package link

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

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

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

// AnnouncedMerge is one merge the forge announced, as the events stream holds it: what it said, and
// when the bus took it.
type AnnouncedMerge struct {
	SourceMoved
	// At is when the bus took the announcement — the stream's own time, not the forge's.
	At time.Time
	// Seq is its place in the events stream.
	Seq uint64
}

// MergedSubject is where the forge's merges land: the controller's own follow of them.
var MergedSubject = broker.ControllerFollows[3]

// readQuiet is how long a read of the stream waits for one more message before it takes the stream
// as read to its end. The stream answers at once when it holds something; this is only the wait at
// the end.
const readQuiet = 2 * time.Second

// AnnouncedMerges is every merge the forge announced since a moment, oldest first, read back from
// the events stream (novox/hq issue 266).
//
// **Read on a consumer of its own, filtered on the one subject.** The controller's durable consumer
// carries several filters, and the bus server the mesh ran when this was written (2.10) skips a
// message now and then on a consumer with more than one filter: it moves its delivered pointer past
// the message without ever handing it over, so the controller never hears of it and nothing says so.
// A consumer filtered on a single subject reads the stream another way and was not seen to skip. This
// one is ordered, ephemeral and acknowledges nothing, so reading it changes nothing on the bus.
func (s *Server) AnnouncedMerges(ctx context.Context, since time.Time) ([]AnnouncedMerge, error) {
	if s.js == nil {
		return nil, errors.New("this control plane is not on the bus, so it cannot read what the forge announced")
	}
	sub, err := s.js.Context().SubscribeSync(MergedSubject, nats.OrderedConsumer(), nats.StartTime(since))
	if err != nil {
		return nil, fmt.Errorf("reading the forge's merges from the events stream: %w", err)
	}
	defer func() { _ = sub.Unsubscribe() }()

	var out []AnnouncedMerge
	for {
		wait, cancel := context.WithTimeout(ctx, readQuiet)
		msg, err := sub.NextMsgWithContext(wait)
		cancel()
		if err != nil {
			if ctx.Err() != nil {
				return nil, ctx.Err()
			}
			// Nothing more within the quiet wait: the stream has been read to its end.
			break
		}
		meta, err := msg.Metadata()
		if err != nil {
			continue
		}
		var moved SourceMoved
		if json.Unmarshal(msg.Data, &moved) == nil && moved.Commit != "" && moved.Repo != "" {
			out = append(out, AnnouncedMerge{SourceMoved: moved, At: meta.Timestamp, Seq: meta.Sequence.Stream})
		}
		if meta.NumPending == 0 {
			break
		}
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Seq < out[j].Seq })
	return out, nil
}