/********************************************************************
_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);
}
}
int _ColorVertices_IdentifyVertices(graphP theGraph, int u, int v, int eBefore)
{
ColorVerticesContext *context = (ColorVerticesContext *) gp_GetExtension(theGraph, COLORVERTICES_ID);
if (context != NULL)
{
int e_v_last, e_v_first;
// First, identify u and v. No point in taking v's degree beforehand
// because some of its incident edges may indicate neighbors of u. This
// causes v to be moved to a lower degree list than deg(v).
if (context->functions.fpIdentifyVertices(theGraph, u, v, eBefore) != OK)
return NOTOK;
// The edges transferred from v to u are indicated on the top of the
// stack, which looks like this after identifying u and v:
// ... e_u_succ e_v_last e_v_first e_u_pred u v
e_v_first = sp_Get(theGraph->theStack, sp_GetCurrentSize(theGraph->theStack)-4);
e_v_last = sp_Get(theGraph->theStack, sp_GetCurrentSize(theGraph->theStack)-5);
// We count the number of edges K transferred from v to u after the
// common edges were hidden
if (gp_IsArc(theGraph, e_v_first))
{
int J, K, degu;
for (J=e_v_first, K=1; J != e_v_last; J=gp_GetNextArc(theGraph, J))
K++;
// Remove v from the degree list K. During IdentifyVertices(), if v had any
// common edges with u, they were "hidden", which reduced the degree of v to K.
_RemoveVertexFromDegList(context, theGraph, v, K);
// We move u from degree list deg(u)-K to degree list deg(u)
degu = gp_GetVertexDegree(theGraph, u);
_RemoveVertexFromDegList(context, theGraph, u, degu-K);
_AddVertexToDegList(context, theGraph, u, degu);
}
return OK;
}
return NOTOK;
}
int gp_ColorVertices(graphP theGraph)
{
ColorVerticesContext *context = NULL;
int v, deg;
int u=0, w=0, contractible;
// Attach the algorithm if it is not already attached
if (gp_AttachColorVertices(theGraph) != OK)
return NOTOK;
// Ensure there is enough stack to perform this operation.
// At a maximum, the graph reduction will push 7N+M integers.
// One integer is pushed per edge that is hidden. Plus, whether
// a vertex is hidden or identified with another vertex, 7 integers
// are used to store enough information to restore it.
if (sp_NonEmpty(theGraph->theStack))
return NOTOK;
if (sp_GetCapacity(theGraph->theStack) < 7*theGraph->N + theGraph->M)
{
stackP newStack = sp_New(7*theGraph->N + theGraph->M);
if (newStack == NULL)
return NOTOK;
sp_Free(&theGraph->theStack);
theGraph->theStack = newStack;
}
// Get the extension context and reinitialize it if necessary
gp_FindExtension(theGraph, COLORVERTICES_ID, (void *)&context);
if (context->color[0] > -1)
_ColorVertices_Reinitialize(context);
// Initialize the degree lists, and provide a color for any trivial vertices
for (v = 0; v < theGraph->N; v++)
{
deg = gp_GetVertexDegree(theGraph, v);
_AddVertexToDegList(context, theGraph, v, deg);
if (deg == 0)
context->color[v] = 0;
}
// Initialize the visited flags so they can be used during reductions
_FillVisitedFlags(theGraph, 0);
// Reduce the graph using minimum degree selection
while (context->numVerticesToReduce > 0)
{
v = _GetVertexToReduce(context, theGraph);
// Find out if v is contractible and the neighbors to contract
contractible = _GetContractibleNeighbors(context, v, &u, &w);
// Remove the vertex from the graph. This calls the fpHideEdge
// overload, which performs the correct _RemoveVertexFromDegList()
// and _AddVertexToDegList() operations on v and its neighbors.
if (gp_HideVertex(theGraph, v) != OK)
return NOTOK;
// If v was contractibile, then identify u and w
if (contractible)
{
if (gp_IdentifyVertices(theGraph, u, w, NIL) != OK)
return NOTOK;
}
}
// Restore the graph one vertex at a time, coloring each vertex distinctly
// from its neighbors as it is restored.
context->colorDetector = (int *) calloc(theGraph->N, sizeof(int));
if (context->colorDetector == NULL)
return NOTOK;
if (gp_RestoreVertices(theGraph) != OK)
return NOTOK;
free(context->colorDetector);
context->colorDetector = NULL;
return OK;
}
