/*
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"
)

// WeightedGraph is an interface so we can construct adjacency matrices for
// both directed and undirected graphs.
type WeightedGraph interface {
	graph.Weighted
	graph.WeightedMultigraph
	graph.WeightedMultigraphBuilder

	WeightedEdges() graph.WeightedEdges
}

// WeightedMatrix is an interface to handle the construction of both directed
// and undirected adjacency matrices.
type WeightedMatrix interface {
	graph.Weighted

	SetWeightedEdge(e graph.WeightedEdge)
	Matrix() mat.Matrix
}

// AdjacencyMatrix returns the graph's adjacency matrix.
func AdjacencyMatrix(g WeightedGraph) *mat.Dense {
	var adj WeightedMatrix
	switch g.(type) {
	case *multi.WeightedDirectedGraph:
		adj = simple.NewDirectedMatrix(g.Nodes().Len(), 0, 0, 0)
	case *multi.WeightedUndirectedGraph:
		adj = simple.NewUndirectedMatrix(g.Nodes().Len(), 0, 0, 0)
	default:
		panic("not a graph type we handle")
	}

	matrix := toAdjMatrix(g, adj)
	return matrix
}

func toAdjMatrix(g WeightedGraph, adj WeightedMatrix) *mat.Dense {
	copyEdges(g, adj)
	matrix := addSelfEdges(g, adj)

	return matrix
}

func copyEdges(g WeightedGraph, adj WeightedMatrix) {
	for edges := g.WeightedEdges(); edges.Next(); {
		e := edges.WeightedEdge()
		if e.From() == e.To() {
			// the simple Matrix classes don't handle self loops
			continue
		}

		adj.SetWeightedEdge(e)
	}
}

func addSelfEdges(g WeightedGraph, adj WeightedMatrix) *mat.Dense {
	matrix := 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 {
			matrix.Set(i, i, e.Weight())
		}
	}

	return matrix
}