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.
	MaxAge 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",
			Subjects:  []string{"mesh.control.*.report", "mesh.control.enrol", "mesh.control.built"},
			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:      "BUILDS",
			Subjects:  []string{"mesh.build.request"},
			Retention: RetentionWorkQueue,
			Why:       "at least once, one builder at a time; a builder that dies mid-build has its message redelivered",
		},
		{
			Name:      "EVENTS",
			Subjects:  []string{"mesh.mod.*.event.>"},
			Retention: RetentionLimits,
			MaxAge:    7 * 24 * 60 * 60,
			Why:       "a subscriber that was down catches up; tool traffic under the same prefix is excluded by the event token",
		},
	}
}

// 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.
//
// Two streams matching one subject is not a warning in NATS; it is accepted, and the message is
// stored twice under two retentions. For the mesh that would mean a declaration kept as both
// state and a work queue, acknowledged in one and lingering in the other — a divergence nothing
// reports and nobody would think to look for. So it is refused here, where the set is written.
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
}