(ledger?) The fair Christmas problem: splitting expenses automatically

[Uwe Brauer]

Hi 

I am not sure whether this is the right place to ask this, but anyhow.


Suppose two friends want to buy Christmas presents, but when they have
finished, they want to have spent the same amount of money. If they
don't a compensation must be transferred.

Here is an example

** Start
#+name: presents
#+begin_src ledger   
2010/01/01 * Starting balance
  assets:bank:savings  0
  income:starting balances
2021/12/01 User1:Jacket
  expenses:User1:Jacket  80
  income:User1:creditcard1
2010/07/24 User1:Pajama
  expenses:User1:Pajama  30
  income:User1:creditcard1
2010/07/23 User2:Flowers
  expenses:User2:Flowers  20
  income:User2:creditcard2
2010/07/23 User2:ChristmasTree
  expenses:User2:ChristmasTree  40.00
  income:User2:creditcard2
#+end_src

#+RESULTS: presents
#+begin_example
                 170  expenses
                 110    User1
                  80      Jacket
                  30      Pajama
                  60    User2
                  40      ChristmasTree
                  20      Flowers
                -170  income
                -110    User1:creditcard1
                 -60    User2:creditcard2
--------------------
                   0
#+end_example

So user1 spent 110, while  user2 spent 60

** Compensation manually
So the compensation is (trivally)

| User1 | User2 | User2-->Uwer1 |
|-------+-------+---------------|
|   110 |    60 |           -25 |
#+TBLFM: $3=($2-$1)/2;f2

Leading to 
#+name: compensated
#+begin_src ledger   
2010/01/01 * Starting balance
  assets:bank:savings  0
  income:starting balances
2021/12/01 User1:Jacket
  expenses:User1:Jacket  80
  income:User1:creditcard1
2010/07/24 User1:Pajama
  expenses:User1:Pajama  30
  income:User1:creditcard1
2010/07/23 User2:Flowers
  expenses:User2:Flowers  20
  income:User2:creditcard2
2010/07/23 User2:ChristmasTree
  expenses:User2:ChristmasTree  40.00
  income:User2:creditcard2
2010/07/23 User2:User1-Compensation
  expenses:User2:User1-Compensation  25.00
  expenses:User1:User1-Compensation
#+end_src

#+RESULTS: compensated
#+begin_example
                 170  expenses
                  85    User1
                  80      Jacket
                  30      Pajama
                 -25      User1-Compensation
                  85    User2
                  40      ChristmasTree
                  20      Flowers
                  25      User1-Compensation
                -170  income
                -110    User1:creditcard1
                 -60    User2:creditcard2
--------------------
                   0
#+end_example




The question is can this be done automatically?

By this I mean
can
2010/07/23 User2:User1-Compensation
  expenses:User2:User1-Compensation  25.00
  expenses:User1:User1-Compensation

Somehow be generated automatically?

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