code: function parseOperand(reader, b0) {
              var b1;
              var b2;
              var b3;
              var b4;
              if (b0 === 28) {
                  b1 = reader.readUint8();
                  b2 = reader.readUint8();
                  return b1 << 8 | b2;
              }
  
              if (b0 === 29) {
                  b1 = reader.readUint8();
                  b2 = reader.readUint8();
                  b3 = reader.readUint8();
                  b4 = reader.readUint8();
                  return b1 << 24 | b2 << 16 | b3 << 8 | b4;
              }
  
              if (b0 === 30) {
                  return parseFloatOperand(reader);
              }
  
              if (b0 >= 32 && b0 <= 246) {
                  return b0 - 139;
              }
  
              if (b0 >= 247 && b0 <= 250) {
                  b1 = reader.readUint8();
                  return (b0 - 247) * 256 + b1 + 108;
              }
  
              if (b0 >= 251 && b0 <= 254) {
                  b1 = reader.readUint8();
                  return -(b0 - 251) * 256 - b1 - 108;
              }
  
              throw new Error('invalid b0 ' + b0 + ',at:' + reader.offset);
          }
docs: /* eslint-enable no-constant-condition 
  解析cff字典数据
  
  @param  {Reader} reader 读取器
  @param  {number} b0     操作码
  @return {number}        数据