calc_boundary_perimeter.md

def calc_boundary_perimeter(rectangles):
    # find external perimeter of all rectangles
    external_perimeter = 0
    for rect in rectangles:
        x1, y1, x2, y2 = rect
        external_perimeter += abs(x2 - x1) + abs(y2 - y1)

    # find internal perimeter of overlapping rectangles
    internal_perimeter = 0
    for i, rect1 in enumerate(rectangles):
        for rect2 in rectangles[i+1:]:
            if is_overlapping(rect1, rect2):
                overlap = calc_overlap(rect1, rect2)
                internal_perimeter += 2 * (abs(overlap[0] - overlap[2]) + abs(overlap[1] - overlap[3]))

    return external_perimeter, internal_perimeter
    ```