function [ t2, s2 ] = triangulation_order3_neighbor ( triangle_num, triangle_node, ...
t1, s1 )
%*****************************************************************************80
%
%% triangulation_order3_neighbor() determines a neighbor of a given triangle.
%
% Discussion:
%
% A set of nodes is given. A triangulation of the nodes has been
% defined and recorded in TRIANGLE_NODE. The TRIANGLE_NODE data structure records
% triangles as sets of three nodes, N1, N2, N3, that implicitly define three
% sides, being the line segments N1-N2, N2-N3, and N3-N1.
%
% The nodes of the triangle are listed in counterclockwise order.
% This means that if two triangles share a side, then the nodes
% defining that side occur in the order (N1,N2) for one triangle,
% and (N2,N1) for the other.
%
% The routine is given a triangle and a side, and asked to find
% another triangle (if any) that shares that side. The routine
% simply searches the TRIANGLE_NODE structure for an occurrence of the
% nodes in the opposite order.
%
% Licensing:
%
% This code is distributed under the GNU LGPL license.
%
% Modified:
%
% 06 March 2005
%
% Author:
%
% John Burkardt
%
% Input:
%
% integer TRIANGLE_NUM, the number of triangles.
%
% integer TRIANGLE_NODE(3,TRIANGLE_NUM), the nodes that define
% each triangle.
%
% integer T1, the index of the triangle.
%
% integer S1, the index of the triangle side.
%
% Output:
%
% integer T2, the index of the triangle which is the neighbor
% to T1 on side S1, or -1 if there is no such neighbor.
%
% integer S2, the index of the side of triangle T2 which
% is shared with triangle T1, or -1 if there is no such neighbor.
%
t2 = -1;
s2 = -1;
n1 = triangle_node(s1,t1);
ss = i4_wrap ( s1+1, 1, 3 );
n2 = triangle_node(ss,t1);
for t = 1 : triangle_num
for s = 1 : 3
if ( triangle_node(s,t) == n1 )
ss = i4_wrap ( s-1, 1, 3 );
if ( triangle_node(ss,t) == n2 )
t2 = t;
s2 = ss;
return
end
end
end
end
return
end