What's the most challenging coding problem you've encountered, and how did you overcome it? 😮

Recently I've written a story about this. This was one of the last challenges I faced and described how solved it. Difficult to say whether this was the most challenging, but definitely not an easy one 😀

I used to work as a consultant for a database company - they wanted to ensure their sql engine arithmetic complied with the ANSI SQL standard (arithmetic involving 26 digit numbers should give correct results)...
I came up with a test that generated arithmetic expressions using 26 digit numbers (again randomly generated) - billions of such expressions were fed through the engine - and the results calculated and stored in a file...
The same calculation were again performed using the bc utility on linux... and the two output files compared...
After a week of testing (aforementioned test running all 24x7) - established the confidence...
The power of Linux, shell scripting and bc - rescued us - Linux & its CLI are just wow! 😊