package link

import (
	"context"
	"crypto/ed25519"
	"encoding/json"
	"errors"
	"fmt"
	"time"
)

// Membership is what a node needs to reach its mesh again, held by the caller.
type Membership struct {
	Node        string
	Broker      string
	Fingerprint string
	Password    string
	Signer      ed25519.PublicKey
	// Transport is which bus this membership was minted for (hearing.go): the mesh's own, and a
	// membership that names another is one this host cannot dial with.
	Transport string
}

// Announce is how the link says what is happening, so a node running unattended leaves an
// account of it. Nil is allowed and means say nothing.
type Announce func(string)

// Hold keeps this node in the mesh, reconnecting for as long as it is asked to.
//
// Disconnection is an ordinary situation and not a failure (novox/hq ADR 0004), so this does not
// give up. A laptop shut for a week comes back and reconnects; it does not come back needing
// somebody to start it again.
//
// The backoff exists because the two common reasons differ in how long they last: a broker
// restarting is back in seconds, and a machine that has moved to a network with no route may be
// hours. Retrying every second for hours is a node shouting into nothing; waiting a minute after
// a broker blip is a node that is needlessly late. So it starts fast and slows down, and resets
// once a connection has actually held.
// Roused is a channel that says the machine has reason to believe its link is stale — it woke
// from suspend, or its network changed.
//
// **The machine knows before any timeout does.** A suspended laptop's connection is dead the
// moment it wakes, and heartbeats find that out in twenty or thirty seconds; for that time the
// node believes it is in the mesh and is not, which is the one state this design says must never
// be indistinguishable from being connected. Nothing new listens on the node to arrange it — the
// signal a service manager already sends is enough (novox/hq ADR 0004).
//
// Nil is allowed and means nothing ever rouses it, which is every machine that does not suspend.
type Roused <-chan struct{}

func Hold(ctx context.Context, m Membership, queue *Session, say Announce, timeout time.Duration) error {
	return HoldRoused(ctx, m, queue, say, timeout, nil)
}

// HoldRoused is Hold, told when the machine has reason to think its link is stale.
//
// The session outlives every link and says the node's facts on whichever is open (novox/hq ADR 0539 §9).
func HoldRoused(ctx context.Context, m Membership, queue *Session, say Announce,
	timeout time.Duration, roused Roused) error {

	return holdWith(ctx, func(ctx context.Context) error {
		return Run(ctx, m, queue, say, timeout)
	}, say, roused)
}

// attempt is one try at holding the link open, returning when it ends for any reason.
//
// Named so the loop below can be driven without a broker. What the loop decides — when to wait,
// how long, what being roused does — is the part with the reasoning in it, and it was reachable
// only through a real connection before.
type attempt func(context.Context) error

func holdWith(ctx context.Context, run attempt, say Announce, roused Roused) error {
	const (
		first = 2 * time.Second
		most  = 2 * time.Minute
		// A connection that lasted this long counts as having worked, so the next failure starts
		// from the bottom again. Without it a node that reconnects and immediately drops climbs
		// to the maximum and stays there, long after whatever caused it went away.
		settled = 30 * time.Second
	)
	wait := first

	for {
		began := time.Now()

		// The link runs under a context this loop can cancel, so being roused ends the current
		// attempt rather than only shortening the wait after it.
		//
		// **That is the whole of it.** After a resume the socket looks perfectly healthy from
		// inside this process — there is no error and no close, because nothing has tried to
		// send anything. It is heartbeats that eventually discover it, twenty or thirty seconds
		// later. A machine that knows it just woke does not have to wait to be told.
		//
		// A rouse that turns out to be spurious costs one reconnect, which is cheap and
		// idempotent: the node redeclares its queue and anything unacknowledged is redelivered.
		// The alternative costs half a minute of believing it is in a mesh it has left.
		trying, done := context.WithCancel(ctx)
		if roused != nil {
			go func() {
				select {
				case <-trying.Done():
				case <-roused:
					say("woken or moved — dropping the link and opening it again")
					done()
				}
			}()
		}
		err := run(trying)
		done()
		if ctx.Err() != nil {
			return nil
		}
		if time.Since(began) > settled {
			wait = first
		}

		switch {
		case errors.Is(err, ErrWrongCertificate):
			// Said in full every time rather than folded into a retry count. This does not mean
			// the network is down; it means what answered is not the mesh this node joined, and
			// no amount of waiting fixes it. The node keeps running what it was last told, which
			// is the right thing to do while somebody works out what happened.
			say("the broker is not the one this node joined: " + err.Error())
			say("this will not fix itself. This node keeps running what it was last told.")
		case err != nil:
			say(fmt.Sprintf("disconnected: %v — trying again in %s", err, wait))
		default:
			say(fmt.Sprintf("the link closed — trying again in %s", wait))
		}

		select {
		case <-ctx.Done():
			return nil
		case <-roused:
			// And it does not serve out a wait computed for a broker that was restarting, either.
			//
			// The backoff is not *reset* by this. Being roused says the machine changed, not that
			// whatever was refusing the connection has stopped — a laptop woken repeatedly on a
			// network with no route would otherwise retry at full speed for as long as somebody
			// keeps opening the lid.
			say("woken or moved — trying again now")
		case <-time.After(wait):
		}
		if wait *= 2; wait > most {
			wait = most
		}
	}
}

// Run holds the link open once, hearing what the controller asks and saying the node's facts.
//
// Outbound only, and nothing listens on this machine. Returns when the link ends, for any reason;
// Hold is what decides whether to open it again.
func Run(ctx context.Context, m Membership, queue *Session, say Announce, timeout time.Duration) error {
	link, err := Open(ctx, m, timeout)
	if err != nil {
		return err
	}
	defer link.Close()
	return serve(ctx, link, m, queue, say, timeout)
}

// serve is Run on a link already open: separated so what the node says, and when, can be tested
// without a broker.
func serve(ctx context.Context, link Link, m Membership, queue *Session, say Announce,
	timeout time.Duration) error {
	if say == nil {
		say = func(string) {}
	}

	// Said, because it is the event anybody watching actually wants. Without it a node logs every
	// failure and nothing on success, so a log full of "trying again" and then silence reads as
	// still broken when it means the opposite.
	say("in the mesh, hearing what the controller asks of this node")

	// The session says this node's facts as the link is handed to it, and is not let go while it is mid-act.
	queue.Attach(ctx, link)
	defer queue.Detach(link)

	// And the mesh asking this node to say its facts again, on a link that hears the question.
	var asks <-chan struct{}
	if a, ok := link.(Asked); ok {
		asks = a.AskedToReport()
	}
	// And the controller asking this node to hand a directory used as found to the mesh (novox/hq issue
	// 356), answered on the link the question came over.
	if h, ok := link.(HandOvers); ok {
		h.HearHandOvers(queue.HandOver)
	}
	// And asking for a fresh search for setuid programs (novox/hq issue 361).
	if s, ok := link.(SetuidSearches); ok {
		s.HearSetuidSearches(queue.SetuidSearch)
	}
	// And asking for a read-only judgement of its directories (novox/hq issue 358).
	if j, ok := link.(DirectoryJudgements); ok {
		j.HearDirectoryJudgements(queue.JudgeDirectories)
	}
	// And asking for the take act on a library, on a warrant (novox/hq ADR 0328 §7).
	if s, ok := link.(AccessActs); ok {
		s.HearAccessActs(queue.AccessAct)
	}
	// And asking to move a value sealed to this machine into the vault (design 57's V3, issue 676).
	if s, ok := link.(MoveSecrets); ok {
		s.HearMoveSecrets(queue.MoveSecret)
	}

	for {
		if ctx.Err() != nil {
			return nil
		}
		select {
		case <-ctx.Done():
			return nil
		case reason := <-link.Lost():
			return reason
		case <-asks:
			queue.FactsAsked(ctx)
		}
	}
}

// publishReport tells the mesh what this node did, and says whether the broker took it.
// Publish sends one report on this node's own connection and returns: the one-shot path for a
// report a command makes rather than the running host — a rekey (novox/hq ADR 0105). The same
// account, the same pinned certificate and the same exchange as the running host's reports.
func Publish(ctx context.Context, m Membership, report Report, timeout time.Duration) error {
	link, err := Open(ctx, m, timeout)
	if err != nil {
		return err
	}
	defer link.Close()
	var said string
	if !publishReport(ctx, link, m, report, func(s string) { said = s }, timeout) {
		return errors.New(said)
	}
	return nil
}

func publishReport(ctx context.Context, bus Bus, m Membership, report Report,
	say Announce, timeout time.Duration) bool {
	report.Node = m.Node
	body, err := json.Marshal(report)
	if err != nil {
		say("cannot encode this node's own report: " + err.Error())
		return false
	}
	publish, cancel := context.WithTimeout(ctx, timeout)
	defer cancel()

	// Said rather than swallowed. A report that fails to publish leaves the mesh believing this
	// node never answered, while the node believes it did — and the two would go on disagreeing
	// with nothing anywhere saying so. That shape of fault is the one this project keeps finding.
	if err := bus.Report(publish, m.Node, body); err != nil {
		say(fmt.Sprintf("applied, and could not tell the mesh: %v", err))
		return false
	}
	return true
}