package overlay

import "encoding/json"

// 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, and Names is the one that gives the machines
// names. Two modules rather than one, because they are two different things: names would be the
// same over any private network, and they are only bundled here by an accident of both being
// computed.
const (
	Name  = "mesh-wireguard"
	Names = "mesh-names"
)

// Resolution is what a module asks for when it needs to reach other machines by name. Separate
// from Requirement because they are separate jobs: one is whether packets arrive, the other is
// whether a name means anything. A machine can want the first without the second.
const Resolution = "name-resolution"

// 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"

// 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
}

// NameGenerator answers what one node's hosts file is.
//
// Separate from the interface and the peers because it is a separate concern. A machine's names
// come from the mesh knowing every machine, not from how the packets travel — over a different
// private network the peers would be written by something else and this would be unchanged.
type NameGenerator struct{ nodes []Node }

// NamesFor builds the name generator over the machines on the private network.
func NamesFor(nodes []Node) *NameGenerator { return &NameGenerator{nodes: nodes} }

// Resources is the one file.
func (g *NameGenerator) Resources(node string) ([]map[string]any, bool, error) {
	var found bool
	for _, n := range g.nodes {
		if n.Name == node {
			found = true
		}
	}
	if !found {
		return nil, false, nil
	}
	hosts, err := Hosts(g.nodes, node)
	if err != nil {
		return nil, false, err
	}
	return []map[string]any{{
		"id": "mesh-names", "type": "file", "path": HostsPath,
		"mode": "0644", "content": hosts,
	}}, 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},
		"claims":   []map[string]any{{"name": TheNetwork, "scope": "node"}},
	}
}

// NamesManifest is the module that gives machines names on the private network.
//
// It requires the network rather than providing it, which is the whole reason it is separate: a
// name resolves to an address on the private wire, so having names without being on it would
// point every machine at somewhere it cannot reach.
func NamesManifest() map[string]any {
	return map[string]any{
		"module":   Names,
		"version":  "1",
		"computed": Names,
		"provides": []string{Resolution},
		"requires": []string{Addressing},
	}
}

// DomainManifest is the module that means "get the network working".
func DomainManifest() map[string]any {
	return map[string]any{
		"module":   Domain,
		"version":  "1",
		"requires": []string{Requirement, Resolution},
	}
}

// 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{}} }