package broker

import (
	"fmt"
	"sort"
)

// The mesh's own streams.
//
// **These four and no more** (novox/hq ADR 0116 task 1.4, as revised by ADR 0118). An earlier
// reading had the controller create *every* stream at genesis, from a fixed set. That is only the
// mesh's own half: a seat's streams are created when the module declaring it is registered, and a
// module's durable consumers when it is assigned — neither of which has happened at genesis. What
// is here is the foundation, which exists before any module does.
//
// The controller is the only writer of stream definitions (design 25 §3). A module declares
// nothing about them and cannot reach the JetStream API to make one.

// Retention is how a stream decides what to keep, which is the whole of what distinguishes the
// mesh's four relationships on the wire (design 29 §4).
type Retention string

const (
	// RetentionWorkQueue: a message is removed once a consumer acknowledges it. Exactly one
	// worker does the work, and a worker that dies has its message redelivered.
	RetentionWorkQueue Retention = "workqueue"
	// RetentionLastPerSubject: only the newest message on each subject survives. This is the
	// state shape — a node that was away gets exactly the current declaration and nothing older.
	RetentionLastPerSubject Retention = "last_per_subject"
	// RetentionLimits: kept until it ages or the stream fills. Events, where a subscriber that
	// was down catches up and nobody is obliged to act.
	RetentionLimits Retention = "limits"
)

// A Stream is one of the mesh's own, as the controller asserts it.
type Stream struct {
	Name      string
	Subjects  []string
	Retention Retention
	// MaxAge in seconds, zero for unbounded. Per stream — JetStream has no per-subject age,
	// which is why differing retention between modules would mean a stream each.
	MaxAge int
	// MaxMsgsPerSubject caps each subject independently, so one noisy emitter cannot push
	// another's events out of a shared stream. Verified: with a cap of 3, ten messages on one
	// subject and one on another leave four in the stream, not three.
	MaxMsgsPerSubject int
	// Why is carried into the assertion so an operator reading the server's own state finds the
	// reason there, rather than only in a repository they may not have.
	Why string
}

// MeshStreams is the foundation set, in the order a person reads it.
//
// **CONTROL names its subjects rather than taking `mesh.control.>`**, because heartbeats live
// under that prefix and must not be persisted: a lost heartbeat is the next heartbeat, and a
// stream of them is a stream of the least valuable messages the mesh sends, competing for the
// same retention as the ones that matter.
//
// **EVENTS filters on the `event` token**, which is the reason that token exists. A module's
// namespace carries both its events and its tool calls; a filter of `mesh.mod.*.>` would persist
// every tool invocation in the mesh, and a tool call must never be persisted (design 25 §3 keeps
// tools on core NATS, where a lost call is a timeout the caller already handles).
func MeshStreams() []Stream {
	return []Stream{
		{
			Name: "CONTROL",
			// A build's outcome is no longer here: it is the build-machine seat's own event, so one
			// publish reaches whoever asked, the controller and the catalogue (novox/hq ADR 0121).
			Subjects:  []string{"mesh.control.*.report", "mesh.control.enrol"},
			Retention: RetentionWorkQueue,
			Why: "the store-window guarantee (ADR 0083): the controller naks with a delay while its " +
				"store is away and the message is redelivered; nothing is dropped",
		},
		{
			Name:      "NODES",
			Subjects:  []string{"mesh.node.*.declare"},
			Retention: RetentionLastPerSubject,
			Why: "one declaration per node, always the newest; a node that sees sequence n refuses " +
				"n-1 by construction (issue 107)",
		},
		{
			Name: "EVENTS",
			// A seat's own events ride here too: they are 1:many like any event, and the
			// `event` token keeps them clear of both the seat's work queue (`accept`) and its
			// tools (`tool`), which must not be persisted.
			Subjects:          []string{"mesh.mod.*.event.>", "mesh.seat.*.event.>"},
			Retention:         RetentionLimits,
			MaxAge:            7 * 24 * 60 * 60,
			MaxMsgsPerSubject: 10000,
			Why: "a subscriber that was down catches up; tool traffic under the same prefix is " +
				"excluded by the event token; per-subject caps keep a noisy emitter from " +
				"evicting a quiet one without splitting the stream",
		},
	}
}

// An Asserter is the part of a JetStream connection stream assertion needs. Narrow on purpose: it
// keeps this testable without a server, and keeps the client library out of everything that only
// wants to know what the streams are.
type Asserter interface {
	// EnsureStream creates the stream if absent and updates it to match if present. It must be
	// idempotent: the controller asserts on every start, not only at genesis.
	EnsureStream(s Stream) error
}

// AssertMeshStreams brings the foundation set into being, in order, and says which one failed
// rather than that something did.
//
// Asserted on every start rather than created once at genesis, because a stream that was deleted,
// or a mesh raised from a restored backup, must converge rather than run without the guarantee
// its messages assume. Idempotence is the whole requirement.
func AssertMeshStreams(a Asserter) error {
	for _, s := range MeshStreams() {
		if err := a.EnsureStream(s); err != nil {
			return fmt.Errorf("asserting stream %s: %w", s.Name, err)
		}
	}
	return nil
}

// Overlaps reports subject filters claimed by more than one stream.
//
// **Corrected against the server**: an earlier version of this comment said NATS accepts
// overlapping streams and stores the message twice. It does not — it refuses the second stream
// with "subjects overlap with an existing stream" (verified against nats-server 2.10). The check
// still earns its place, for a different reason: the server's refusal arrives when the controller
// is applying, naming one stream, at a moment when the mesh is half-configured. This one arrives
// where the set is written, names both, and cannot reach a running mesh.
//
// It also decides a design question. Because overlap is refused rather than merged, a shared
// EVENTS stream and a per-module stream cannot coexist — the module's would be refused — so
// "one stream for most, its own for a module that wants different retention" is not an option
// the server allows. It is all of one or all of the other.
func Overlaps() []string {
	seen := map[string]string{}
	var clashes []string
	for _, s := range MeshStreams() {
		for _, subject := range s.Subjects {
			if first, ok := seen[subject]; ok {
				clashes = append(clashes, fmt.Sprintf("%s and %s both claim %s", first, s.Name, subject))
				continue
			}
			seen[subject] = s.Name
		}
	}
	sort.Strings(clashes)
	return clashes
}

// The mesh's own consumers.
//
// A seat's streams and a module's consumers are derived from declarations (derived.go). These two
// are not: **the controller is not a module and files no manifest**, so its authority and its
// subscriptions cannot come from a declaration that does not exist. They are named here, where the
// mesh's own streams are named, and narrowly — a controller subscribing `mesh.mod.*.event.>` would
// hear every event in the mesh, which it has no business doing and which would make its permission
// list stop explaining anything.

// ControllerName is the controller's durable consumer on each stream it reads, and the name its
// ack subject is derived from (nats.go: `$JS.ACK.<stream>.controller.>`).
const ControllerName = "controller"

// ControllerSeat is the role the control plane holds, and ControllerStates are the facts it states
// under it (novox/hq ADR 0134).
//
// **Written here as well as in `link`, and a test keeps them agreeing.** `link` imports `broker`, so
// `broker` cannot import `link`; a grant naming a subject the controller never publishes is authority
// nobody uses, and a controller publishing one the grant omits is refused at the moment it has
// something to say.
const ControllerSeat = "mesh-controller"

var ControllerStates = []string{"applied", "refused", "built-before"}

// ControllerFollows are the events the controller reacts to: the catalogue saying a module's
// current version moved, and a catalogue that has just started saying it may have missed builds.
//
// **Derived the same way a module's subscription is**, from the emitter and the bare local event
// name, rather than written out. They were written out while the catalogue still spelled its events
// as the old bus's routing keys, and the moment those were converted (novox/hq 04-ISSUES/127) a
// hard-coded pair became a controller listening to a subject nothing publishes — the same fault, from
// the other side. Deriving them means the conversion could not leave these behind.
var ControllerFollows = []string{
	moduleEventSubject("mesh-catalog", "upgraded"),
	moduleEventSubject("mesh-catalog", "catching-up"),
	// A build's outcome, which is the build-machine role's own event now (ADR 0121) rather than a
	// message on the control branch. Same three audiences, one publish: whoever asked, this, and the
	// catalogue.
	seatEventSubject("mesh-build-machine", "built"),
	// The forge's merges: what moved a source, so the mesh builds what that source produces
	// without anybody telling it (novox/hq 04-ISSUES/131). Appended, because the index is a name.
	moduleEventSubject("gitea", "pull.merged"),
}

// moduleEventSubject is where one module's event lands. The same derivation PermissionsFor uses, so
// what the controller subscribes and what the emitter is permitted to publish cannot drift apart.
func moduleEventSubject(module, event string) string {
	return "mesh.mod." + module + ".event." + event
}

// seatEventSubject is where a role's own event lands, derived the same way a holder's permission is.
func seatEventSubject(seat, verb string) string {
	return "mesh.seat." + seat + ".event." + verb
}

// MeshConsumers is what the controller consumes, in the order a person reads it.
//
// **Unlimited redelivery on CONTROL, deliberately.** The store window's bound is the controller's,
// not the server's (window.go): a message is held with a nak-and-delay until the controller either
// takes it or gives up and says so. A max-deliver here would dead-letter a push that was being
// held through a store restart — the exact message the stream exists to protect — some minutes
// before the controller had finished deciding about it.
func MeshConsumers() []Consumer {
	return []Consumer{
		{
			Name:           ControllerName,
			Stream:         "CONTROL",
			Push:           true,
			AckWaitSeconds: 30,
			Why: "the controller is the single consumer of what nodes say; explicit ack and no " +
				"max-deliver, because the store window's bound is the controller's own",
		},
		{
			Name:           ControllerName,
			Stream:         "EVENTS",
			Filters:        ControllerFollows,
			Push:           true,
			AckWaitSeconds: 30,
			MaxDeliver:     5,
			Why: "the two events the mesh's own controller reacts to; after max-deliver it " +
				"dead-letters, because an announcement it cannot act on will not become actionable",
		},
	}
}

// Ensurer is the part of a JetStream connection consumer assertion needs, narrow for the reason
// Asserter is.
type Ensurer interface {
	EnsureConsumer(c Consumer) error
}

// AssertMeshConsumers brings the controller's own consumers into being, and says which one failed.
//
// After the streams, necessarily: a consumer on a stream that does not exist is refused, and the
// refusal names the stream rather than the order.
func AssertMeshConsumers(e Ensurer) error {
	for _, c := range MeshConsumers() {
		if err := e.EnsureConsumer(c); err != nil {
			return fmt.Errorf("asserting consumer %s on %s: %w", c.Name, c.Stream, err)
		}
	}
	return nil
}