Connect2D.m

function [EToE, EToF] = Connect2D(EToV)

% function [EToE, EToF] = Connect2D(EToV)
% Purpose  : Build global connectivity arrays for grid based on
%            standard EToV input array from grid generator

Nfaces = 3;

% Find number of elements and vertices
K = size(EToV,1); Nv = max(max(EToV));

% Create face to node connectivity matrix
TotalFaces = Nfaces*K;

% List of local face to local vertex connections
vn = [[1,2];[2,3];[1,3]];

% Build global face to node sparse array
SpFToV = spalloc(TotalFaces, Nv, 2*TotalFaces);
sk = 1;
for k=1:K
  for face=1:Nfaces
    SpFToV( sk, EToV(k, vn(face,:))) = 1;
    sk = sk+1;
  end
end

% Build global face to global face sparse array
SpFToF = SpFToV*SpFToV' - 2*speye(TotalFaces);

% Find complete face to face connections
[faces1, faces2] = find(SpFToF==2);

% Convert face global number to element and face numbers
element1 = floor( (faces1-1)/Nfaces )  + 1; face1    =   mod( (faces1-1), Nfaces ) + 1;
element2 = floor( (faces2-1)/Nfaces )  + 1; face2    =   mod( (faces2-1), Nfaces ) + 1;

% Rearrange into Nelements x Nfaces sized arrays
ind = sub2ind([K, Nfaces], element1, face1);

EToE = (1:K)'*ones(1,Nfaces); EToF = ones(K,1)*(1:Nfaces);
EToE(ind) = element2; EToF(ind) = face2;
return;