package buscompose

import (
	"strings"
)

// 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
	// Direct lets a client read a subject's last message without a consumer, which is how a
	// runtime reads its own membership with no JetStream API beyond one request (ADR 0160).
	Direct bool
	// MaxBytes bounds the stream's size, zero for unbounded. With DiscardNew a full stream refuses
	// what comes next rather than dropping what it holds: the publisher is told, and says so.
	MaxBytes   int64
	DiscardNew bool
	// DuplicatesSeconds is the window in which a message id published twice is kept once; zero for the
	// server's default (two minutes).
	DuplicatesSeconds int
	// AllowMsgTTL lets a message carry a time to live of its own (`Nats-TTL`), for what must not outlive the
	// stream it came from (novox/hq ADR 0376 §3, §5).
	AllowMsgTTL bool
	// Restricted marks a module's stream restricted by a grant (novox/hq ADR 0363 §4), in the stream's own
	// metadata, so whoever keeps one of its messages reads it from the bus (ADR 0376 §3).
	Restricted bool
	// WorkQueue marks a module stream declared retention: work-queue (novox/hq ADR 0436 §6), in the
	// stream's own metadata, so a dead letter kept from it is read as drop-only from the bus, by the
	// declaration and not by the bus's work-queue retention alone — which SEAT_* and other mesh-internal
	// streams share without being one of ADR 0436's, and whose dead letters are still delivered again
	// (ADR 0264 §4).
	WorkQueue bool
}

// RestrictedMetadata is the key a restricted stream's metadata says it under.
const RestrictedMetadata = "mesh-restricted"

// WorkQueueMetadata is the key a module stream declared retention: work-queue (novox/hq ADR 0436 §6) says
// it under: never set by a mesh-internal work-queue stream (a seat's, a check's), only by Stream.WorkQueue.
const WorkQueueMetadata = "mesh-work-queue"

// What a durable consumer gave up on is kept (novox/hq issue 330, design 25 §3).
//
// **The server says it and keeps it; the controller copies it.** A consumer that handed a message over
// as often as it may stops offering it and publishes `$JS.EVENT.ADVISORY.CONSUMER.MAX_DELIVERIES` with
// the stream and the message's sequence. DeadLetterNoticesStream captures those advisories as the
// server publishes them, so one said while no controller listens is still there when one starts. The
// controller's consumer on it fetches the given-up message by its sequence, while the source stream
// still holds it, and keeps a copy in DeadLettersStream under DeadLetterSubject, with the consumer,
// the subject, how often it was handed over and when it was given up. It stays there until a person
// delivers it again or drops it, with why; a condition is open for as long as it does.
const (
	DeadLetterNoticesStream = "DEAD_LETTER_NOTICES"
	DeadLettersStream       = "DEAD_LETTERS"
	// MaxDeliveriesAdvisories is the subject the server says a given-up message on.
	MaxDeliveriesAdvisories = "$JS.EVENT.ADVISORY.CONSUMER.MAX_DELIVERIES.>"
	deadLetterPrefix        = "mesh.events.dead."
	againPrefix             = "mesh.again."
	// DeadLettersBytes bounds the kept copies. Full, the stream refuses the next copy, which is said
	// as the consumer's condition; it never drops one it holds.
	DeadLettersBytes = 256 << 20
)

// DeadLetterSubject is where a message one consumer gave up on is kept: `mesh.events.dead.<stream>.<consumer>`.
func DeadLetterSubject(stream, consumer string) string {
	return deadLetterPrefix + stream + "." + consumer
}

// DeadLetterOf is the stream and consumer a kept message's subject names; false for any other subject.
func DeadLetterOf(subject string) (stream, consumer string, ok bool) {
	rest, found := strings.CutPrefix(subject, deadLetterPrefix)
	if !found {
		return "", "", false
	}
	stream, consumer, ok = strings.Cut(rest, ".")
	return stream, consumer, ok && stream != "" && consumer != "" && !strings.Contains(consumer, ".")
}

// MeshStreams is the foundation set, in the order a person reads it.
//
// **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{
		// CONTROL, the ended controller's work queue of what nodes reported, is no longer the mesh's (novox/hq ADR 0590
		// clause 6): not declared here, which would make it again; the bus module's tools remove it.
		{
			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)",
		},
		// The assignments stream (ADR 0160) is no longer the mesh's: a runtime reads its membership from the grants
		// part's state memberships (ADR 0571 clause 7), and the bus module's tools delete the stream at the lease write
		// (research 072 row 33), so it is not declared here, which would make it again.
		{
			Name: EventsStream,
			// 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.
			// And an event given up on, delivered again to the one consumer that gave it up
			// (novox/hq issue 330): under `mesh.again.<consumer>.`, which only that consumer filters.
			Subjects:          []string{"mesh.mod.*.event.>", "mesh.seat.*.event.>", againPrefix + ">"},
			Retention:         RetentionLimits,
			MaxAge:            7 * 24 * 60 * 60,
			MaxMsgsPerSubject: 10000,
			// A restricted dead letter delivered again keeps what is left of its stream's age (ADR 0376 §5).
			AllowMsgTTL: true,
			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",
		},
		{
			Name:      DeadLetterNoticesStream,
			Subjects:  []string{MaxDeliveriesAdvisories},
			Retention: RetentionWorkQueue,
			MaxAge:    7 * 24 * 60 * 60,
			Why: "the server's word that a consumer gave up on a message, kept until the controller has " +
				"copied the message into DEAD_LETTERS (novox/hq issue 330); a week, the longest the source " +
				"streams keep what they are about",
		},
		{
			Name:              DeadLettersStream,
			Subjects:          []string{deadLetterPrefix + ">"},
			Retention:         RetentionLimits,
			MaxBytes:          DeadLettersBytes,
			DiscardNew:        true,
			DuplicatesSeconds: 24 * 60 * 60,
			AllowMsgTTL:       true,
			Why: "every message a consumer gave up on, with its consumer, subject, deliveries and when, kept " +
				"until a person delivers it again or drops it with why (novox/hq issue 330); no age but a restricted " +
				"stream's message, which ages out with its stream (ADR 0376), and full it refuses the next copy " +
				"rather than drop one it holds",
		},
		{
			Name:      ProvAppliedStream,
			Subjects:  []string{ProvAppliedSubjects},
			Retention: RetentionLastPerSubject,
			Direct:    true,
			MaxAge:    7 * 24 * 60 * 60,
			Why: "each machine's provisioner client's newest word on each module's files, applied, waiting or " +
				"failed with the reason (novox/hq ADR 0349 §4): what a walk's end and a gate wait for (ADR 0362, " +
				"ADR 0349 §15), read by subject; a week, as the events are",
		},
	}
}

// ProvAppliedStream keeps what each machine's provisioner client last said of each module's files, newest per
// subject (novox/hq ADR 0349 §4, ADR 0362, design 57 P6): `prov.applied.<node>.<module>` lies outside `mesh.*`, so
// no other stream captures it, and a report said while no controller listened is still there to be read.
const ProvAppliedStream = "PROV_APPLIED"

// ProvAppliedSubjects is every machine's client's reports: `prov.applied.<node>.<module>`, a module's name dots and
// all (ProvisionerAppliedSubject).
const ProvAppliedSubjects = "prov.applied.>"

// The ended controller's consumers' names (novox/hq ADR 0590 clause 6): kept only so its leftovers are named for their
// removal (ControllerLeftoverConsumers) and the watchman can read whether `controller` on DEAD_LETTER_NOTICES still
// stands.
const (
	ControllerName     = "controller"
	MergesConsumerName = "controller-merges"
)

// EventsStream holds every module's and every role's events, a build's log among them.
const EventsStream = "EVENTS"