I Implemented Myers Diff in 130 Lines, Then Lost Half a Day to an Off-by-One Bug

I Didn't Want Another Dependency I could have installed a diff library and been done in five minutes. I know. But my developer tool suite PureMark has a strict zero-dependency policy. The JSON Formatter, Base64 decoder, URL Encoder — all hand-written. Letting the Diff Checker be the one exception didn't sit right. Besides, I use git diff every single day, and I couldn't explain what was actually happening behind the scenes. That bothered me. Turns out, it's a 1986 algorithm by Eugene Myers. git diff , the Unix diff command — they all use it under the hood. Nearly 40 years old and still the standard. When I implemented it, it fit in about 130 lines of TypeScript . This article covers how the algorithm works and the off-by-one bug that cost me half a day. Try the finished product here: PureMark Diff Checker Paste two blocks of text and see the diff highlighted instantly. Reframing Diff as a Shortest Path Problem The core insight of Myers' algorithm is turning a diff problem into a shorte