Object orientation

Rework the code to use OOP concepts and make the code more flexibla

The issue is to rework the codebase to use object-oriented programming concepts for greater flexibility. Start by examining the existing calculator logic in the repository to understand the current structure. Determine which parts can be encapsulated into classes and how to design the new OOP architecture. There is no specific guidance on files or tests, so this requires significant design work and domain knowledge.

索引モデルが issue の本文から書いたものです。