package link

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"regexp"
	"sort"
	"time"

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

	"github.com/novox/mesh-controller/internal/broker"
)

// The build queue, read and changed by hand (novox/hq ADR 0219).
//
// **A queue nobody can see is a queue nobody can trust.** Until this, the build seat's work queue
// was visible only as its consequences: a plan LATE with nothing to say why, a build that never
// started because five asks ahead of it were waiting on a laptop that was shut. ADR 0219 makes the
// queue the controller's to show and change, and the build running on one machine that machine's
// holder's to end.
//
// Three kinds of ask are in the seat's stream at any moment, told apart by the worker consumer
// every holder pulls from:
//
//   - **waiting** — after the last one the worker handed out (stream seq > delivered.stream_seq);
//   - **in flight** — handed out and not yet settled, which the holder that took it said with its
//     `started` event, and which the worker counts among its acks pending;
//   - **dead** — handed out as many times as the worker allows and never settled. A work queue
//     drops what it settles, so one still in the stream behind the worker is either in flight or
//     dead; the started events and the worker's pending count say which.
//
// Everything that drops an ask leaves a failed outcome for it, recorded the way a failed build's is
// (`cancelled by hand`, `killed by hand`), so a plan waiting on it fails visibly rather than waiting
// for ever on an ask nobody will answer.

// The words an outcome says when a person ended the ask.
const (
	CancelledByHand = "cancelled by hand"
	KilledByHand    = "killed by hand"
)

// Ask states in a queue.
const (
	AskWaiting  = "waiting"
	AskInFlight = "in flight"
	AskDead     = "dead"
)

// QueuedAsk is one ask in the seat's work queue.
type QueuedAsk struct {
	Seq     uint64       `json:"seq"`
	Request BuildRequest `json:"-"`
	ID      string       `json:"id"`
	// Repository, Path and Ref are the ask's, as the request carried them; Held never travels out of
	// here — it is every artifact the mesh has built, and a queue listing is not where that belongs.
	Repository string    `json:"repository"`
	Path       string    `json:"path,omitempty"`
	Ref        string    `json:"ref,omitempty"`
	AskedAt    time.Time `json:"asked-at,omitempty"`
	State      string    `json:"state"`
	// On and Started are the holder building an in-flight ask and when it started, from its started
	// event. Was is the holder that started an in-flight ask and is no longer building it — handed
	// back, to be handed out again — with Started its start there.
	On      string    `json:"on,omitempty"`
	Was     string    `json:"was-on,omitempty"`
	Started time.Time `json:"started,omitempty"`
}

// Queue is the seat's work queue as it stands.
type Queue struct {
	Seat string `json:"seat"`
	// Delivered is the last stream sequence the worker handed out; AckPending how many it handed out
	// and has not had settled; MaxDeliver how many times it hands one ask out.
	Delivered  uint64      `json:"delivered"`
	AckPending int         `json:"ack-pending"`
	MaxDeliver int         `json:"max-deliver"`
	Asks       []QueuedAsk `json:"asks"`
}

// Of is the asks in one state, in queue order.
func (q Queue) Of(state string) []QueuedAsk {
	var out []QueuedAsk
	for _, a := range q.Asks {
		if a.State == state {
			out = append(out, a)
		}
	}
	return out
}

// Find is the ask with this id.
func (q Queue) Find(id string) (QueuedAsk, bool) {
	for _, a := range q.Asks {
		if a.ID == id {
			return a, true
		}
	}
	return QueuedAsk{}, false
}

// BuildHeard is what the seat's events say about builds: the latest start of each id, and every id
// whose outcome was heard.
type BuildHeard struct {
	Started  map[string]BuildStart
	Outcomes map[string]bool
}

// ClassifyQueue says which state each ask is in. Pure: the stream's asks in sequence order, what the
// worker says, and what the seat's events said.
//
// **In flight is what the worker counts handed out and unsettled**, at most AckPending of the asks
// behind it, given out in this order:
//
//  1. what a machine is building now — its latest start, no outcome heard (a holder builds one ask
//     at a time, ADR 0190), newest start first;
//  2. what a machine started and is no longer building — a holder restarted mid-build, the ask
//     handed back and waiting to be handed out again while it has deliveries left — newest start
//     first, naming the machine it was last on;
//  3. what was taken and not yet said started.
//
// Only what is left after that is dead: so nothing behind the worker is dead while there are no more
// of them than the worker counts pending. A machine that died mid-build keeps a latest start for
// ever; the worker's count is what says the ask was handed out as often as it may be.
//
// **The worker's count is the server's, and it lags in one case**: an ask handed out its last time
// and handed back is still counted pending until some holder pulls again, when the server finds it
// past its deliveries and lets it go. Holders pull whenever they are idle, so this is a moment —
// except while every holder is paused, when such an ask reads as in flight with no machine named.
func ClassifyQueue(asks []QueuedAsk, delivered uint64, ackPending int, heard BuildHeard) []QueuedAsk {
	// Each machine's latest start.
	latest := map[string]BuildStart{}
	for _, s := range heard.Started {
		at := startedAt(s)
		if was, ok := latest[s.On]; !ok || at.After(startedAt(was)) {
			latest[s.On] = s
		}
	}
	current := map[string]BuildStart{}
	for _, s := range latest {
		if !heard.Outcomes[s.ID] {
			current[s.ID] = s
		}
	}

	out := make([]QueuedAsk, len(asks))
	copy(out, asks)
	var running, handedBack, unsaid []int
	for i := range out {
		a := &out[i]
		if a.Seq > delivered {
			a.State = AskWaiting
			continue
		}
		a.State = AskDead
		if s, ok := current[a.ID]; ok {
			a.On, a.Started = s.On, startedAt(s)
			running = append(running, i)
			continue
		}
		if s, ever := heard.Started[a.ID]; ever {
			a.Was, a.Started = s.On, startedAt(s)
			handedBack = append(handedBack, i)
			continue
		}
		unsaid = append(unsaid, i)
	}
	newestFirst := func(ix []int) {
		sort.SliceStable(ix, func(x, y int) bool { return out[ix[x]].Started.After(out[ix[y]].Started) })
	}
	newestFirst(running)
	newestFirst(handedBack)
	slots := ackPending
	for _, group := range [][]int{running, handedBack, unsaid} {
		for _, i := range group {
			if slots == 0 {
				// Past what the worker counts: handed out as often as it may be.
				out[i].On, out[i].Started = "", time.Time{}
				continue
			}
			out[i].State = AskInFlight
			slots--
		}
	}
	return out
}

func startedAt(s BuildStart) time.Time {
	t, _ := time.Parse(time.RFC3339Nano, s.At)
	return t
}

// ReadQueue reads a build seat's work queue: the stream's asks, the worker's position, and what the
// seat's events said about the builds behind it. Through the JetStream API alone (STREAM.INFO,
// CONSUMER.INFO, STREAM.MSG.GET), which only the controller's account reaches.
func ReadQueue(ctx context.Context, js *broker.JetStream, seat string) (Queue, error) {
	worker, _ := broker.HolderConsumerFor("", "", broker.DeclaredSeat{Name: seat, Accepts: []string{"build"}})
	q := Queue{Seat: seat}
	api, err := jetstream.New(js.Conn())
	if err != nil {
		return q, err
	}
	stream, err := api.Stream(ctx, worker.Stream)
	if err != nil {
		return q, fmt.Errorf("the %s work queue (%s) cannot be read: %w", seat, worker.Stream, err)
	}
	consumer, err := stream.Consumer(ctx, worker.Name)
	switch {
	case errors.Is(err, jetstream.ErrConsumerNotFound):
		// No holder has ever been given a worker: everything in the stream is waiting.
	case err != nil:
		return q, fmt.Errorf("the worker of %s cannot be read: %w", seat, err)
	default:
		info, err := consumer.Info(ctx)
		if err != nil {
			return q, fmt.Errorf("the worker of %s cannot be read: %w", seat, err)
		}
		q.Delivered = info.Delivered.Stream
		q.AckPending = info.NumAckPending
		q.MaxDeliver = info.Config.MaxDeliver
	}

	// Every ask, by next-by-subject from the first: the stream holds only what is unsettled, so this
	// is a handful of reads, never the history.
	var asks []QueuedAsk
	var seq uint64 = 1
	for n := 0; n < 10000; n++ {
		msg, err := stream.GetMsg(ctx, seq, jetstream.WithGetMsgSubject(BuildWorkOf(seat)))
		if errors.Is(err, jetstream.ErrMsgNotFound) {
			break
		}
		if err != nil {
			return q, fmt.Errorf("reading the %s work queue: %w", seat, err)
		}
		ask := QueuedAsk{Seq: msg.Sequence}
		if json.Unmarshal(msg.Data, &ask.Request) == nil {
			r := ask.Request
			ask.ID, ask.Repository, ask.Path, ask.Ref = r.ID, r.Repository, r.Path, r.Ref
			if at, ok := BuildAskedAt(r.ID); ok {
				ask.AskedAt = at
			}
		}
		asks = append(asks, ask)
		seq = msg.Sequence + 1
	}

	// What the events say, from shortly before the oldest ask behind the worker: its start, if any,
	// came after it was asked.
	var since time.Time
	for _, a := range asks {
		if a.Seq <= q.Delivered && (since.IsZero() || (!a.AskedAt.IsZero() && a.AskedAt.Before(since))) {
			since = a.AskedAt
		}
	}
	heard := BuildHeard{Started: map[string]BuildStart{}, Outcomes: map[string]bool{}}
	if len(asks) > 0 && q.Delivered > 0 {
		if since.IsZero() {
			since = time.Now().Add(-7 * 24 * time.Hour)
		}
		if heard, err = ReadBuildEvents(ctx, js, seat, since.Add(-time.Minute)); err != nil {
			return q, err
		}
	}
	q.Asks = ClassifyQueue(asks, q.Delivered, q.AckPending, heard)
	return q, nil
}

// ReadBuildEvents is every start and outcome the seat announced since a moment, from the events
// stream, with a consumer of its own that is gone when this returns.
func ReadBuildEvents(ctx context.Context, js *broker.JetStream, seat string, since time.Time) (BuildHeard, error) {
	heard := BuildHeard{Started: map[string]BuildStart{}, Outcomes: map[string]bool{}}
	// One token after `event`: started and built, never a build's log lines, which are two.
	subject := "mesh.seat." + seat + ".event.*"
	sub, err := js.Context().PullSubscribe(subject, "",
		nats.BindStream(broker.EventsStream), nats.StartTime(since), nats.AckNone())
	if err != nil {
		return heard, fmt.Errorf("cannot read %s from the bus: %w", subject, err)
	}
	defer func() { _ = sub.Unsubscribe() }()
	for {
		batch, err := sub.Fetch(500, nats.MaxWait(2*time.Second))
		if err != nil && !errors.Is(err, nats.ErrTimeout) && !errors.Is(err, context.DeadlineExceeded) {
			return heard, fmt.Errorf("reading %s: %w", subject, err)
		}
		for _, msg := range batch {
			switch msg.Subject {
			case BuildStartedOf(seat):
				var s BuildStart
				if json.Unmarshal(msg.Data, &s) == nil && s.ID != "" {
					if was, ok := heard.Started[s.ID]; !ok || startedAt(s).After(startedAt(was)) {
						heard.Started[s.ID] = s
					}
				}
			case BuildOutcomeOf(seat):
				var r BuildResult
				if json.Unmarshal(msg.Data, &r) == nil && r.ID != "" {
					heard.Outcomes[r.ID] = true
				}
			}
		}
		if len(batch) < 500 {
			break
		}
		if ctx.Err() != nil {
			return heard, ctx.Err()
		}
	}
	return heard, nil
}

// --- the cancelled set ----------------------------------------------------------------------

// safeKey is an id that can be a key, and a subject token, as it is: a build id, never a pattern.
var safeKey = regexp.MustCompile(`^[A-Za-z0-9_-]+$`)

// Cancelled is what the controller writes for an ask it cancelled.
type Cancelled struct {
	At  string `json:"at"`
	Why string `json:"why"`
}

// MarkCancelled puts an ask's id into the seat's cancelled set (novox/hq ADR 0219). The controller's
// act, before it deletes the ask from the queue.
func MarkCancelled(ctx context.Context, js *broker.JetStream, seat, id string) error {
	if !safeKey.MatchString(id) {
		return fmt.Errorf("%q is not a build id", id)
	}
	api, err := jetstream.New(js.Conn())
	if err != nil {
		return err
	}
	kv, err := api.KeyValue(ctx, broker.CancelledSetName(seat))
	if err != nil {
		return fmt.Errorf("the cancelled set of %s is not on the bus — the controller asserts it at its "+
			"start, so one older than this has not: %w", seat, err)
	}
	body, err := json.Marshal(Cancelled{At: time.Now().UTC().Format(time.RFC3339Nano), Why: CancelledByHand})
	if err != nil {
		return err
	}
	_, err = kv.Put(ctx, id, body)
	return err
}

// UnmarkCancelled takes an ask's id out of the cancelled set: a cancel withdrawn, because the ask
// was taken as it was being cancelled.
func UnmarkCancelled(ctx context.Context, js *broker.JetStream, seat, id string) error {
	if !safeKey.MatchString(id) {
		return nil
	}
	api, err := jetstream.New(js.Conn())
	if err != nil {
		return err
	}
	kv, err := api.KeyValue(ctx, broker.CancelledSetName(seat))
	if err != nil {
		return err
	}
	return kv.Delete(ctx, id)
}

// IsCancelled asks the seat's cancelled set whether an ask was cancelled: one direct read of one key,
// which is all a holder is granted of it.
func IsCancelled(conn *nats.Conn, seat, id string) (bool, error) {
	if !safeKey.MatchString(id) {
		// Not a key the controller could have written.
		return false, nil
	}
	set := broker.CancelledSetName(seat)
	reply, err := conn.Request("$JS.API.DIRECT.GET.KV_"+set+".$KV."+set+"."+id, nil, 3*time.Second)
	if err != nil {
		return false, err
	}
	return cancelledFrom(reply)
}

// cancelledFrom reads a direct get's answer: a value is a cancel; not found, or a deletion marker,
// is not.
func cancelledFrom(reply *nats.Msg) (bool, error) {
	if reply.Header != nil {
		switch reply.Header.Get("Status") {
		case "":
		case "404":
			return false, nil
		default:
			return false, fmt.Errorf("the cancelled set answered %s %s",
				reply.Header.Get("Status"), reply.Header.Get("Description"))
		}
		if op := reply.Header.Get("KV-Operation"); op == "DEL" || op == "PURGE" {
			return false, nil
		}
	}
	return len(reply.Data) > 0, nil
}

// --- a holder's verbs -----------------------------------------------------------------------

// NodeSeatToolSubject is where a node-scoped seat's verb is asked of one machine's holder (design 33
// §4): the seat's verb subject with the machine as its last token.
func NodeSeatToolSubject(seat, verb, node string) string {
	return SeatToolSubject(seat, verb) + "." + node
}

// AskSeatTool asks one machine's holder of a node seat one of its verbs and reads its answer.
func AskSeatTool(ctx context.Context, conn *nats.Conn, seat, verb, node string, args any,
	timeout time.Duration) (Answer, error) {
	body, err := json.Marshal(args)
	if err != nil {
		return Answer{}, err
	}
	asking, cancel := context.WithTimeout(ctx, timeout)
	defer cancel()
	subject := NodeSeatToolSubject(seat, verb, node)
	// **A refused question is known at once** (novox/hq to-be 45, D8 live on 2026-10-06): the bus
	// says it to the connection and the request would otherwise wait out its whole timeout, reading
	// as a machine that did not answer.
	refused, stop := refusalsOf(conn, subject)
	defer stop()
	type replied struct {
		msg *nats.Msg
		err error
	}
	done := make(chan replied, 1)
	go func() {
		msg, err := conn.RequestWithContext(asking, subject, body)
		done <- replied{msg, err}
	}()
	var reply *nats.Msg
	select {
	case r := <-done:
		reply, err = r.msg, r.err
	case why := <-refused:
		cancel()
		return Answer{}, fmt.Errorf("the bus refused the controller asking %s.%s of %s — its grants do not "+
			"name %s: %v", seat, verb, node, subject, why)
	}
	switch {
	case errors.Is(err, nats.ErrNoResponders):
		return Answer{}, fmt.Errorf("nothing on %s answers %s.%s: its holder is not running, or is "+
			"older than the verb", node, seat, verb)
	case errors.Is(err, context.DeadlineExceeded), errors.Is(err, nats.ErrTimeout):
		return Answer{}, fmt.Errorf("%s did not answer %s.%s within %s", node, seat, verb, timeout)
	case err != nil:
		return Answer{}, err
	}
	var answer Answer
	if err := json.Unmarshal(reply.Data, &answer); err != nil {
		return Answer{}, fmt.Errorf("%s answered %s.%s with something unreadable: %w", node, seat, verb, err)
	}
	return answer, nil
}

// --- a holder saying whether it takes work ----------------------------------------------------

// HolderState is what a holder says about itself whenever it is paused or resumed, at its start,
// and while it stays paused: whether it takes work (novox/hq ADR 0219). Retained on the events
// stream under the machine's own subject, so the last one is the machine's state.
type HolderState struct {
	On     string `json:"on"`
	Paused bool   `json:"paused"`
	At     string `json:"at"`
}

// BuildPausedOf is where one machine's holder of a build seat says whether it takes work.
func BuildPausedOf(seat, node string) string { return "mesh.seat." + seat + ".event.paused." + node }

// PausedSaid reads what each machine last said about taking work. A machine that never said is not
// in the answer — a holder older than ADR 0219 takes work and never pauses.
func PausedSaid(js *broker.JetStream, seat string, nodes []string) (map[string]HolderState, error) {
	out := map[string]HolderState{}
	for _, node := range nodes {
		msg, err := js.Context().GetLastMsg(broker.EventsStream, BuildPausedOf(seat, node))
		if errors.Is(err, nats.ErrMsgNotFound) {
			continue
		}
		if err != nil {
			return out, err
		}
		var s HolderState
		if json.Unmarshal(msg.Data, &s) == nil {
			out[node] = s
		}
	}
	return out, nil
}