Link to Guide on How to Build Spreadsheet Software from Ground Up?

I saw a guide some time ago on the topic of building a spreadsheet software. It is not anonymously written, and it's not on spam platforms like medium or substack, it's hosted on its own (a personal blog? a dedicated site? a github page? I'm not sure).

The guide is in written form, very profound, and discusses entire process from ground up. The author tells about their own experience, explains about different trade-offs and edge-cases.

Would really appreciate, if someone shares the link with me.