  0x02d0, 0x82d5, 0x82df, 0x02da, 0x82cb, 0x02ce, 0x02c4, 0x82c1,
  0x8243, 0x0246, 0x024c, 0x8249, 0x0258, 0x825d, 0x8257, 0x0252,
  0x0270, 0x8275, 0x827f, 0x027a, 0x826b, 0x026e, 0x0264, 0x8261,
  0x0220, 0x8225, 0x822f, 0x022a, 0x823b, 0x023e, 0x0234, 0x8231,
  0x8213, 0x0216, 0x021c, 0x8219, 0x0208, 0x820d, 0x8207, 0x0202
]

const crc16CheckSum = (data) => {
  if (!data.length) return 0x00
  if (!Buffer.isBuffer(data)) data = Buffer.from(data)

  let crc = 0x00
  let tableIdx

  for (let d of data) {
    tableIdx = (crc >> 8) ^ d
    crc = ((crc << 8) ^ table[tableIdx]) & 0xffff;
  }

  return crc.toString(16).toUpperCase()
}

const hex2Arr = (hex) => {
  //清除所有空格
  hex = hex.replace(/\s/g, "");
  //若字符个数为奇数，补一个空格
  hex += hex.length % 2 ? " " : "";

  const arr = [];
  for (var i = 0; i < hex.length / 2; i++) {
    arr.push(hex.substr(i * 2, 2));
  }
  return ('0x' + arr.join(' 0x')).split(' ');
};

let gen485Data = (fnCode = 10, word = '', subcmd = '', loop = 1) => {
//   const iconv = require('iconv-lite')

  const hexStart = 'AA55'
  const hexFirst = 'FFFF' + String(fnCode).toUpperCase().padStart(2, '0')
  const hexSubcmd = subcmd && String(subcmd).toString(16).toUpperCase().padStart(2, '0')
  const hexLoop = loop && String(loop).toString(16).toUpperCase().padStart(2, '0')
  const hexData = iconvLite.encode(word, 'GBK').toString('hex').toUpperCase()
  const hexLen = (~~!!hexSubcmd + ~~!!hexLoop + hexData.length / 2 + 2).toString(16).toUpperCase().padStart(2, '0')
  let hexStr = hexFirst + hexLen + hexSubcmd + hexLoop + hexData
  const hexCrc = crc16CheckSum(hex2Arr(hexStr))
  
  const targetData = hex2Base64(hexStart + hexStr + hexCrc)

  return targetData
}