I love how the first example is "use the common interfaces for new code". If only! That assumes there _is_ a common interface for doing a common task, and things aren't just a copy-paste of similar code and tweaked to fit the use case.
So the only tweak I'd make here, is that if you are tempted to copy a bit of code that is already in 100 places, but with maybe 1% of a change - please, for the love of god, make a common function and parameterize out the differences. Pick a dozen or so instances throughout the codebase and replace it with your new function, validating the abstraction. So begins the slow work of improving an old code base created by undisciplined hands.
Oh, and make sure you have regression tests. The stupider the better. For a given input, snapshot the output. If that changes, audit the change. If the program only has user input, consider capturing it and playing it back, and if the program has no data as output, consider snapshotting the frames that have been rendered.
So the only tweak I'd make here, is that if you are tempted to copy a bit of code that is already in 100 places, but with maybe 1% of a change - please, for the love of god, make a common function and parameterize out the differences. Pick a dozen or so instances throughout the codebase and replace it with your new function, validating the abstraction. So begins the slow work of improving an old code base created by undisciplined hands.
Oh, and make sure you have regression tests. The stupider the better. For a given input, snapshot the output. If that changes, audit the change. If the program only has user input, consider capturing it and playing it back, and if the program has no data as output, consider snapshotting the frames that have been rendered.