Fast Search adjacent cells in an unstructured grid
 

Hello!
I write my own solver. I use a grid format .NEU - gambit file format. This format as the other I know has the following structure fo tetrahedron mesh:
Matrix points. Each row are 3 numbers - the coordinates of the point.
Мatrix cells. Each row is 4 numbers - the points that form this cell.

I need the following matrix:
1 22 33 44 56
2 21 45 67 89
.
.

Each row is 4 number - cells with which this cell contacts through its surface. For example, cell 2 is in contact with the cells 21 45 67 89.

In the case of the .NEU file format, I have to do a nested loop that for each cell goes through all cells. I Get N * N "IF" comparisons. It is too long.

My question1. Is there a file format in which the matrix that I need is contained explicitly?
My question2. Is there a file format in which the matrix that I need I can get a faster way.