add lab3

lab3/README.md

@@ -0,0 +1,73 @@
+# Lab 3: Stencil-Based Grid Subdivision
+
+## How to Build
+
+### Windows
+
+Use Visual Studio (Developer Command Prompt) to compile the code.
+
+```bat
+.\make.bat
+```
+
+> **Note:** `cl` is only available in the Developer Command Prompt, change the path of `VsDevCmd.bat` in `make.bat` first.
+
+### Linux / macOS — GCC via Makefile
+
+```
+make
+```
+
+This compiles all targets including `subdivide_grid`.
+
+## How to Run
+
+### Windows
+
+```bat
+subdivide_grid.exe {num_iter} {input_file}
+```
+
+### Linux / macOS
+
+```bash
+./subdivide_grid {num_iter} {input_file}
+```
+
+- `{num_iter}`: integer number of subdivision iterations (>= 0)
+- `{input_file}`: a `.grid2D` file
+
+The output file is written to the same directory as the input, with the suffix
+`-subdiv.grid2D` appended (replacing the `.grid2D` extension).
+
+### Optional `-off` flag
+
+```bash
+./subdivide_grid -off {num_iter} {input_file}
+```
+
+With `-off`, the program additionally writes a Geomview `.off` file named
+`{basename}-subdiv.off`.
+
+### Examples
+
+```bash
+./subdivide_grid 1 "Lab3 simple grids/grid2D-2x3-A.grid2D"
+./subdivide_grid 3 "Lab3 grids/wave-A.grid2D"
+./subdivide_grid -off 3 "Lab3 grids/sphere-A.grid2D"
+```
+
+## Visualization
+
+Convert a `.grid2D` file to Geomview `.off` format:
+
+```bash
+./grid2off {input.grid2D}
+```
+
+## AI Tools Used
+
+**GitHub Copilot** (via VS Code) was used for:
+
+- Auto generating `Makefile` and this `README.md`.
+