Design to keep complexity low from day one.
Targets
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Cyclomatic complexity per function ≤ 7 (start splitting at 5)
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Function length ≤ 80 lines (aim for ≤ 30)
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One responsibility per function; one axis of variation per module
Tactics
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Use early returns and guard clauses; avoid deep nesting
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Extract branch bodies into named helpers
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Replace long if/else/switch with tables (maps) or strategies
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Separate phases: parse → validate → normalize → act
Signs to refactor now
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Hard-to-test code paths
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Repeated conditionals with subtle differences
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Mixed concerns (IO, validation, transformation) in one method