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/* Copyright (c) 1997-2024
Ewgenij Gawrilow, Michael Joswig, and the polymake team
Technische Universität Berlin, Germany
https://polymake.org
This program is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the
Free Software Foundation; either version 2, or (at your option) any
later version: http://www.gnu.org/licenses/gpl.txt.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
--------------------------------------------------------------------------------
*/
#pragma once
#include "polymake/SparseMatrix.h"
#include "polymake/Graph.h"
namespace pm {
// the node-edge incidence matrix of a directed graph is totally unimodular
template <typename Coord, typename Graph> inline
SparseMatrix<Coord> node_edge_incidences(const GenericGraph<Graph, graph::Directed>& G)
{
SparseMatrix<Coord> ne(G.top().nodes(), G.top().edges());
Int k = 0;
for (auto e = entire(edges(G)); !e.at_end(); ++e, ++k) {
ne(e.to_node(), k) = 1;
ne(e.from_node(), k) =-1;
}
return ne;
}
// this is the undirected case
// the node-edge incidence matrix of a /bipartite/ graph is also totally unimodular
template <typename Coord, typename Graph> inline
SparseMatrix<Coord> node_edge_incidences(const GenericGraph<Graph, graph::Undirected>& G)
{
SparseMatrix<Coord> ne(G.top().nodes(), G.top().edges());
Int k = 0;
for (auto e=entire(edges(G)); !e.at_end(); ++e, ++k) {
ne(e.to_node(), k) = 1;
ne(e.from_node(), k) = 1;
}
return ne;
}
}
namespace polymake {
using pm::node_edge_incidences;
}
// Local Variables:
// mode:C++
// c-basic-offset:3
// indent-tabs-mode:nil
// End:
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