大改修のコードレビュー

間違えないように変更したつもりですがミスはつきもの．どなたかレビューお願いできませんか？　当たり前ですが，元の関数と同じ結果を返すという動作確認もお願いしたいです．
I tried to change it so that there would be no mistakes, but mistakes are bound to happen. Can someone please review it for me? Of course, I would also like to confirm that it returns the same result as the original function.
[d656121d]

Start by inspecting commit d656121d and identify the files and original functions changed by the large refactor. Compare the modified behavior with the original implementation, then add or run checks showing that each function returns the same results. Done means the review findings are resolved and behavioral equivalence is demonstrated.

Written by the indexing model from the issue text.