package overlay

import (
	"fmt"
	"os"
	"regexp"
	"sort"
	"strings"
)

// Names are how one node reaches another by name rather than by address.
//
// novox/hq 08-connectivity: what the host receives is the resolver's configuration, as files,
// listing every peer's internal name and overlay address. Computed centrally for the same reason
// the peer graph is — it needs every node at once.

// SuffixVar lets a mesh choose what its internal names end in.
const SuffixVar = "MESH_INTERNAL_SUFFIX"

// DefaultSuffix is `.internal`, which IANA reserved for exactly this in 2024. A name under it can
// never collide with a public one, so an internal name that leaks into a public resolver fails
// rather than reaching a stranger's machine.
const DefaultSuffix = "internal"

// HostsPath is where the names go.
//
// This is not the `/etc/hosts` floor the design removes. That floor existed because a node had to
// reach the mesh's database before its own DNS worked — a fallback for a circularity, and the
// circularity is gone. This is the mechanism itself: the complete set of names in this mesh,
// generated whole and owned by the mesh (novox/hq ADR 0011), rather than a patch written
// underneath something else.
//
// A file rather than a resolver daemon, deliberately, for now: it works on every Linux, needs no
// package, and has no failure mode of its own. A daemon becomes necessary when names are wanted
// that are not one-per-node — service names, wildcards — and that is not yet true.
const HostsPath = "/etc/hosts"

// Suffix is what internal names end in.
func Suffix() string {
	if v := strings.TrimSpace(os.Getenv(SuffixVar)); v != "" {
		return strings.TrimPrefix(v, ".")
	}
	return DefaultSuffix
}

// nodeName is what a node may be called, so that it can also be a hostname.
var nodeName = regexp.MustCompile(`^[a-z0-9]([a-z0-9-]{0,61}[a-z0-9])?$`)

// InternalName is a node's name inside the mesh.
func InternalName(node string) string { return node + "." + Suffix() }

// Hosts writes the name file for one node.
//
// Every node in the mesh, including this one. Including itself because a machine referring to
// itself by its mesh name should get its overlay address rather than a loopback — otherwise a
// service that binds to the name it was given ends up unreachable from everywhere else.
//
// The machine's own loopback lines come first and are not the mesh's to have an opinion about,
// but they have to be here: this file is generated whole, so anything left out is removed.
func Hosts(nodes []Node, self string) (string, error) {
	var b strings.Builder
	b.WriteString("# Generated by the mesh. Do not edit — this file is replaced whenever a node\n")
	b.WriteString("# joins or leaves, and an edit would survive until then and vanish.\n\n")

	// The floor every Linux expects, and which removing would break things that have nothing to
	// do with the mesh.
	b.WriteString("127.0.0.1\tlocalhost\n")
	b.WriteString("::1\t\tlocalhost ip6-localhost ip6-loopback\n")
	if self != "" {
		fmt.Fprintf(&b, "127.0.1.1\t%s\n", self)
	}

	named := make([]Node, 0, len(nodes))
	for _, n := range nodes {
		if n.Address == "" {
			// A node with no address on the network has no name here. Writing one that resolves
			// to nothing is worse than not writing it: a connection to an address that does not
			// answer hangs, where a name that does not resolve fails at once and says so.
			continue
		}
		if !nodeName.MatchString(n.Name) {
			return "", fmt.Errorf(
				"%q cannot be a mesh name: it becomes a hostname, so it is lower-case letters, "+
					"digits and dashes", n.Name)
		}
		named = append(named, n)
	}
	sort.Slice(named, func(i, j int) bool { return named[i].Name < named[j].Name })

	if len(named) > 0 {
		fmt.Fprintf(&b, "\n# the mesh, %d node(s)\n", len(named))
	}
	for _, n := range named {
		line := fmt.Sprintf("%s\t%s\t%s", n.Address, InternalName(n.Name), n.Name)
		if n.Name == self {
			line += "\t# this machine"
		}
		b.WriteString(line + "\n")
	}
	return b.String(), nil
}