package broker

import (
	"fmt"
	"sort"
	"strings"

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

// A module's log on the bus (novox/hq ADR 0297): its record of operations, each entry appended under
// a key and kept as long as the log is.
//
// A log follows a bucket in every respect (ADR 0201): the module names it locally, the mesh derives
// its stream and subjects, the controller creates it from the catalogue on every raise and from
// registration, never a module, and **no path of the mesh removes one** — a log whose declaration
// is gone is reported, as a bucket is. Unlike a bucket, a log is its owner's alone: no other module
// reads it, so there is no read of a log to grant or issue.
//
// Pure, but for the stream's configuration, which is the server's own type so that what is tested
// is exactly what is sent; jetstream.go is the part that asks a server.

// The mesh's caps on a log: what an entry's value may weigh, what a message on the log's stream may
// weigh — the value and its headers, which the server counts in a message's size, so a value of the
// full size still fits with the expected-last-sequence header an append carries — and what a log holds
// when its module says nothing and at most.
const (
	LogMaxEntryBytes   = 256 * 1024
	LogMaxMessageBytes = LogMaxEntryBytes + 4*1024
	LogDefaultMiB      = 1024
	LogMostMiB         = 8192
)

// A Log is one module's declared log as the bus holds it.
type Log struct {
	Module string
	Name   string
	// MaxMiB is its cap in MiB; zero is LogDefaultMiB.
	MaxMiB int
}

// LogStreamPrefix starts every log's stream name, which no other stream of the mesh's starts with.
const LogStreamPrefix = "LOG_"

// LogStreamName is the stream a module's log lives in: `LOG_<module>_<name>`. The module and the
// local name are each one token with no underscore, so two modules can never derive one stream.
func LogStreamName(module, name string) string { return LogStreamPrefix + module + "_" + name }

// LogSubject is the subject a log's entries are published under, without the key: an entry for key K
// is on `<LogSubject>.<K>`.
func LogSubject(module, name string) string { return "mesh.log." + module + "." + name }

// Stream is this log's stream name.
func (l Log) Stream() string { return LogStreamName(l.Module, l.Name) }

// Subject is this log's subject, without the key.
func (l Log) Subject() string { return LogSubject(l.Module, l.Name) }

// MaxBytes is this log's cap in bytes.
func (l Log) MaxBytes() int64 {
	mib := l.MaxMiB
	if mib <= 0 {
		mib = LogDefaultMiB
	}
	return int64(mib) * 1024 * 1024
}

// Why is carried into the server's description of the stream, so somebody reading the server's own
// state finds whose it is and why it is kept.
func (l Log) Why() string {
	return fmt.Sprintf("%s's log %q (novox/hq ADR 0297): its record of operations, one entry per operation "+
		"under its key, appended by %s alone and kept as long as the log; never removed by the mesh, because "+
		"it is data", l.Module, l.Name, l.Module)
}

// Config is the stream a log is, field by field as novox/hq ADR 0297 fixes it: a file stream kept by
// limits, with no maximum age and no cap per key, whose message is an entry's value and 4 KiB of
// headers at most, that refuses a new entry when full rather than
// drop an old one, and that refuses deleting an entry or purging it. Direct gets are allowed, which
// is how the runtime reads it.
func (l Log) Config() jetstream.StreamConfig {
	return jetstream.StreamConfig{
		Name:              l.Stream(),
		Description:       l.Why(),
		Subjects:          []string{l.Subject() + ".>"},
		Storage:           jetstream.FileStorage,
		Retention:         jetstream.LimitsPolicy,
		MaxAge:            0,
		MaxMsgs:           -1,
		MaxMsgsPerSubject: -1,
		MaxBytes:          l.MaxBytes(),
		MaxMsgSize:        LogMaxMessageBytes,
		Discard:           jetstream.DiscardNew,
		AllowDirect:       true,
		DenyDelete:        true,
		DenyPurge:         true,
		Replicas:          1,
	}
}

// LogIssued is one log an assignment may reach, by the name its module uses for it (novox/hq ADR
// 0297): its stream, the subject its entries go under, and whether it may append. Only the owner's
// instances are issued a log, so Writes is always true today; it is said so the runtime need not
// assume it.
type LogIssued struct {
	Name    string `json:"name"`
	Stream  string `json:"stream"`
	Subject string `json:"subject"`
	Writes  bool   `json:"writes"`
}

// logsIssuedFor is every log a module's code may reach, as its membership lists them: its own.
func logsIssuedFor(d Declared) []LogIssued {
	var out []LogIssued
	for _, l := range d.Logs {
		if !safeSubject.MatchString(d.Module) || !safeSubject.MatchString(l.Name) {
			continue
		}
		out = append(out, LogIssued{Name: l.Name, Stream: LogStreamName(d.Module, l.Name),
			Subject: LogSubject(d.Module, l.Name), Writes: true})
	}
	return out
}

// logGrants is what a principal publishes to reach the logs its module keeps: for each, appending
// under the log's subjects, binding to its stream, and reading it directly. Nothing more — the
// runtime reads a log by direct gets alone and makes no consumer on it — and nothing of any other
// module's log, because a log is its owner's alone. Replies come on the principal's inbox, as for a
// bucket.
func logGrants(module string, names []string) []string {
	if !safeSubject.MatchString(module) {
		return nil
	}
	var out []string
	for _, name := range names {
		if !safeSubject.MatchString(name) {
			continue
		}
		stream := LogStreamName(module, name)
		out = append(out,
			LogSubject(module, name)+".>",
			"$JS.API.STREAM.INFO."+stream,
			"$JS.API.DIRECT.GET."+stream,
			"$JS.API.DIRECT.GET."+stream+".>")
	}
	return out
}

// logNames is the local names of a module's logs.
func logNames(logs []Log) []string {
	out := make([]string, 0, len(logs))
	for _, l := range logs {
		out = append(out, l.Name)
	}
	return out
}

// A LogAsserter is the part of a JetStream connection log assertion needs. It has no way to remove a
// log, by design: nothing the mesh runs asks for one.
type LogAsserter interface {
	// EnsureLog creates the log's stream if absent and brings its configuration to match if present,
	// never deleting or recreating it.
	EnsureLog(l Log) error
	// LogStreams is every log stream on the server: every stream named with LogStreamPrefix.
	LogStreams() ([]string, error)
}

// RaiseLogs asserts every declared log and answers the log streams on the server that nothing
// declares any more.
//
// **Those are reported, never removed** (novox/hq ADR 0297 §2, ADR 0201 §15–16): a log is a module's
// record, and a manifest edited, a module renamed or unassigned is an ordinary day's work that must
// not take a record with it. Removing one is a person's act, outside the mesh.
func RaiseLogs(a LogAsserter, logs []Log) (undeclared []string, err error) {
	sorted := append([]Log(nil), logs...)
	sort.Slice(sorted, func(i, j int) bool { return sorted[i].Stream() < sorted[j].Stream() })
	declared := map[string]bool{}
	for _, l := range sorted {
		if err := a.EnsureLog(l); err != nil {
			return nil, fmt.Errorf("asserting %s's log %q: %w", l.Module, l.Name, err)
		}
		declared[l.Stream()] = true
	}
	names, err := a.LogStreams()
	if err != nil {
		return nil, fmt.Errorf("listing the bus's logs: %w", err)
	}
	for _, n := range names {
		if strings.HasPrefix(n, LogStreamPrefix) && !declared[n] {
			undeclared = append(undeclared, n)
		}
	}
	sort.Strings(undeclared)
	return undeclared, nil
}