Transfer files over TCP

• Send file or folder
• resume from break-point
• update existing copy
• GUI

A folder is treated as multiple files in it.
To start, client sends the file header including structure as follows:

• name_size indicates the length of the name that follows.
• name is the UTF-8 encoded file name, with its relative path if sent in folder, it uses the same unix seperator in header and is connverted before sending and after receiving.
• md5 is the calculated MD5 of the original file.
• compress indicates weather the following data is compressed or not.
• data_size is the size of data(compressed size if compress)

On server side, it first read name_size, if 0 then the transfer process is over, else it reads all the header then sends shift back to client, which tells client the starting position of file and determines the length of data that follows.
Here's how shift is determined:

1. if [name].download exits and md5==[name].md5, then shift=[name].download
2. if [name] exits and its md5 is identical, then shift=0xffffffffffffffff
3. else shift=0

Once client receives shift, it will send data starting from shift.

Python 3 is required.
Tested on both Linux and Windows.