// 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: every instance answers one request, and
// how many will is what is being found out.
var Window = 750 * time.Millisecond

// 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),
	}
	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] != "$SRV" {
			return nil
		}
		if len(parts) >= 3 && parts[2] != s.Name {
			return nil // another service's
		}
		if len(parts) >= 4 && parts[3] != s.ID {
			return nil // another instance's
		}
		var v any
		switch parts[1] {
		case "PING":
			v = micro.Ping{ServiceIdentity: s.identity(), Type: micro.PingResponseType}
		case "INFO":
			v = Info(s, current())
		case "STATS":
			st := micro.Stats{ServiceIdentity: s.identity(), Type: micro.StatsResponseType, Started: started,
				Endpoints: []*micro.EndpointStats{}}
			for _, e := range current() {
				st.Endpoints = append(st.Endpoints, &micro.EndpointStats{Name: e.Name(), Subject: e.Subject, QueueGroup: e.Queue})
			}
			v = st
		default:
			return nil
		}
		body, err := json.Marshal(v)
		if err != nil {
			return nil
		}
		return body
	}
	var stops []func()
	for _, verb := range []string{"PING", "INFO", "STATS"} {
		for _, subject := range []string{"$SRV." + verb, "$SRV." + verb + ".>"} {
			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
}

// Gather asks every service on the bus what it serves and answers what came back within the window.
// An answer that is not an info_response is skipped.
func Gather(conn *bus.Conn) ([]micro.Info, error) {
	raw, err := conn.Gather("$SRV.INFO", nil, Window)
	if err != nil {
		return nil, err
	}
	var out []micro.Info
	for _, b := range raw {
		var i micro.Info
		if json.Unmarshal(b, &i) != nil || i.Type != micro.InfoResponseType {
			continue
		}
		out = append(out, i)
	}
	return out, 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
}