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if it's not in git / SVC, add it as is. Create a "refactor" branch, and liberally use commits
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Treat it like a decompilation
Figure out what something does, and rename it (with a stupidly verbose name, if you have to). Use the IDE refactor tools to rename all instances of that identifier
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Take a function, figure out what it does, and refactor it in a way that makes sense to you
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Use the editor's diff mode to compare duplicate code, extract out anything different into a variable or callback, and combine the code into a function call. Vscode's "select for compare" and "compare with selected" are useful for this
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Track what you're doing / keep notes in something like Obsidian. You can use
[[Wikilinks]]syntax to link between notes, which lets you build a graph structure using your notes as nodes -
be cognizant of "Side Effects"
For example, a function or property, or class might be invoked using Reflection, via a string literal (or even worse, a constructed string). And renaming it can cause a reflective invocation somewhere else random to fail
Or function or operator overloading/overiding doing something bizarre
Or two tightly coupled objects that mutate each other, and expect certain unstated invariants to be held (like, foo() can only be called once, or thingyA.len() must equal thingyB.len()
- write tests if you can, either using a testing framework or custom Python scripts
You can use these to more thoroughly compare behavior between the original and a refactor