Region Copy: Implement an Efficient Rectangular Region Copy in a 2D Grid

Question

## Problem

You are given a 2D grid (matrix) of integers. Implement an operation that copies a rectangular region defined by its top-left and bottom-right corners to a new destination position, handling overlapping source and destination correctly.

```python
def copy_region(
    grid: list[list[int]],
    src_r1: int, src_c1: int,  # top-left of source
    src_r2: int, src_c2: int,  # bottom-right of source
    dst_r: int, dst_c: int     # top-left of destination
) -> list[list[int]]:
    # Modifies grid in place; returns grid
    pass
```

**Example:**
```
grid = [[1,2,3],[4,5,6],[7,8,9]]
copy_region(grid, 0,0, 1,1, 1,1)
# Copies top-left 2x2 [[1,2],[4,5]] to position (1,1)
# Result: [[1,2,3],[4,1,2],[7,4,5]]
```

## Follow-ups

1. When source and destination overlap, what copy order (row direction) prevents data corruption?
2. How does `memmove` handle overlapping regions in C — and what is the analogous logic here?
3. How would you support undo of a copy operation efficiently?
4. If the grid is stored as a flat array, how do you compute the 1D index from (row, col)?

Region Copy: Implement an Efficient Rectangular Region Copy in a 2D Grid

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