doc(rfel): refactoring plan

Signed-off-by: Jiefeng Li <jiefeng_li@hust.edu.cn>

Author: Jiefeng Li

rfel/plan.md

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+# xfel to Rust rfel Refactoring To-Do List
+
+## Phase 1: Foundational Work
+
+- [ ] **Set up the basic Rust project structure.**
+    - Define modules and file layout.
+- [ ] **Define core data structures and constants.**
+- [ ] **Implement command-line argument parsing.**
+    - Define the structure for holding parsed arguments.
+- [ ] **Re-implement simple, self-contained utility functions.**
+    such as:
+    - `filedump`
+    - `hexdump`
+
+## Phase 2: Core Logic and Library Migration (Could be LLM-Assisted)
+
+- [ ] **See the "Core Logic Refactoring Workflow" section for detailed tasks.**
+
+## Phase 3: Integration and Verification
+
+- [ ] **Integrate all refactored modules.**
+- [ ] **Write unit and integration tests.**
+- [ ] **Perform end-to-end testing to ensure functionality matches the original C project.**
+
+## Low Priority
+
+- [ ] **Investigate and understand the `payload` directory.**
+
+
+# LLM-Assisted Refactoring Workflow
+
+## 1. Library and Dependency Replacement
+
+* **Goal:** Replace C libraries and custom functions with modern, idiomatic Rust crates. An LLM can excel here due to its broad knowledge of libraries across different languages.
+* **Workflow:**
+    * [ ] **Identify Candidates:** Use an LLM to scan the C source code and identify all external library calls (e.g. crypto libraries for `sha256.h/.c`, `ecdsa256.h/.c`) and custom utility functions that have common equivalents.
+    * [ ] **Suggest Alternatives:** Prompt the LLM to suggest popular and well-maintained Rust crates that provide equivalent functionality.
+    * [ ] **Generate Wrapper Code:** Once a crate is chosen, ask the LLM to help generate the necessary wrapper code or usage examples to integrate the new crate into our project structure.
+
+## 2. Automated Translation and Idiomatic Refactoring
+
+* **Goal:** Translate the core C logic into safe, idiomatic Rust, improving upon the direct output of automated tools like `c2rust`.
+* **Workflow:**
+    * [ ] **Generate Baseline with `c2rust`:** Run the `c2rust` tool on a target C module to get an initial, albeit rough and unsafe, Rust translation.
+    * [ ] **Semantic Refactoring with LLM:** Provide the LLM with both the **original C source code** (for semantic context) and the **`c2rust`-generated Rust code** (as a starting point).
+    * [ ] **Prompt for Idiomatic Rewrite:** Instruct the LLM to refactor the `c2rust` output into safe, idiomatic Rust. This is the most critical step. (Notice that the context could not be too long, so maybe we need to break it into smaller parts.)
+    * [ ] **Iterative Review:** This will be an iterative process. Start with small, pure functions and gradually move to more complex code. **Human oversight is critical** to validate the correctness and safety of the LLM's output.