One thing (amongst many) that Remacs has taught me: there's a lot of well documented C code in Emacs.
miniblog.
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I've documented several additional interesting challenges for syntactic diffing: meaningful comparisons of large string literals, understanding blank line changes, and autoformatters adding minor punctuation!
Usually, npmtrends is a great way of choosing a library that's widely used and probably more reliable and better documented. When choosing a JS parser, the options are remarkably close!
An interesting commit in GNU Emacs: optimising the format command by allowing (format "%s" x) to just return x, saving a string allocation. This was already the place in some parts of elisp, but it's now documented and taken advantage of :)
https://github.com/emacs-mirror/emacs/commit/3db388b0bf83d3138562f09ce25fab8ba89bcc81

