Measuring how much I write in vim by comparing word count on each save

Question

Suppose I want to write N words related to my studies in Vim everyday as my daily goal.

Every time I am done writing something I can do g+Ctrl-g and then I can send that number variable to a file. I want help in automating this.

If at each save vim can execute a shell command like Words diff (where Words is a bash script in my path) with diff being the word difference between the two saves, this can be automated.

The problem is that I don't know vim-script. With the basic structure of the function I think I will be able to tweak it to my needs.

Can you help?

Edit: full explanation

I imagine something like a :Write mode which sends the new added words (diff: difference between saves) on each save to a file.

Similar to this mode, I imagine an :Edit mode, which would do almost the same thing.. instead of sending the difference diff, it would send 2*abs(diff).

I use vim splits very often, so a full shell script solution which uses the only original file name can't solve my problem. For each split I would like to invoke these command functions separately.

I can do basic bash scripts, so you can leave the writing to file part to me.

Answer 1

I modified this answer by Martin Tournoij to my needs.

fun! s:write() abort
     if get(b:, 'write_start', 0) is 0
        let b:write_start = wordcount()
     else
        let l:words = wordcount()['words'] - b:write_start['words']
        let b:write_start = wordcount()
        execute !BashScript l:words
     endif
endfun

command! Write call s:write()

where the BashScript is in the path and can be as simple as

#!/bin/bash
num=$1
echo $num >> path/to/your/file

Now I can call :Write at the beginning to initialize the word count and thereafter, on each :Write will append the added words to my file. This can of course be made fast using a custom remap.

It's trivial to repeat this for :Edit mode now.