// Copyright ©2014 The gonum Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package simple

import (
	"sort"

	"gonum.org/v1/gonum/graph"
	"gonum.org/v1/gonum/graph/internal/ordered"
	"gonum.org/v1/gonum/mat"
)

// UndirectedMatrix represents an undirected graph using an adjacency
// matrix such that all IDs are in a contiguous block from 0 to n-1.
// Edges are stored implicitly as an edge weight, so edges stored in
// the graph are not recoverable.
type UndirectedMatrix struct {
	mat   *mat.SymDense
	nodes []graph.Node

	self   float64
	absent float64
}

// NewUndirectedMatrix creates an undirected dense graph with n nodes.
// All edges are initialized with the weight given by init. The self parameter
// specifies the cost of self connection, and absent specifies the weight
// returned for absent edges.
func NewUndirectedMatrix(n int, init, self, absent float64) *UndirectedMatrix {
	matrix := make([]float64, n*n)
	if init != 0 {
		for i := range matrix {
			matrix[i] = init
		}
	}
	for i := 0; i < len(matrix); i += n + 1 {
		matrix[i] = self
	}
	return &UndirectedMatrix{
		mat:    mat.NewSymDense(n, matrix),
		self:   self,
		absent: absent,
	}
}

// NewUndirectedMatrixFrom creates an undirected dense graph with the given nodes.
// The IDs of the nodes must be contiguous from 0 to len(nodes)-1, but may
// be in any order. If IDs are not contiguous NewUndirectedMatrixFrom will panic.
// All edges are initialized with the weight given by init. The self parameter
// specifies the cost of self connection, and absent specifies the weight
// returned for absent edges.
func NewUndirectedMatrixFrom(nodes []graph.Node, init, self, absent float64) *UndirectedMatrix {
	sort.Sort(ordered.ByID(nodes))
	for i, n := range nodes {
		if int64(i) != n.ID() {
			panic("simple: non-contiguous node IDs")
		}
	}
	g := NewUndirectedMatrix(len(nodes), init, self, absent)
	g.nodes = nodes
	return g
}

// Node returns the node in the graph with the given ID.
func (g *UndirectedMatrix) Node(id int64) graph.Node {
	if !g.has(id) {
		return nil
	}
	if g.nodes == nil {
		return Node(id)
	}
	return g.nodes[id]
}

// Has returns whether the node exists within the graph.
func (g *UndirectedMatrix) Has(n graph.Node) bool {
	return g.has(n.ID())
}

func (g *UndirectedMatrix) has(id int64) bool {
	r := g.mat.Symmetric()
	return 0 <= id && id < int64(r)
}

// Nodes returns all the nodes in the graph.
func (g *UndirectedMatrix) Nodes() []graph.Node {
	if g.nodes != nil {
		nodes := make([]graph.Node, len(g.nodes))
		copy(nodes, g.nodes)
		return nodes
	}
	r := g.mat.Symmetric()
	nodes := make([]graph.Node, r)
	for i := 0; i < r; i++ {
		nodes[i] = Node(i)
	}
	return nodes
}

// Edges returns all the edges in the graph.
func (g *UndirectedMatrix) Edges() []graph.Edge {
	var edges []graph.Edge
	r, _ := g.mat.Dims()
	for i := 0; i < r; i++ {
		for j := i + 1; j < r; j++ {
			if w := g.mat.At(i, j); !isSame(w, g.absent) {
				edges = append(edges, Edge{F: g.Node(int64(i)), T: g.Node(int64(j)), W: w})
			}
		}
	}
	return edges
}

// From returns all nodes in g that can be reached directly from n.
func (g *UndirectedMatrix) From(n graph.Node) []graph.Node {
	id := n.ID()
	if !g.has(id) {
		return nil
	}
	var neighbors []graph.Node
	r := g.mat.Symmetric()
	for i := 0; i < r; i++ {
		if int64(i) == id {
			continue
		}
		// id is not greater than maximum int by this point.
		if !isSame(g.mat.At(int(id), i), g.absent) {
			neighbors = append(neighbors, g.Node(int64(i)))
		}
	}
	return neighbors
}

// HasEdgeBetween returns whether an edge exists between nodes x and y.
func (g *UndirectedMatrix) HasEdgeBetween(u, v graph.Node) bool {
	uid := u.ID()
	if !g.has(uid) {
		return false
	}
	vid := v.ID()
	if !g.has(vid) {
		return false
	}
	// uid and vid are not greater than maximum int by this point.
	return uid != vid && !isSame(g.mat.At(int(uid), int(vid)), g.absent)
}

// Edge returns the edge from u to v if such an edge exists and nil otherwise.
// The node v must be directly reachable from u as defined by the From method.
func (g *UndirectedMatrix) Edge(u, v graph.Node) graph.Edge {
	return g.EdgeBetween(u, v)
}

// EdgeBetween returns the edge between nodes x and y.
func (g *UndirectedMatrix) EdgeBetween(u, v graph.Node) graph.Edge {
	if g.HasEdgeBetween(u, v) {
		// u.ID() and v.ID() are not greater than maximum int by this point.
		return Edge{F: g.Node(u.ID()), T: g.Node(v.ID()), W: g.mat.At(int(u.ID()), int(v.ID()))}
	}
	return nil
}

// Weight returns the weight for the edge between x and y if Edge(x, y) returns a non-nil Edge.
// If x and y are the same node or there is no joining edge between the two nodes the weight
// value returned is either the graph's absent or self value. Weight returns true if an edge
// exists between x and y or if x and y have the same ID, false otherwise.
func (g *UndirectedMatrix) Weight(x, y graph.Node) (w float64, ok bool) {
	xid := x.ID()
	yid := y.ID()
	if xid == yid {
		return g.self, true
	}
	if g.has(xid) && g.has(yid) {
		// xid and yid are not greater than maximum int by this point.
		return g.mat.At(int(xid), int(yid)), true
	}
	return g.absent, false
}

// SetEdge sets e, an edge from one node to another. If the ends of the edge are not in g
// or the edge is a self loop, SetEdge panics.
func (g *UndirectedMatrix) SetEdge(e graph.Edge) {
	fid := e.From().ID()
	tid := e.To().ID()
	if fid == tid {
		panic("simple: set illegal edge")
	}
	if int64(int(fid)) != fid {
		panic("simple: unavailable from node ID for dense graph")
	}
	if int64(int(tid)) != tid {
		panic("simple: unavailable to node ID for dense graph")
	}
	// fid and tid are not greater than maximum int by this point.
	g.mat.SetSym(int(fid), int(tid), e.Weight())
}

// RemoveEdge removes e from the graph, leaving the terminal nodes. If the edge does not exist
// it is a no-op.
func (g *UndirectedMatrix) RemoveEdge(e graph.Edge) {
	fid := e.From().ID()
	if !g.has(fid) {
		return
	}
	tid := e.To().ID()
	if !g.has(tid) {
		return
	}
	// fid and tid are not greater than maximum int by this point.
	g.mat.SetSym(int(fid), int(tid), g.absent)
}

// Degree returns the degree of n in g.
func (g *UndirectedMatrix) Degree(n graph.Node) int {
	id := n.ID()
	if !g.has(id) {
		return 0
	}
	var deg int
	r := g.mat.Symmetric()
	for i := 0; i < r; i++ {
		if int64(i) == id {
			continue
		}
		// id is not greater than maximum int by this point.
		if !isSame(g.mat.At(int(id), i), g.absent) {
			deg++
		}
	}
	return deg
}

// Matrix returns the mat.Matrix representation of the graph.
func (g *UndirectedMatrix) Matrix() mat.Matrix {
	// Prevent alteration of dimensions of the returned matrix.
	m := *g.mat
	return &m
}