How to determine normal indices to complex bodies using loops?

Question

if (i >= 0 && i <= NX - 1 && j == 0)
{
   phase[i*NY + j] = phase[i * NY + (j + 1)];
}

NX and NY are integers. As you can see, phase[i * NY + (j + 1)] accesses the vertices next to the wall, where wall vertices are accessed by the if loop as if (i >= 0 && i <= NX - 1 && j == 0) (bottom wall). Here it is simple, but for complex bodies (square, triangle, circle), how can I access the NORMAL/Perpendicular vertices outside the body using for loop? In image, how can I access red vertices with the help of black vertices?

square with unknown