package overlay

import (
	"encoding/json"
	"fmt"
	"strconv"

	"github.com/novox/mesh-control/internal/catalogue"
)

// The private network as a module rather than as code beside the module system.
//
// A machine's peer list is derived from every other machine, so it differs on each one and
// changes when any of them changes — there is nothing that could be written in a manifest. So the
// module says its resources are computed, and this computes them.
//
// What that buys, beyond one mechanism instead of two: **a machine is on the private network
// because it was assigned the module.** Before this, every machine with a key and an address was
// on it, and there was no way to say a machine should stay off.
//
// It is worth saying what is *not* here, because networking looks like it should be foundational.
// The host needs none of this. It has an address and a route to the broker before the mesh exists
// — that is the machine's own networking — and the broker's address is carried in the enrolment
// token rather than resolved. So the private network is something the mesh installs on top, like
// anything else, and a node without it is an ordinary node that nothing reaches directly.

// What a module asks for when it needs machines to reach each other. **This is the name that
// matters** — WireGuard is one way to answer it, and naming the requirement after the answer is
// how a mesh ends up unable to have a second one.
const Requirement = "private-network"

// Name is the module that answers it with WireGuard.
//
// **The names went with it.** A mesh-names module used to sit beside this — it wrote /etc/hosts
// and ran nothing, which is not a module. Being on the private network is what gives a machine a
// name, so this module asks for the `node-names` fact and the mesh writes the file. The
// name-resolution provision went the same way: names are facts the mesh computes, not something a
// module that runs nowhere can provide.
const Name = "mesh-wireguard"

// Addressing is the mesh handing out addresses on the private network itself.
//
// Names are computed from it, which is why they require this rather than a private network in
// general. A different VPN that hands out its own addresses would come with its own names — the
// mesh has nothing to write about a machine whose address it did not choose. Saying so here is
// what keeps a machine from being given a hosts file full of addresses that mean nothing.
const Addressing = "mesh-addressing"

// TheNetwork is what a machine can only have one of.
//
// Providing a private network is not the singular part — a machine could reasonably run two VPNs
// for two different purposes. Being **the** one the mesh runs over is singular, and without
// saying so a person who chose a different VPN can still end up with this one dragged back in by
// something that needed the mesh's own addresses. Which is exactly what happened, once.
const TheNetwork = "the-private-network"

// Resolver is the module that answers every name under a machine, and the claim it holds.
//
// A claim because a machine has one resolver: two daemons answering the same names on one machine
// is a coin toss about which one a query reaches, and the answer differing between them is the
// kind of fault nobody finds by looking at either.
const (
	Resolver = "mesh-resolver"
	// ResolverData is what a module running a resolver requires: the mesh's own account of which
	// machines exist and where, in a file.
	ResolverData = "resolver-data"
)

// Domain is the module for people who want a network and do not want to choose one.
//
// It has no files of its own — it is requirements and nothing else. Assigning it finds one
// answer to each and takes them silently, so getting a mesh onto a private network is one word.
// The day the catalogue holds a second VPN there are two answers, the resolver refuses and names
// both, and choosing is assigning the one you want. **That is the whole mechanism**: picking an
// implementation is assigning a module, and there is no flavor field, no configuration language,
// and nothing to learn.
const Domain = "networking"

// Generator answers what one node's network configuration is.
type Generator struct {
	nodes []Node
	graph Graph
	// keyPath is where each node keeps the private half it generated. Named rather than carried:
	// the mesh has never seen it and never will.
	keyPath string
}

// From builds a generator over the machines that are part of the network.
//
// The nodes given are the ones assigned the module — not every node the mesh knows. A machine
// that was never given it is absent from everybody's peer list and from the names, which is what
// "not on the network" has to mean.
func From(nodes []Node, cidr, keyPath string) (*Generator, error) {
	graph, err := Compute(nodes, cidr)
	if err != nil {
		return nil, err
	}
	return &Generator{nodes: nodes, graph: graph, keyPath: keyPath}, nil
}

// Resources is one node's interface, peers and names.
func (g *Generator) Resources(node string) ([]map[string]any, bool, error) {
	peers, part := g.graph[node]
	if !part {
		// Assigned and not yet placed on the network. Ordinary and brief, so it is an answer
		// rather than an error.
		return nil, false, nil
	}
	var self Node
	for _, n := range g.nodes {
		if n.Name == node {
			self = n
		}
	}

	raw, err := Declaration(self, peers, g.keyPath)
	if err != nil {
		return nil, false, err
	}
	var parsed struct {
		Resources []map[string]any `json:"resources"`
	}
	if err := json.Unmarshal(raw, &parsed); err != nil {
		return nil, false, err
	}
	return parsed.Resources, true, nil
}

// Nodes are the machines this generator was built over, so a caller can say who is on the network.
func (g *Generator) Nodes() []Node { return g.nodes }

// Graph is the peer list per node, for showing.
func (g *Generator) Graph() Graph { return g.graph }

// Manifest is the module the mesh provides for itself.
//
// It ships with the control plane rather than coming from a repository, because the thing that
// computes it ships with the control plane. That is the only way it is unusual: it is assigned,
// unassigned, resolved and settled exactly like a module somebody wrote.
func Manifest() map[string]any {
	return map[string]any{
		"module":   Name,
		"version":  "1",
		"computed": Name,
		"provides": []string{Requirement, Addressing},
		// Being on the private network is what gives a machine a name, so the module that puts it
		// there is what writes them. Asked for rather than generated by a module of its own: the
		// mesh knows which machines exist and where; writing that into a hosts file is not a thing
		// that needs a module to run nowhere.
		"facts":  map[string]string{"node-names": "/etc/hosts"},
		"claims": []map[string]any{{"name": TheNetwork, "scope": "node"}},
	}
}

// DomainManifest is the module that means "get the network working".
func DomainManifest() map[string]any {
	return map[string]any{
		"module":  Domain,
		"version": "1",
		// **Only the network now.** It used to require name-resolution as well, answered by a
		// module that wrote a hosts file and ran nothing. Names are not a provision — they are a
		// fact the mesh computes, and whatever puts a machine on the private network writes them,
		// because a mesh name IS an address on that network.
		"requires": []string{Requirement},
	}
}

// Empty is a network nobody is on.
//
// A mesh where no machine was given the module. Legitimate rather than broken — every node still
// reaches the broker, which is what being in the mesh is — so it answers "not part of this" for
// everyone instead of refusing for want of a hub.
func Empty() *Generator { return &Generator{graph: Graph{}} }

// Listens is the port this node accepts the private network on, which only a hub has.
//
// **A fact about this machine's place in the mesh, not about the module.** Every machine on the
// network runs the same module; a hub is dialled by every node at other sites and needs its port
// open, and a machine that is not a hub dials out and needs nothing open at all. A static field in
// a manifest is one answer for every machine that runs it, so it cannot say this — and the machine
// it would get wrong is the one facing the public internet, which is the machine that most needs
// filtering.
//
// The port is the one in the endpoint, which is also where the interface takes its ListenPort
// from. One source, so a rule set cannot open a port the interface is not on.
func (g *Generator) Listens(node string) ([]catalogue.Listening, error) {
	for _, n := range g.nodes {
		if n.Name != node {
			continue
		}
		if !n.Reachable() {
			// It dials out and nothing dials it. Opening a port here would be opening one on a
			// machine nothing connects to, which is not harmless — it is a rule with no source
			// that somebody later has to work out the reason for.
			return nil, nil
		}
		port := portOf(n.Endpoint)
		if port == "" {
			return nil, fmt.Errorf(
				"%s is reachable at %q and no port can be read from it, so what it must accept "+
					"the private network on is unknown", node, n.Endpoint)
		}
		number, err := strconv.Atoi(port)
		if err != nil {
			return nil, fmt.Errorf("%s is reachable at %q, and %q is not a port", node, n.Endpoint, port)
		}
		return []catalogue.Listening{{
			Port: number, Protocol: "udp", From: catalogue.FromEverywhere,
			// From everywhere, and deliberately: a node at another site is not on the private
			// network until this port lets it on, so restricting this to the mesh would be a
			// rule that can never be satisfied by the thing it exists for.
			Why: "the private network — a node at another site has no other way in",
		}}, nil
	}
	return nil, nil
}