// Package announce answers the NATS services protocol's discovery for what a runtime serves, and
// gathers the answers (novox/hq ADR 0197).
//
// A runtime does not re-serve its tools through a services library: serving is unchanged. It answers
// `$SRV.PING`, `$SRV.INFO` and `$SRV.STATS` — and the same followed by its service name, and by its
// name and id — in the format NATS's own tools read, with what it is serving at the moment it is
// asked. One service per runtime process: the bus admits one reply per request from each responder,
// so a runtime serving many modules and seats answers once, one endpoint per tool per subject, and
// says in each endpoint's metadata which module, seat, scope and machine it is.
package announce

import (
	"encoding/json"
	"strings"
	"time"

	"github.com/nats-io/nats.go/micro"

	"github.com/novox/mesh-tools/node-tools/internal/bus"
)

// Version is the announced service version (semver, as the protocol requires).
const Version = "0.1.0"

// Window is how long the console gathers discovery answers at the least: every instance answers one
// request, and how many will is what is being found out.
var Window = 750 * time.Millisecond

// Patience is how long the console waits, at the most, for the full answer of a service that has
// said it is there. **A fixed window lost the largest runtime** (2026-10-05): the laptop's answer —
// 48 modules, 341 endpoints, 164 kB — went up to the broker on another machine and back, arrived
// last every time (a median of 365 ms, one in 25 after 813 ms on a quiet link, later still while the
// runtime re-served), and when it missed the 750 ms window the console said its modules ran nowhere.
// A PING answer is a hundred bytes and arrives at once, so who is there is known early; what each
// serves is waited for until it arrives or this passes, and a service that never sends it is named.
var Patience = 5 * time.Second

// Shortened is the service metadata key a runtime sets when its announcement was cut to fit the bus.
const Shortened = "shortened"

// Kinds of endpoint.
const (
	KindTool = "tool" // a module's own tool
	KindSeat = "seat" // a seat's verb, served by the module holding it
)

// Endpoint is one tool served on one subject, as it is announced.
type Endpoint struct {
	Kind            string
	Module          string // the module whose code answers
	Tool            string // the tool's or the verb's name
	Seat            string // for a seat's verb
	Scope           string // "mesh" or "node", for a seat's verb
	Node            string
	Description     string
	Schema          json.RawMessage
	Interchangeable bool
	Subject         string
	Queue           string
}

// Name is the endpoint's name as the protocol allows it — letters, digits, `-` and `_` — the
// prefix and the tool joined by `__`; the metadata, not the name, is what identifies it.
func (e Endpoint) Name() string {
	prefix := e.Module
	if e.Kind == KindSeat {
		prefix = e.Seat
	}
	return clean(prefix) + "__" + clean(e.Tool)
}

func clean(s string) string {
	var b strings.Builder
	for _, r := range s {
		if r == '-' || r == '_' || (r >= 'a' && r <= 'z') || (r >= 'A' && r <= 'Z') || (r >= '0' && r <= '9') {
			b.WriteRune(r)
		} else {
			b.WriteRune('_')
		}
	}
	return b.String()
}

func (e Endpoint) info() micro.EndpointInfo {
	md := map[string]string{
		"kind": e.Kind, "module": e.Module, "tool": e.Tool, "node": e.Node,
		"description": e.Description, "interchangeable": boolWord(e.Interchangeable),
	}
	// **A seat's verb carries its schema; a module's tool does not.** Every tool's schema, once per
	// subject it answers on, made a runtime's one answer outgrow the bus's largest message (1 MiB) once
	// machines served a couple of hundred tools — and the answer that cannot be sent is silence: the
	// machine vanished from discovery (2026-10-04). A module's tool's schema is asked of the runtime that
	// serves it when a person describes it (its `tools` verb); a seat's verbs are few, and a seat has no
	// `tools` verb of its own to ask.
	if e.Kind == KindSeat {
		schema := strings.TrimSpace(string(e.Schema))
		if schema == "" || schema == "null" {
			schema = "{}"
		}
		md["schema"] = schema
	}
	if e.Kind == KindSeat {
		md["seat"] = e.Seat
		md["scope"] = e.Scope
	}
	return micro.EndpointInfo{Name: e.Name(), Subject: e.Subject, QueueGroup: e.Queue, Metadata: md}
}

func boolWord(b bool) string {
	if b {
		return "true"
	}
	return "false"
}

// Service is who answers: the runtime's name, its instance, and a word about it.
type Service struct {
	Name        string
	ID          string
	Description string
	Metadata    map[string]string
}

func (s Service) identity() micro.ServiceIdentity {
	md := s.Metadata
	if md == nil {
		md = map[string]string{}
	}
	return micro.ServiceIdentity{Name: s.Name, ID: s.ID, Version: Version, Metadata: md}
}

// Info is the info_response for these endpoints.
func Info(s Service, endpoints []Endpoint) micro.Info {
	out := micro.Info{ServiceIdentity: s.identity(), Type: micro.InfoResponseType,
		Description: s.Description, Endpoints: []micro.EndpointInfo{}}
	for _, e := range endpoints {
		out.Endpoints = append(out.Endpoints, e.info())
	}
	return out
}

// Serve answers discovery for one service until stopped, asking `current` for its endpoints each
// time — so what is announced is what is served now, re-served memberships included.
func Serve(conn *bus.Conn, s Service, current func() []Endpoint) (func(), error) {
	started := time.Now().UTC()
	answer := func(subject string, _ []byte) []byte {
		parts := strings.Split(subject, ".")
		if len(parts) < 2 || parts[0] != "$SRV" {
			return nil
		}
		if len(parts) >= 3 && parts[2] != s.Name {
			return nil // another service's
		}
		if len(parts) >= 4 && parts[3] != s.ID {
			return nil // another instance's
		}
		var v any
		switch parts[1] {
		case "PING":
			v = micro.Ping{ServiceIdentity: s.identity(), Type: micro.PingResponseType}
		case "INFO":
			v = Info(s, current())
		case "STATS":
			st := micro.Stats{ServiceIdentity: s.identity(), Type: micro.StatsResponseType, Started: started,
				Endpoints: []*micro.EndpointStats{}}
			for _, e := range current() {
				st.Endpoints = append(st.Endpoints, &micro.EndpointStats{Name: e.Name(), Subject: e.Subject, QueueGroup: e.Queue})
			}
			v = st
		default:
			return nil
		}
		body, err := json.Marshal(v)
		if err != nil {
			return nil
		}
		// Too large for the bus is no answer at all, and was silent: shorten every description to its
		// first line and say so, rather than vanish; still too large, leave descriptions out.
		if limit := conn.MaxPayload(); limit > 0 && int64(len(body)) > limit {
			if info, ok := v.(micro.Info); ok {
				body = shorten(conn.Logf, info, body, limit)
			}
		}
		return body
	}
	var stops []func()
	for _, verb := range []string{"PING", "INFO", "STATS"} {
		// Exactly the questions asked of every service and of this one by name and instance — what the
		// grants allow (novox/hq ADR 0197). A wildcard is refused by the bus.
		for _, subject := range []string{"$SRV." + verb, "$SRV." + verb + "." + s.Name, "$SRV." + verb + "." + s.Name + "." + s.ID} {
			stop, err := conn.Raw(subject, answer)
			if err != nil {
				for _, st := range stops {
					st()
				}
				return func() {}, err
			}
			stops = append(stops, stop)
		}
	}
	return func() {
		for _, st := range stops {
			st()
		}
	}, nil
}

// shorten is an info_response cut to fit the bus: descriptions to their first line, then none, the
// service's metadata saying which (so discovery can tell a person what it was given).
func shorten(logf func(string, ...any), info micro.Info, body []byte, limit int64) []byte {
	md := map[string]string{}
	for k, v := range info.Metadata {
		md[k] = v
	}
	info.Metadata = md
	for _, step := range []string{"descriptions cut to their first line", "descriptions left out"} {
		for i := range info.Endpoints {
			e := info.Endpoints[i].Metadata
			if step == "descriptions left out" {
				delete(e, "description")
			} else {
				e["description"] = firstLine(e["description"])
			}
		}
		md[Shortened] = step
		shorter, err := json.Marshal(info)
		if err != nil {
			return body
		}
		logf("[mesh-tools] what this runtime serves is %d bytes, beyond the bus's %d; announced with %s (%d bytes)",
			len(body), limit, step, len(shorter))
		if int64(len(shorter)) <= limit {
			return shorter
		}
		body = shorter
	}
	return body
}

// Heard is one service instance's answer to discovery: what it serves, how long it took to arrive,
// and how large it was.
type Heard struct {
	Info      micro.Info
	Took      time.Duration
	Bytes     int
	Shortened string // why its descriptions are not whole, or ""
}

// Machine is the machine an instance runs on: its metadata's, or the one its endpoints name.
func (h Heard) Machine() string {
	if n := h.Info.Metadata["node"]; n != "" {
		return n
	}
	for _, e := range h.Info.Endpoints {
		if n := e.Metadata["node"]; n != "" {
			return n
		}
	}
	return ""
}

// Instance names one service instance.
type Instance struct {
	Name    string
	ID      string
	Machine string
}

// Discovery is what one round of discovery heard, and who said it was there but did not say what it
// serves in time.
type Discovery struct {
	At     time.Time
	Waited time.Duration
	Heard  []Heard
	Silent []Instance
}

// Gather asks every service on the bus who it is and what it serves, and waits at least the window
// and at most the patience: until every instance that answered PING has answered INFO. An answer
// that is not the protocol's is skipped.
func Gather(conn *bus.Conn) (Discovery, error) {
	start := time.Now()
	d := Discovery{At: start.UTC()}
	pings, stopPings, err := conn.Collect("$SRV.PING", nil)
	if err != nil {
		return d, err
	}
	defer stopPings()
	infos, stopInfos, err := conn.Collect("$SRV.INFO", nil)
	if err != nil {
		return d, err
	}
	defer stopInfos()

	type key struct{ name, id string }
	pinged := map[key]int{}
	answered := map[key]int{}
	where := map[key]string{}
	var order []key
	complete := func() bool {
		for k, n := range pinged {
			if answered[k] < n {
				return false
			}
		}
		return true
	}
	least := time.NewTimer(Window)
	defer least.Stop()
	most := time.NewTimer(Patience)
	defer most.Stop()
	windowOver := false
	for {
		select {
		case a := <-pings:
			var p micro.Ping
			if json.Unmarshal(a.Data, &p) != nil || p.Type != micro.PingResponseType {
				continue
			}
			k := key{p.Name, p.ID}
			if _, seen := pinged[k]; !seen {
				order = append(order, k)
			}
			pinged[k]++
			if where[k] == "" {
				where[k] = p.Metadata["node"]
			}
		case a := <-infos:
			var i micro.Info
			if json.Unmarshal(a.Data, &i) != nil || i.Type != micro.InfoResponseType {
				continue
			}
			answered[key{i.Name, i.ID}]++
			d.Heard = append(d.Heard, Heard{Info: i, Took: a.At.Sub(start), Bytes: len(a.Data), Shortened: i.Metadata[Shortened]})
		case <-least.C:
			windowOver = true
		case <-most.C:
			d.Waited = time.Since(start)
			for _, k := range order {
				if answered[k] < pinged[k] {
					d.Silent = append(d.Silent, Instance{Name: k.name, ID: k.id, Machine: where[k]})
				}
			}
			return d, nil
		}
		if windowOver && complete() {
			d.Waited = time.Since(start)
			return d, nil
		}
	}
}

// Endpoints reads an info_response's endpoints back into what they announce.
func Endpoints(i micro.Info) []Endpoint {
	var out []Endpoint
	for _, e := range i.Endpoints {
		md := e.Metadata
		out = append(out, Endpoint{
			Kind: md["kind"], Module: md["module"], Tool: md["tool"], Seat: md["seat"], Scope: md["scope"],
			Node: md["node"], Description: md["description"], Schema: json.RawMessage(md["schema"]),
			Interchangeable: md["interchangeable"] == "true", Subject: e.Subject, Queue: e.QueueGroup,
		})
	}
	return out
}

// firstLine is a description's first sentence or line, at most 160 characters.
func firstLine(s string) string {
	if i := strings.IndexAny(s, "\n"); i >= 0 {
		s = s[:i]
	}
	if i := strings.Index(s, ". "); i >= 0 {
		s = s[:i+1]
	}
	if len(s) > 160 {
		s = s[:157] + "..."
	}
	return s
}