LedgerSplit vim function

I manage my finances using ledger. Periodically, I export my bank statements and push them through a make pipeline to get a ledger file. The conversion assigns transactions to accounts based on a set of rules, e.g: If the payee is “Foo Shop”, the account is going to be “Expenses:Food”:

2023-03-09 Foo Shop
    Expenses:Food           1000 HUF
    Assets:Card

However, sometimes I buy e.g: toothpaste in “Foo Shop”, that I don’t want to eat. I do not want to contaminate my ledger, so I save the receipt and mark the item. During the periodic export, I manually fix the ledger file in vim:

2023-03-09 Foo Shop
    Expenses:Food            750 HUF
    Expenses:Health          250 HUF
    Assets:Card

Since I have an engineering degree, most of the time I get this subtraction right. For the same reason, I made a tiny vim script, that automates some of this. Standing on this line:

    Expenses:Food           1000 HUF

Issuing this command:

:LedgerSplit 100+200

Modifies the posting like this:

    Expenses:Food           700 HUF
    Expenses:Food           300 HUF

It splits the given amount from the current posting, and moves it to the next line. It handles expressions (as shown) and decimal fractions. After this I just need to fix the second account name.

Installation

To .vimrc, add:

au BufRead,BufNewFile *.ledger set filetype=ledger

To .vim/after/ftplugin/ledger.vim:

function! LedgerSplit(amount) abort
  let pat = "[0-9\\.]\\+"
  let line = getline(".")
  let old = str2float(matchstr(line, pat, 0))
  let new = substitute(printf("%.2f", old-a:amount), "\\.00$", "", "")
  let split = substitute(printf("%.2f", a:amount), "\\.00$", "", "")
  call setline(".", substitute(line, pat, new, ""))
  call append(".", substitute(line, pat, split, ""))
endfunction

command! -nargs=1 LedgerSplit call LedgerSplit(<args>)