[Bug]: Loan interest paid at end of quarter is accounted in next quarter's expenses

[James103]

Version of OpenTTD

Since before svn r1 (via git blame)

Expected result

Loan interest paid at the end of a quarter should be accounted for in the quarterly operating expenses for that quarter.
For example, loan interest paid for March 2022 should be accounted for in the quarterly operating expenses for Q1 2022.

Actual result

Loan interest paid at the end of a quarter is accounted for in the quarterly operating expenses for next quarter.
For example, loan interest paid for March 2022 is accounted for in the quarterly operating expenses for Q2 2022.
Furthermore, if loan interest is paid for December of a given year, that interest gets accounted for in the yearly totals for that year but the quarterly expenses for next year, causing a discrepancy between the annual totals and the quarterly totals (the sum of the quarterlies don't add up to the annual).

Steps to reproduce

1. Take out a loan in December of a given year.
2. Wait until the following January, then repay the loan in full.

This is best reproduced in a scenario where you otherwise would not need a loan (such as in an established company or in a new game with the money cheat).

See JGRennison/OpenTTD-patches#524 for more detailed reproduction steps and a screenshot of the erroneous accounting. One fix, as implemented in JGRPP by @JGRennison, is simply to move CompaniesPayInterest() such that it occurs before CompaniesGenStatistics().

The alternative fix would be to somehow make the loan interest paid for December of a given year to be posted in the annual totals for next year. That would also solve the discrepancy, but in the other direction (loan interest payments for a given month are reported at the beginning of the month after).

[Swastik-Chakravorty]

I wanna work on this issue, can anyone help me, where I can get the code and how can I solve this. If you help me, this is going to be my first contribution in open-source.