int _GetContractibleNeighbors(ColorVerticesContext *context, int v, int *pu, int *pw)
{
int lowDegreeNeighbors[5], i, j, n=0, J;
graphP theGraph = context->theGraph;
// This method is only applicable to degree 5 vertices
if (_GetVertexDegree(context, v) != 5)
return FALSE;
// Get all neighbors of degree at most 7
J = gp_GetFirstArc(theGraph, v);
while (gp_IsArc(theGraph, J))
{
if (_GetVertexDegree(context, theGraph->G[J].v) <= 7)
lowDegreeNeighbors[n++] = theGraph->G[J].v;
J = gp_GetNextArc(theGraph, J);
}
// Seek the pair of *non-adjacent* low degree neighbors
for (i=0; i < (n-1); i++)
for (j=i+1; j < n; j++)
if (!gp_IsNeighbor(theGraph, lowDegreeNeighbors[i], lowDegreeNeighbors[j]))
{
*pu = lowDegreeNeighbors[i];
*pw = lowDegreeNeighbors[j];
return TRUE;
}
// The desired pair of neighbors was not found
return FALSE;
}
/********************************************************************
_ColorVertices_HideEdge()
An overload to perform the degree list updates for the edge endpoints.
This routine also covers the work done by _HideVertex() and part of
the work done by _ContractEdge() and _IdentifyVertices().
********************************************************************/
void _ColorVertices_HideEdge(graphP theGraph, int e)
{
ColorVerticesContext *context = (ColorVerticesContext *) gp_GetExtension(theGraph, COLORVERTICES_ID);
if (context != NULL)
{
int u, v, udeg, vdeg;
// Get the endpoint vertices of the edge
u = theGraph->G[e].v;
v = theGraph->G[gp_GetTwinArc(theGraph, e)].v;
// Get the degrees of the vertices
udeg = _GetVertexDegree(context, u);
vdeg = _GetVertexDegree(context, v);
// Remove them from the degree lists that contain them
_RemoveVertexFromDegList(context, theGraph, u, udeg);
_RemoveVertexFromDegList(context, theGraph, v, vdeg);
// Hide the edge
context->functions.fpHideEdge(theGraph, e);
// Decrement the degrees of the endpoint vertices
udeg--;
vdeg--;
// Add them to the new degree lists
_AddVertexToDegList(context, theGraph, u, udeg);
_AddVertexToDegList(context, theGraph, v, vdeg);
}
}