On my first day in a new company, not even senior dev yet, I met with the head accountant. I asked about her top problems, she said her top problem was that the application would produce different invoice on screen, different invoice when printed as PDF and a different invoice in the accounting software. About 1% of all invoices were affected but due to amount of billing they were doing (telecommunications and advertising) they needed to have 3 FTEs just to correct the invoices.
And correcting the invoices meant playing with numbers so that at least the PDF and accounting software agreed on the total value and tax.
She also said they had at least 2 different employees and an external company look at it and not able to fix it. She also told me not to bother because she does not believe the problem can be fixed (that's what she was told).
I looked at the software, it had two separate copies of the invoice calculation (separate for on screen and for printing to PDF). And of course it would send the invoice to the accounting software which calculated the invoice in a different way still.
I ran couple of experiments to reverse engineer how the accounting software did the calculations -- the exact order of them and the exact rounding rules. Then I built a small module that captured those calculations. Then I changed all doubles to arbitrary precision.
It took two days and the problem was fixed but it took couple more days before accounting department actually believed it.
> I looked at the software, it had two separate copies of the invoice calculation (separate for on screen and for printing to PDF).
That's actually kinda normal, in some industries.
For example, an amazon marketplace seller's warehouse management system might not be tightly integrated with Amazon's basket/checkout display logic.
In some situations the results of recalculating are supposed to be different. For example, if there's a "5% off when you buy 3 widgets" offer and you check out with 2 widgets in your basket, the offer doesn't apply. But if you checked out 3 widgets, thus getting the offer, then the seller found they were low on stock and could only send you 2, you should get the 5% discount on those 2.
> I looked at the software, it had two separate copies of the invoice calculation (separate for on screen and for printing to PDF). And of course it would send the invoice to the accounting software which calculated the invoice in a different way still.
Your setup made it sound like something crazy and inane, like "the PDF printer used on those machines changed floating point rounding modes" or what have you.
In olden times we would "print to PDF" as in have a little piece of code to format the document that would be sent to a printer which could be a PDF document.
In this particular case we had two separate pieces of code, one running on the client (for on screen presentation) and one on the backend (to create the PDF on a shared location and produce a download URL).
It is easy to be a hero when the company is shitty.
My career advice is to work in a field / company / job where you can be somewhere in the top 10-20% of all employees. Just don't overdo it, if you are top 1% you are probably aiming too low and could be working for better paying, more rewarding field / company / job.
For a lot of my career I was working for financial institutions like banks. A lot of really badly managed projects with definitely not top level developers. Easy to be a top performer. I really like helping people and projects and it was working well for me especially when it was easy for me to provide valuable help.
I got hired once for a really good company with really top performers and suddenly I lost the status that I was so used to. I was keeping up with my work, sure, but I was no longer a shiny star. I got back to working for banks.
And correcting the invoices meant playing with numbers so that at least the PDF and accounting software agreed on the total value and tax.
She also said they had at least 2 different employees and an external company look at it and not able to fix it. She also told me not to bother because she does not believe the problem can be fixed (that's what she was told).
I looked at the software, it had two separate copies of the invoice calculation (separate for on screen and for printing to PDF). And of course it would send the invoice to the accounting software which calculated the invoice in a different way still.
I ran couple of experiments to reverse engineer how the accounting software did the calculations -- the exact order of them and the exact rounding rules. Then I built a small module that captured those calculations. Then I changed all doubles to arbitrary precision.
It took two days and the problem was fixed but it took couple more days before accounting department actually believed it.