// Package bus is the node runtime's connection to the mesh bus, on NATS (novox/hq design 25, design
// 29, ADR 0160, ADR 0175). It is the Go port of node-tools' broker-nats.ts, and speaks the same
// wire: the same subjects, the same JSON request and reply bodies, the same event headers.
//
//	mesh.mod.<module>.event.<type>   an event a module emits
//	mesh.mod.<module>.tool.<tool>    a tool a module serves
//	mesh.seat.<seat>.tool.<verb>     a role's verb, answered by whoever holds the seat
package bus

import (
	"crypto/sha256"
	"crypto/tls"
	"crypto/x509"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"log"
	"regexp"
	"strings"
	"sync"
	"time"

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

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

// RequestTimeout is how long a call waits for its answer — what modules already expect.
const RequestTimeout = 30 * time.Second

const assignmentsStream = "ASSIGNMENTS"

// Credential is a broker credential as the mesh delivers it (novox/hq ADR 0120).
type Credential struct {
	URL         string  `json:"url"`
	Fingerprint string  `json:"fingerprint,omitempty"`
	Node        string  `json:"node,omitempty"`
	Module      string  `json:"module,omitempty"`
	User        string  `json:"user,omitempty"`
	Password    string  `json:"password,omitempty"`
	Claims      []Claim `json:"claims,omitempty"`
}

// Claim is a seat a module claims, with the verbs it promises (novox/hq ADR 0159).
type Claim struct {
	Seat   string   `json:"seat"`
	Scope  string   `json:"scope,omitempty"`
	Serves []string `json:"serves,omitempty"`
}

// Membership is what the mesh issued one assignment (novox/hq ADR 0160).
type Membership struct {
	Node    string              `json:"node"`
	Module  string              `json:"module"`
	Serves  []Served            `json:"serves"`
	Seats   []SeatVerb          `json:"seats,omitempty"`
	Emits   string              `json:"emits"`
	Reaches map[string][]string `json:"reaches,omitempty"`
	Tools   string              `json:"tools"`
}

// Served is an address a tool is answered on; `{tool}` stands for the tool's name.
type Served struct {
	Subject string `json:"subject"`
	Queue   string `json:"queue,omitempty"`
}

// SeatVerb is one verb of a seat a module holds, where it is answered.
type SeatVerb struct {
	Seat    string `json:"seat"`
	Verb    string `json:"verb"`
	Subject string `json:"subject"`
}

// Envelope is an event as a module emits it: the body is the payload, the metadata rides as headers
// (novox/hq ADR 0042).
type Envelope struct {
	Key     string            `json:"key"`
	Node    string            `json:"node,omitempty"`
	Body    json.RawMessage   `json:"body"`
	Headers map[string]string `json:"headers,omitempty"`
}

// Answered is a call's result and the machine that gave it (novox/hq ADR 0159).
type Answered struct {
	Result json.RawMessage
	Node   string
}

// Handler answers one request body.
type Handler func(body json.RawMessage) (any, error)

// ErrPin is a bus whose certificate is not the one the mesh pinned: final, never retried.
var ErrPin = errors.New("the bus's certificate does not match the pin")

// Fatal is why a connection failure is final rather than "not yet", or "" when waiting may fix it —
// the same classification the TypeScript runtime makes.
func Fatal(err error) string {
	if err == nil {
		return ""
	}
	msg := err.Error()
	if errors.Is(err, ErrPin) || strings.Contains(msg, ErrPin.Error()) {
		return "the bus's certificate does not match the pin"
	}
	if regexp.MustCompile(`(?i)invalid url|no servers available for connection: .*url`).MatchString(msg) ||
		strings.Contains(msg, "nats: invalid url") {
		return "the bus address is not a usable URL"
	}
	if regexp.MustCompile(`(?i)authorization violation|user authentication expired|permissions violation`).MatchString(msg) {
		return "the bus refused this account"
	}
	return ""
}

func normalizeFingerprint(f string) string {
	f = strings.TrimSpace(f)
	if len(f) > 7 && strings.EqualFold(f[:7], "sha256:") {
		f = f[7:]
	}
	return strings.ToLower(strings.ReplaceAll(f, ":", ""))
}

// pinned accepts exactly the certificate with this SHA-256 and no other. The pin is the only check:
// the bus's certificate names the seat, not the address a machine dials it by.
func pinned(want string) *tls.Config {
	want = normalizeFingerprint(want)
	return &tls.Config{
		InsecureSkipVerify: true, //nolint:gosec // replaced by the pin, which is stricter
		MinVersion:         tls.VersionTLS12,
		VerifyPeerCertificate: func(raw [][]byte, _ [][]*x509.Certificate) error {
			if len(raw) == 0 {
				return fmt.Errorf("%w: it presented none", ErrPin)
			}
			sum := sha256.Sum256(raw[0])
			if got := hex.EncodeToString(sum[:]); got != want {
				return fmt.Errorf("%w: it presented %s, not the pinned %s", ErrPin, got, want)
			}
			return nil
		},
	}
}

// Conn is the runtime's connection: the module it is, the memberships it follows, the subjects it
// answers.
type Conn struct {
	nc     *nats.Conn
	js     nats.JetStreamContext
	self   string
	node   string
	cred   Credential
	mu     sync.Mutex
	issued map[string]*Membership // module → membership; present with nil = followed, none issued
	onNew  []func(Membership)
	subs   []*nats.Subscription
	Logf   func(format string, args ...any)
}

// Connect dials the bus as the credential's module. A module's subjects come from its credential,
// never from its calls (ADR 0074).
func Connect(cred Credential) (*Conn, error) {
	if cred.Module == "" {
		return nil, errors.New("a broker credential with no module: the runtime derives its subjects " +
			"from the account the mesh issued, and cannot guess which module it is")
	}
	node := cred.Node
	if node == "" {
		node = "?"
	}
	opts := []nats.Option{
		nats.Name(node + "." + cred.Module),
		// Reconnect forever: the bus restarting is an upgrade, not a reason to exit.
		nats.MaxReconnects(-1),
	}
	if cred.User != "" {
		opts = append(opts, nats.UserInfo(cred.User, cred.Password),
			// Its own inbox: every user's inbox is private to it (design 25 §4).
			nats.CustomInboxPrefix("_INBOX."+cred.User))
	}
	if strings.TrimSpace(cred.Fingerprint) != "" {
		opts = append(opts, nats.Secure(pinned(cred.Fingerprint)))
	}
	nc, err := nats.Connect(cred.URL, opts...)
	if err != nil {
		return nil, err
	}
	js, err := nc.JetStream()
	if err != nil {
		nc.Close()
		return nil, err
	}
	c := &Conn{nc: nc, js: js, self: cred.Module, node: cred.Node, cred: cred,
		issued: map[string]*Membership{}, Logf: log.Printf}
	c.Follow(cred.Module)
	return c, nil
}

// Module is what this connection is.
func (c *Conn) Module() string { return c.self }

// Node is the machine this connection's account is scoped to.
func (c *Conn) Node() string { return c.node }

// Credential is what this connection was opened with.
func (c *Conn) Credential() Credential { return c.cred }

// MembershipSubject is the one address a runtime derives for an assignment (ADR 0160).
func MembershipSubject(node, module string) string {
	return "mesh.assignment." + node + "." + module
}

// Follow reads a module's membership on this machine once and follows it live, so its tools are
// served where the mesh issued them (ADR 0175).
func (c *Conn) Follow(module string) {
	c.mu.Lock()
	if c.node == "" {
		c.mu.Unlock()
		return
	}
	if _, has := c.issued[module]; has {
		c.mu.Unlock()
		return
	}
	c.issued[module] = nil
	c.mu.Unlock()
	subject := MembershipSubject(c.node, module)
	// The subject-addressed direct get: the one address the mesh grants this account on the
	// stream's API.
	if got, err := c.nc.Request("$JS.API.DIRECT.GET."+assignmentsStream+"."+subject, nil, 5*time.Second); err == nil {
		if got.Header.Get("Status") == "" && len(got.Data) > 0 {
			var m Membership
			if json.Unmarshal(got.Data, &m) == nil {
				c.mu.Lock()
				c.issued[module] = &m
				c.mu.Unlock()
			}
		}
	}
	if c.Membership(module) == nil {
		c.Logf("[mesh-tools] no membership issued for %s on %s yet; serving the derived shape until one arrives", module, c.node)
	}
	sub, err := c.nc.Subscribe(subject, func(msg *nats.Msg) {
		var m Membership
		if err := json.Unmarshal(msg.Data, &m); err != nil {
			c.Logf("[mesh-tools] a membership arrived that is not one: %v", err)
			return
		}
		c.mu.Lock()
		c.issued[module] = &m
		handlers := append([]func(Membership){}, c.onNew...)
		c.mu.Unlock()
		c.Logf("[mesh-tools] %s on %s was issued a new membership; re-serving on it", module, c.node)
		for _, h := range handlers {
			h(m)
		}
		_ = c.nc.Flush()
	})
	if err == nil {
		c.track(sub)
	}
}

// Membership is what the mesh issued a module here, or nil when nothing has been issued.
func (c *Conn) Membership(module string) *Membership {
	c.mu.Lock()
	defer c.mu.Unlock()
	return c.issued[module]
}

// Following says whether this connection follows a module's membership.
func (c *Conn) Following(module string) bool {
	c.mu.Lock()
	defer c.mu.Unlock()
	_, has := c.issued[module]
	return has
}

// Serving is every module whose membership this connection follows.
func (c *Conn) Serving() []string {
	c.mu.Lock()
	defer c.mu.Unlock()
	out := make([]string, 0, len(c.issued))
	for m := range c.issued {
		out = append(out, m)
	}
	return out
}

// OnMembership is called with every new membership any followed module is issued.
func (c *Conn) OnMembership(h func(Membership)) {
	c.mu.Lock()
	c.onNew = append(c.onNew, h)
	c.mu.Unlock()
}

func (c *Conn) track(s *nats.Subscription) {
	c.mu.Lock()
	c.subs = append(c.subs, s)
	c.mu.Unlock()
}

// servedOn is where a served module's tool is answered: its membership's subjects when issued, the
// derived shape otherwise (the shape the mesh issues on day one).
func (c *Conn) servedOn(module, tool string) []Served {
	if m := c.Membership(module); m != nil {
		out := make([]Served, 0, len(m.Serves)+1)
		for _, s := range m.Serves {
			out = append(out, Served{Subject: strings.ReplaceAll(s.Subject, "{tool}", tool), Queue: s.Queue})
		}
		if tool == "tools" && m.Tools != "" {
			found := false
			for _, s := range out {
				found = found || s.Subject == m.Tools
			}
			if !found {
				out = append([]Served{{Subject: m.Tools, Queue: "serve." + module}}, out...)
			}
		}
		return out
	}
	base := "mesh.mod." + module + ".tool." + tool
	out := []Served{{Subject: base, Queue: "serve." + module}}
	if c.node != "" {
		out = append(out, Served{Subject: base + "." + c.node})
	}
	return out
}

type reply struct {
	Result any    `json:"result,omitempty"`
	Error  string `json:"error,omitempty"`
	Node   string `json:"node,omitempty"`
}

// answerOn answers one subject with one handler, and says which machine answered (ADR 0159).
func (c *Conn) answerOn(subject, queue string, h Handler) (func(), error) {
	cb := func(msg *nats.Msg) {
		go func() {
			var r reply
			result, err := h(json.RawMessage(msg.Data))
			if err != nil {
				r.Error = err.Error()
			} else {
				r.Result = nullable(result)
			}
			r.Node = c.node
			body, _ := wire.Marshal(r)
			_ = msg.Respond(body)
		}()
	}
	var sub *nats.Subscription
	var err error
	if queue != "" {
		sub, err = c.nc.QueueSubscribe(subject, queue, cb)
	} else {
		sub, err = c.nc.Subscribe(subject, cb)
	}
	if err != nil {
		return func() {}, err
	}
	c.track(sub)
	return func() { _ = sub.Unsubscribe() }, nil
}

// nullable keeps a nil result as JSON null rather than dropping the key: the TypeScript reply always
// carries `result` when the handler did not throw.
func nullable(v any) any {
	if v == nil {
		return json.RawMessage("null")
	}
	return v
}

// HandleSubject answers a subject outright: a seat's verb where the mesh issued it.
func (c *Conn) HandleSubject(subject string, h Handler) (func(), error) {
	return c.answerOn(subject, "", h)
}

// Handle serves `<module>.<tool>` where the mesh issued that module, and follows its membership:
// when a new one arrives, it serves where it now says and stops where it no longer does.
func (c *Conn) Handle(key string, h Handler) (func(), error) {
	if strings.HasPrefix(key, "seat:") {
		subject, err := ToolSubject(key, c.self)
		if err != nil {
			return func() {}, err
		}
		return c.answerOn(subject, "", h)
	}
	module, tool := c.self, key
	if dot := strings.Index(key, "."); dot >= 0 {
		module, tool = key[:dot], key[dot+1:]
	}
	if module != c.self && !c.Following(module) {
		return func() {}, fmt.Errorf("%s cannot serve %s: a module serves its own tools, and a runtime "+
			"those of the modules it follows", c.self, key)
	}
	var mu sync.Mutex
	var stops []func()
	serve := func() {
		mu.Lock()
		defer mu.Unlock()
		for _, s := range stops {
			s()
		}
		stops = nil
		for _, s := range c.servedOn(module, tool) {
			if stop, err := c.answerOn(s.Subject, s.Queue, h); err == nil {
				stops = append(stops, stop)
			} else {
				c.Logf("[mesh-tools] cannot serve %s on %s: %v", key, s.Subject, err)
			}
		}
	}
	serve()
	c.OnMembership(func(m Membership) {
		if m.Module == module {
			serve()
		}
	})
	return func() {
		mu.Lock()
		defer mu.Unlock()
		for _, s := range stops {
			s()
		}
		stops = nil
	}, nil
}

// reachedAt is where a call by key goes: a subject this connection's own membership says it
// reaches — the machine's when named — else the derived shape.
func (c *Conn) reachedAt(key string) (string, error) {
	name, wanted, _ := strings.Cut(key, "@")
	if m := c.Membership(c.self); m != nil {
		if reach := m.Reaches[name]; len(reach) > 0 {
			if wanted == "" {
				return reach[0], nil
			}
			for _, s := range reach {
				if strings.HasSuffix(s, "."+wanted) {
					return s, nil
				}
			}
		}
	}
	return ToolSubject(key, c.self)
}

// Ask calls a tool by key — or on a subject the mesh listed for it — and learns which machine
// answered. Core request/reply: a tool call is never persisted (design 25 §3).
func (c *Conn) Ask(key string, body any, on string) (Answered, error) {
	subject := on
	if subject == "" {
		s, err := c.reachedAt(key)
		if err != nil {
			return Answered{}, err
		}
		subject = s
	}
	data, err := wire.Marshal(body)
	if err != nil {
		return Answered{}, err
	}
	msg, err := c.nc.Request(subject, data, RequestTimeout)
	if err != nil {
		if errors.Is(err, nats.ErrNoResponders) {
			return Answered{}, errors.New("503 no responders")
		}
		if errors.Is(err, nats.ErrTimeout) {
			return Answered{}, errors.New("timeout")
		}
		return Answered{}, err
	}
	var r struct {
		Result json.RawMessage `json:"result"`
		Error  string          `json:"error"`
		Node   string          `json:"node"`
	}
	if err := json.Unmarshal(msg.Data, &r); err != nil {
		return Answered{}, err
	}
	if r.Error != "" {
		return Answered{}, errors.New(r.Error)
	}
	return Answered{Result: r.Result, Node: r.Node}, nil
}

// PublishAs emits an event as a module: published into JetStream and awaited, de-duplicated by its
// own id (ADR 0042). The body is the payload; the metadata rides as headers.
func (c *Conn) PublishAs(module string, env Envelope) error {
	msg := nats.NewMsg("mesh.mod." + module + ".event." + env.Key)
	for k, v := range env.Headers {
		msg.Header.Set(k, v)
	}
	if env.Headers["content-type"] == "" {
		msg.Header.Set("content-type", "application/json")
	}
	if env.Node != "" {
		msg.Header.Set("x-node", env.Node)
	}
	body := env.Body
	if len(body) == 0 {
		body = json.RawMessage("null")
	}
	msg.Data = body
	var opts []nats.PubOpt
	if id := env.Headers["x-event-id"]; id != "" {
		opts = append(opts, nats.MsgId(id))
	}
	_, err := c.js.PublishMsg(msg, opts...)
	return err
}

// Flush waits until the bus has every subscription made so far, so what is served is answerable
// when this returns.
func (c *Conn) Flush() { _ = c.nc.Flush() }

// Close unsubscribes everything and drains, so an in-flight reply is finished rather than dropped.
func (c *Conn) Close() {
	c.mu.Lock()
	subs := c.subs
	c.subs = nil
	c.mu.Unlock()
	for _, s := range subs {
		_ = s.Unsubscribe()
	}
	_ = c.nc.Drain()
}

// ToolSubject is a tool's subject. A bare name is this module's own; `<module>.<tool>` another's;
// `seat:<seat>.<verb>` a role's, with `@<node>` for a node-scoped seat (design 33 §4).
func ToolSubject(key, self string) (string, error) {
	if strings.HasPrefix(key, "seat:") {
		rest := strings.TrimPrefix(key, "seat:")
		dot := strings.Index(rest, ".")
		if dot < 0 {
			return "", fmt.Errorf("%q names a seat and no verb: seat:<seat>.<verb>", key)
		}
		seat := rest[:dot]
		verb, node, _ := strings.Cut(rest[dot+1:], "@")
		if node != "" {
			return "mesh.seat." + seat + ".tool." + verb + "." + node, nil
		}
		return "mesh.seat." + seat + ".tool." + verb, nil
	}
	name, node, _ := strings.Cut(key, "@")
	var base string
	if dot := strings.Index(name, "."); dot < 0 {
		base = "mesh.mod." + self + ".tool." + name
	} else {
		base = "mesh.mod." + name[:dot] + ".tool." + name[dot+1:]
	}
	if node != "" {
		return base + "." + node, nil
	}
	return base, nil
}

// SeatToolSubject is a seat's verb as its holder serves it: flat for a mesh seat, carrying the
// machine for a node-scoped one.
func SeatToolSubject(seat, verb, scope, node string) string {
	base := "mesh.seat." + seat + ".tool." + verb
	if scope == "node" && node != "" {
		return base + "." + node
	}
	return base
}