// Package overlay computes the private network every node runs on.
//
// novox/hq 08-connectivity. This is control-plane work by definition: a peer list is derived from
// every node at once, and no node has that. A node computes nothing about the mesh — it generates
// a keypair, publishes the public half, and receives the rest.
//
// The shape is a hub, with direct peering between nodes at the same site. Not a full mesh, and
// the reason is a property of WireGuard rather than a preference: there is no failover. A more
// specific route to a dead endpoint blackholes; it does not fall back to the general one. So a
// node gets exactly one path to any peer, because two would mean one of them silently swallowing
// traffic.
package overlay

import (
	"errors"
	"fmt"
	"sort"
	"strings"
)

// Node is one machine's place on the network, as the mesh holds it.
type Node struct {
	Name     string
	Key      string
	Endpoint string
	Site     string
	Hub      bool
	Address  string
}

// Reachable reports whether other nodes can dial this one. Declared, never inferred.
func (n Node) Reachable() bool { return strings.TrimSpace(n.Endpoint) != "" }

// Peer is one entry in a node's peer list.
type Peer struct {
	Name string
	Key  string
	// Endpoint is empty when this peer cannot be dialled — it must dial us instead.
	Endpoint string
	// Allowed is what traffic goes down this tunnel. A single address for a direct peer; the
	// whole overlay for the hub, which is what makes it the route of last resort.
	Allowed string
	// Keepalive matters only on the side behind NAT: a node that cannot be dialled has to keep
	// the path open from its end, or the peer's first packet arrives at a mapping that has
	// already expired.
	Keepalive bool
	// Why this peer is in the list, for a person reading a generated file and wondering.
	Why string
}

// Graph is every node's peer list.
type Graph map[string][]Peer

// ErrNoHub means nobody has said which node is the hub.
//
// Its own error rather than an empty graph: a mesh with no hub has no path between sites, and
// answering with "no peers" would look like a working mesh where nothing can reach anything.
var ErrNoHub = errors.New("this mesh has no hub, so there is no path between sites")

// Compute derives every node's peer list.
//
// Nodes without a key or an address are skipped rather than refused: a node that has enrolled and
// not yet been given a place on the network is an ordinary in-between state, and failing the whole
// graph because one node is half-configured would mean no node gets a network.
func Compute(nodes []Node, overlayCIDR string) (Graph, error) {
	var hub *Node
	usable := make([]Node, 0, len(nodes))
	for i := range nodes {
		n := nodes[i]
		if n.Key == "" || n.Address == "" {
			continue
		}
		usable = append(usable, n)
		if n.Hub {
			hub = &usable[len(usable)-1]
		}
	}
	if len(usable) == 0 {
		return Graph{}, nil
	}
	if hub == nil {
		return nil, ErrNoHub
	}
	if !hub.Reachable() {
		return nil, fmt.Errorf(
			"%s is the hub and has no endpoint, so nothing can dial it. The hub is the one node "+
				"that must be reachable from wherever the others are", hub.Name)
	}

	graph := Graph{}
	for _, self := range usable {
		var peers []Peer

		for _, other := range usable {
			if other.Name == self.Name {
				continue
			}
			// Two nodes at the same site peer directly. A site is where a machine physically is,
			// and machines that share one have a path that does not need the hub — so using it
			// keeps their traffic off a link that may be somewhere else entirely.
			if self.Site != "" && self.Site == other.Site {
				peers = append(peers, Peer{
					Name: other.Name, Key: other.Key,
					Endpoint:  other.Endpoint,
					Allowed:   other.Address + "/32",
					Keepalive: !self.Reachable(),
					Why:       "at the same site",
				})
			}
		}

		if !self.Hub {
			// Everything else goes through the hub, including a node that roams. AllowedIPs is
			// the whole overlay, so this is the route of last resort — and because direct peers
			// above are single addresses, they win on specificity without either being ambiguous.
			peers = append(peers, Peer{
				Name: hub.Name, Key: hub.Key,
				Endpoint:  hub.Endpoint,
				Allowed:   overlayCIDR,
				Keepalive: !self.Reachable(),
				Why:       "the hub — everything not at this site",
			})
		} else {
			// The hub holds every node that does not share a site with it, because those nodes
			// route through it and it must know where to send the replies. Ones it cannot dial
			// will dial it.
			for _, other := range usable {
				if other.Name == self.Name || (self.Site != "" && self.Site == other.Site) {
					continue
				}
				peers = append(peers, Peer{
					Name: other.Name, Key: other.Key,
					Endpoint: other.Endpoint,
					Allowed:  other.Address + "/32",
					Why:      "routes through this hub",
				})
			}
		}

		sort.Slice(peers, func(i, j int) bool { return peers[i].Name < peers[j].Name })
		graph[self.Name] = peers
	}
	return graph, nil
}