I was lead developer for a company ten years ago who developed tools for industrial companies. We had a steel rack manufacturer approach us with a panic inducing spreadsheet of the calculations they needed. It was 8 very full worksheets and included everything you could think of, including seismic data by zip code.

To build this proper was going to take our small team at least a couple months. The sales people didn’t like this. I had an idea however to extend a SpreadsheetML parser I’d written a few years prior with a formula evaluator. I had a prototype working within a couple days, I reimplemented a large number of Excel functions warts and all, and since Excel saved all the last computed values along side the formulas it was easy to automatically test. It was the first project that really taught me the value of unit testing.

It was a fun project and took around a month in the end, which was still over the amount of time sales had given us. The client actually sent us a fair number of revisions to the spreadsheet such that had we built it out any other way would have been a major project, but was as easy as swapping the spreadsheet the tool read. I left the company a few months later but I was sad to hear from coworkers they never did anything else with the tool. I thought it was a really neat hack.