将C校验和函数转换为Lua

I am writing a script to allow my host device to send data files to a slave device. The slave requires a checksum calculation to be made and added to the end of my requests prior to sending the file(s). My problem is that not only am I fairly new to programming, but I'm still trying to fully grasp bit manipulation. I'm currently in a Java class so the checksum function so portions of the functions do have a familiar format, but since I'm still scratching my head with bits and the bit library, I'm having problems translating the provided checksum function into Lua. I am writing a script to allow my host device t