/*
Copyright © 2025 filifa

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 3 of the License, or
(at your option) any later version.

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.

You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
package graph

import (
	"gonum.org/v1/gonum/graph"
	"gonum.org/v1/gonum/graph/multi"
	"gonum.org/v1/gonum/graph/simple"
	"gonum.org/v1/gonum/mat"
)

// DirectedGraph embeds a multi.WeightedDirectedGraph (as opposed to
// simple.WeightedDirectedGraph) to handle self loops.
type DirectedGraph struct {
	*multi.WeightedDirectedGraph
}

// NewDirectedGraph returns a graph with no nodes or edges.
func NewDirectedGraph() *DirectedGraph {
	return &DirectedGraph{WeightedDirectedGraph: multi.NewWeightedDirectedGraph()}
}

// AbsorbingNodes returns all the nodes in the Markov chain identified as
// absorbing nodes.
func (g *DirectedGraph) AbsorbingNodes() []graph.Node {
	absorbingNodes := make([]graph.Node, 0)
	for nodes := g.Nodes(); nodes.Next(); {
		u := nodes.Node()
		successors := g.From(u.ID())
		if successors.Len() != 1 {
			continue
		}

		successors.Next()
		v := successors.Node()
		if u == v {
			absorbingNodes = append(absorbingNodes, u)
		}
	}

	return absorbingNodes
}

// AdjacencyMatrix returns the graph's adjacency matrix.
func (g *DirectedGraph) AdjacencyMatrix() *mat.Dense {
	adj := simple.NewDirectedMatrix(g.Nodes().Len(), 0, 0, 0)
	for edges := g.WeightedEdges(); edges.Next(); {
		e := edges.WeightedEdge()
		if e.From() == e.To() {
			continue
		}

		adj.SetWeightedEdge(e)
	}

	a := mat.DenseCopyOf(adj.Matrix())

	nodes := adj.Nodes()
	for i := 0; nodes.Next(); i++ {
		id := nodes.Node().ID()
		u := g.Node(id)

		e := g.WeightedEdge(u.ID(), u.ID())
		if e != nil {
			a.Set(i, i, e.Weight())
		}
	}

	return a
}