package bus

import (
	"context"
	"crypto/rand"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"strings"
	"sync"
	"time"

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

// A bundle's traffic on seats that name their caller or their kind (novox/hq ADR 0259 §3): submitting an
// accept and saying an event under its own name or kind, taking a held seat's work from its worker, asking a
// proof and answering one, and reading a record kept for it.
//
// **The runtime keeps each module to its own name and kind.** One account per machine carries every module
// on it, so the bus enforces only the union; what one module may reach is its membership's `seat-traffic`,
// and anything else is refused here, with the reason, before it is sent. A bundle reaches this only through
// its own standard input and output: no tool call does.

// SeatTraffic is what a membership lists of a module's seat traffic (the controller's broker.SeatTraffic).
type SeatTraffic struct {
	Publish   []string     `json:"publish,omitempty"`
	Subscribe []string     `json:"subscribe,omitempty"`
	Answers   []string     `json:"answers,omitempty"`
	Workers   []SeatWorker `json:"workers,omitempty"`
	Records   []string     `json:"records,omitempty"`
}

// SeatWorker is one work queue a holder takes from.
type SeatWorker struct {
	Stream   string `json:"Stream"`
	Consumer string `json:"Consumer"`
	Filter   string `json:"Filter"`
}

// ProofTimeout bounds how long a proof waits for its answer.
var ProofTimeout = 15 * time.Second

// SubjectMatches is a subject against a permission pattern, in the server's wildcards.
func SubjectMatches(pattern, subject string) bool {
	p, s := strings.Split(pattern, "."), strings.Split(subject, ".")
	for i, part := range p {
		if part == ">" {
			return len(s) > i
		}
		if i >= len(s) || (part != "*" && part != s[i]) {
			return false
		}
	}
	return len(p) == len(s)
}

func (c *Conn) trafficOf(module string) (*SeatTraffic, error) {
	m := c.Membership(module)
	if m == nil {
		return nil, fmt.Errorf("%s has no membership issued on %s yet, so it reaches no seat (novox/hq ADR 0259)",
			module, c.node)
	}
	if m.SeatTraffic == nil {
		return nil, fmt.Errorf("%s holds, uses and watches no seat that names its caller or its kind (novox/hq "+
			"ADR 0259): its manifest says so under `uses`, `claims` and `consumes`", module)
	}
	return m.SeatTraffic, nil
}

func listed(patterns []string, subject string) bool {
	for _, p := range patterns {
		if SubjectMatches(p, subject) {
			return true
		}
	}
	return false
}

// mayPublish refuses a subject the module's seat traffic does not list. A wildcard is never published.
func (c *Conn) mayPublish(module, subject string) error {
	t, err := c.trafficOf(module)
	if err != nil {
		return err
	}
	if strings.ContainsAny(subject, "*>") || !listed(t.Publish, subject) {
		return fmt.Errorf("%s may not publish %s: a module submits and says only under its own name or kind, "+
			"and its membership lists %s (novox/hq ADR 0259)", module, subject, strings.Join(t.Publish, ", "))
	}
	return nil
}

func newID() string {
	var b [16]byte
	_, _ = rand.Read(b[:])
	return hex.EncodeToString(b[:])
}

// SeatPublish submits an accept or says an event on a seat, as the module, into the stream that keeps it,
// awaited and de-duplicated by its id: the same id twice is one message. A proof is never published here.
func (c *Conn) SeatPublish(module, subject string, body json.RawMessage, id string) (uint64, error) {
	if strings.Contains(subject, ".proof.") {
		return 0, fmt.Errorf("%s is a proof, asked and answered, never kept: ask it as one", subject)
	}
	if err := c.mayPublish(module, subject); err != nil {
		return 0, err
	}
	if len(body) == 0 || !json.Valid(body) {
		return 0, fmt.Errorf("what is said on %s is JSON", subject)
	}
	if id == "" {
		id = newID()
	}
	msg := nats.NewMsg(subject)
	msg.Header.Set("x-event-id", id)
	msg.Header.Set("x-source", module)
	msg.Header.Set("x-node", c.node)
	msg.Header.Set("x-time", time.Now().UTC().Format(time.RFC3339Nano))
	msg.Header.Set("content-type", "application/json")
	msg.Data = body
	ack, err := c.js.PublishMsg(msg, nats.MsgId(id))
	if err != nil {
		return 0, err
	}
	return ack.Sequence, nil
}

// SeatProve asks a proof and answers its reply: core request and reply, on no stream.
func (c *Conn) SeatProve(module, subject string, body json.RawMessage) (json.RawMessage, error) {
	if !strings.Contains(subject, ".proof.") {
		return nil, fmt.Errorf("%s is not a proof", subject)
	}
	if err := c.mayPublish(module, subject); err != nil {
		return nil, err
	}
	reply, err := c.nc.Request(subject, body, ProofTimeout)
	if errors.Is(err, nats.ErrNoResponders) {
		return nil, fmt.Errorf("nothing answers %s: the module watching the seat is not running", subject)
	}
	if err != nil {
		return nil, err
	}
	if !json.Valid(reply.Data) {
		return nil, fmt.Errorf("the answer to %s is not JSON", subject)
	}
	return json.RawMessage(reply.Data), nil
}

// SeatAnswer answers every proof on a subject the module's seat traffic lists among its answers, with what
// answer returns. A returned error is answered as `{"error": "<words>"}`, never as silence.
func (c *Conn) SeatAnswer(module, subject string, answer func(subject string, body json.RawMessage) (json.RawMessage, error)) (func(), error) {
	t, err := c.trafficOf(module)
	if err != nil {
		return nil, err
	}
	allowed := false
	for _, a := range t.Answers {
		allowed = allowed || a == subject
	}
	if !allowed {
		return nil, fmt.Errorf("%s does not answer %s: its membership lists %s (novox/hq ADR 0259)", module,
			subject, strings.Join(t.Answers, ", "))
	}
	sub, err := c.nc.Subscribe(subject, func(msg *nats.Msg) {
		if msg.Reply == "" {
			return
		}
		body := json.RawMessage(msg.Data)
		if !json.Valid(body) {
			_ = msg.Respond([]byte(`{"error":"a proof is JSON"}`))
			return
		}
		out, err := answer(msg.Subject, body)
		if err != nil {
			b, _ := json.Marshal(map[string]string{"error": err.Error()})
			_ = msg.Respond(b)
			return
		}
		if len(out) == 0 {
			out = json.RawMessage(`{}`)
		}
		if err := msg.Respond(out); err != nil {
			c.Logf("[mesh-tools] %s could not answer the proof on %s: %v", module, msg.Subject, err)
		}
	})
	if err != nil {
		return nil, err
	}
	c.track(sub)
	var once sync.Once
	return func() { once.Do(func() { _ = sub.Unsubscribe() }) }, nil
}

// Work is one message a holder took from a seat's work queue.
type Work struct {
	Subject string            `json:"subject"`
	Body    json.RawMessage   `json:"body"`
	Headers map[string]string `json:"headers,omitempty"`
}

// SeatTake reads a worker the module's seat traffic lists and hands each message to deliver: acknowledged
// when deliver returns nil, offered again after NakDelay when it returns an error, ended when it is not
// JSON. The worker is the controller's to create; this binds to it by name and never makes one.
func (c *Conn) SeatTake(module, consumer string, deliver func(Work) error) (func(), error) {
	t, err := c.trafficOf(module)
	if err != nil {
		return nil, err
	}
	var w *SeatWorker
	for i := range t.Workers {
		if t.Workers[i].Consumer == consumer {
			w = &t.Workers[i]
		}
	}
	if w == nil {
		var names []string
		for _, x := range t.Workers {
			names = append(names, x.Consumer)
		}
		return nil, fmt.Errorf("%s takes no work from %s: its membership lists %s (novox/hq ADR 0259)", module,
			consumer, strings.Join(names, ", "))
	}
	js, err := jetstream.New(c.nc)
	if err != nil {
		return nil, err
	}
	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
	defer cancel()
	bound, err := js.Consumer(ctx, w.Stream, w.Consumer)
	if err != nil {
		return nil, fmt.Errorf("binding %s's worker %s on %s: %w", module, w.Consumer, w.Stream, err)
	}
	reading, err := bound.Consume(func(msg jetstream.Msg) {
		if !json.Valid(msg.Data()) {
			_ = msg.Term()
			return
		}
		headers := map[string]string{}
		for k := range msg.Headers() {
			headers[k] = msg.Headers().Get(k)
		}
		if err := deliver(Work{Subject: msg.Subject(), Body: json.RawMessage(msg.Data()), Headers: headers}); err != nil {
			_ = msg.NakWithDelay(NakDelay)
			return
		}
		_ = msg.Ack()
	})
	if err != nil {
		return nil, fmt.Errorf("reading %s's worker %s: %w", module, w.Consumer, err)
	}
	var once sync.Once
	return func() { once.Do(reading.Stop) }, nil
}

// SeatRecord reads one key of a record a seat keeps for the module, under its own name: the bucket's
// current value of `<module>.<key>`, or nil when there is none.
func (c *Conn) SeatRecord(module, bucket, key string) (*StateEntry, error) {
	t, err := c.trafficOf(module)
	if err != nil {
		return nil, err
	}
	if err := checkKey(key); err != nil {
		return nil, err
	}
	subject := "$JS.API.DIRECT.GET.KV_" + bucket + ".$KV." + bucket + "." + module + "." + key
	if !listed(t.Records, subject) {
		return nil, fmt.Errorf("%s reads no record of %s under its name: its membership lists %s (novox/hq ADR 0259)",
			module, bucket, strings.Join(t.Records, ", "))
	}
	reply, err := c.nc.Request(subject, nil, StateTimeout)
	if err != nil {
		return nil, err
	}
	if status := reply.Header.Get("Status"); status == "404" {
		return nil, nil
	} else if status != "" {
		return nil, fmt.Errorf("the record %s.%s could not be read: %s %s", module, key, status, reply.Header.Get("Description"))
	}
	if op := reply.Header.Get("KV-Operation"); op == "DEL" || op == "PURGE" {
		return nil, nil
	}
	var revision uint64
	fmt.Sscan(reply.Header.Get("Nats-Sequence"), &revision)
	return &StateEntry{Key: module + "." + key, Value: valueOf(reply.Data), Revision: revision}, nil
}

// StateCreate writes one key only when it has no value, and answers its revision; a key that has one is
// refused, naming it, so two writers racing to make it are one winner and one refusal.
func (c *Conn) StateCreate(module, name, key string, value json.RawMessage) (uint64, error) {
	return c.stateWrite(module, name, key, value, func(ctx context.Context, kv jetstream.KeyValue) (uint64, error) {
		return kv.Create(ctx, key, value)
	})
}

// StateUpdate writes one key only when its revision is still the one given, and answers the new revision;
// a key changed meanwhile is refused, so of two writers that read one value only the first writes.
func (c *Conn) StateUpdate(module, name, key string, value json.RawMessage, revision uint64) (uint64, error) {
	return c.stateWrite(module, name, key, value, func(ctx context.Context, kv jetstream.KeyValue) (uint64, error) {
		return kv.Update(ctx, key, value, revision)
	})
}

// ErrStateChanged is a compare-and-set that lost: the key was made or changed by another writer first.
var ErrStateChanged = errors.New("the key was written by another writer first")

func (c *Conn) stateWrite(module, name, key string, value json.RawMessage,
	write func(context.Context, jetstream.KeyValue) (uint64, error)) (uint64, error) {
	s, err := c.writable(module, name)
	if err != nil {
		return 0, err
	}
	if err := checkKey(key); err != nil {
		return 0, err
	}
	if len(value) == 0 || !json.Valid(value) {
		return 0, fmt.Errorf("a state value is JSON")
	}
	if field := credentialField(value); field != "" {
		return 0, fmt.Errorf("%s's %s.%s carries a field %q, which names a credential: no secret is kept in state "+
			"(novox/hq ADR 0201)", module, name, key, field)
	}
	ctx, cancel := context.WithTimeout(context.Background(), StateTimeout)
	defer cancel()
	kv, err := c.bucket(ctx, s)
	if err != nil {
		return 0, err
	}
	rev, err := write(ctx, kv)
	if errors.Is(err, jetstream.ErrKeyExists) || isWrongSequence(err) {
		return 0, fmt.Errorf("%s.%s: %w", name, key, ErrStateChanged)
	}
	return rev, err
}

// isWrongSequence is the server's refusal of an update whose revision is not the key's last.
func isWrongSequence(err error) bool {
	var api *jetstream.APIError
	return errors.As(err, &api) && api.ErrorCode == jetstream.JSErrCodeStreamWrongLastSequence
}