// Package emission delivers the decision-record emission guarantee declared in
// cadence.yaml (FLEX-WP-0031). A decision is released only after its event is
// durably committed to a local outbox (FLEX-DEC-2026-018); draining to
// audit-core is asynchronous.
package emission

import (
	"bufio"
	"bytes"
	"crypto/rand"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"sync"
	"time"

	"github.com/netkingdom/access-engine/pkg/api"
)

// ClassPrefix prefixes every decision event class; the suffix is the effect.
const ClassPrefix = "flex-auth.decision."

// PlatformTenant is sent when a decision names no tenant, because audit-core
// requires one on every event.
const PlatformTenant = "tenant:platform"

const outboxFile = "outbox.jsonl"
const MaxOutboxBytes int64 = 64 * 1024 * 1024
const MaxEntryBytes = 2 * 1024 * 1024
const MaxBatchBytes = MaxEntryBytes

var ErrCapacity = errors.New("outbox capacity exceeded; committed evidence retained")

// Class returns the event class for a decision effect.
func Class(effect api.DecisionEffect) string {
	return ClassPrefix + string(effect)
}

// Event is one audit-core.event envelope: exactly the fields audit-core's
// normalize() requires. Data carries the decision record itself.
type Event struct {
	ID            string `json:"id"`
	Type          string `json:"type"`
	Source        string `json:"source"`
	Subject       string `json:"subject"`
	Tenant        string `json:"tenant"`
	CorrelationID string `json:"correlation_id"`
	OccurredAt    string `json:"occurred_at"`
	Data          any    `json:"data"`
}

// entry is one committed outbox line.
type entry struct {
	Seq         uint64 `json:"seq"`
	CommittedAt string `json:"committed_at"`
	Event       Event  `json:"event"`
}

// Status is the local half of reconciliation: what this outbox committed, per
// class, and what it released without committing.
type Status struct {
	Source              string            `json:"source"`
	MaxBytes            int64             `json:"max_bytes"`
	CommittedBytes      int64             `json:"committed_bytes"`
	RemainingBytes      int64             `json:"remaining_bytes"`
	WriterFenced        bool              `json:"writer_fenced"`
	CapacityExhausted   bool              `json:"capacity_exhausted"`
	Committed           map[string]uint64 `json:"committed"`
	ReleasedUncommitted map[string]uint64 `json:"released_uncommitted"`
	// ReleasedUncommitted is held in memory: a restart during an outbox outage
	// loses it (FLEX-DEC-2026-018, consequences).
	Means           string `json:"means"`
	Pending         uint64 `json:"pending"`
	OldestPendingAt string `json:"oldest_pending_at,omitempty"`
	LagExceeded     bool   `json:"lag_exceeded"`
}

// Outbox is a durable, append-only, fsync-per-commit event log.
type Outbox struct {
	source string
	path   string
	clock  func() time.Time

	mu          sync.Mutex
	file        *os.File
	writerLock  *os.File
	maxBytes    int64
	closed      bool
	writeFault  bool
	seq         uint64
	size        int64 // bytes committed; a failed commit is cut back to it
	committed   map[string]uint64
	uncommitted map[string]uint64
	drainMu     sync.Mutex
	cursor      drainCursor
	heartbeats  map[string]time.Time
}

// Open opens or creates the outbox in dir and rebuilds committed counts from it.
func Open(dir, source string) (*Outbox, error) { return OpenBounded(dir, source, MaxOutboxBytes) }

// OpenBounded acquires OS writer exclusion before inspecting/recovering evidence.
// The selected limit cannot exceed the reviewed 64MiB source ceiling.
func OpenBounded(dir, source string, maxBytes int64) (*Outbox, error) {
	if maxBytes < 1 || maxBytes > MaxOutboxBytes {
		return nil, errors.New("outbox byte limit must be between 1 and 67108864")
	}

	if source == "" {
		return nil, errors.New("emission source is required")
	}
	if err := os.MkdirAll(dir, 0o750); err != nil {
		return nil, fmt.Errorf("create outbox directory: %w", err)
	}
	writerLock, err := acquireWriterLock(filepath.Join(dir, "writer.lock"))
	if err != nil {
		return nil, err
	}
	success := false
	defer func() {
		if !success {
			writerLock.Close()
		}
	}()
	o := &Outbox{
		source:      source,
		writerLock:  writerLock,
		maxBytes:    maxBytes,
		path:        filepath.Join(dir, outboxFile),
		clock:       time.Now,
		committed:   map[string]uint64{},
		uncommitted: map[string]uint64{},
		heartbeats:  map[string]time.Time{},
	}
	if err := o.loadCursor(); err != nil {
		return nil, err
	}
	if err := o.load(); err != nil {
		return nil, err
	}
	if o.cursor.Seq > o.seq || o.cursor.Offset > o.size {
		return nil, errors.New("drain cursor is ahead of committed outbox")
	}
	file, err := openOwnedFile(o.path, os.O_CREATE|os.O_APPEND|os.O_WRONLY, 0o640)
	if err != nil {
		return nil, fmt.Errorf("open outbox: %w", err)
	}
	o.file = file
	if err := syncOutboxDirectory(dir); err != nil {
		file.Close()
		return nil, err
	}
	success = true
	return o, nil
}

func (o *Outbox) load() error {
	file, err := openOwnedFile(o.path, os.O_RDONLY, 0)
	if errors.Is(err, os.ErrNotExist) {
		return nil
	}
	if err != nil {
		return fmt.Errorf("read outbox: %w", err)
	}
	defer file.Close()
	info, err := file.Stat()
	if err != nil {
		return err
	}
	if !info.Mode().IsRegular() {
		return errors.New("outbox must be a regular file")
	}
	if info.Size() > o.maxBytes {
		return ErrCapacity
	}
	reader := bufio.NewReaderSize(file, 64*1024)
	var good int64
	for line := 1; ; line++ {
		raw, err := boundedLine(reader)
		if err == io.EOF {
			// Only an unterminated final write is uncommitted. Never discard a
			// newline-terminated corrupt or oversized record.
			o.size = good
			if len(raw) > 0 {
				if err := os.Truncate(o.path, good); err != nil {
					return err
				}
				if err := file.Sync(); err != nil {
					return err
				}
			}
			break
		}
		if err != nil {
			return fmt.Errorf("outbox line %d: %w", line, err)
		}
		var e entry
		decoder := json.NewDecoder(bytes.NewReader(raw))
		decoder.UseNumber()
		if err := decoder.Decode(&e); err != nil {
			return fmt.Errorf("outbox line %d is corrupt: %w", line, err)
		}
		if _, err := decoder.Token(); err != io.EOF {
			return errors.New("trailing outbox entry data")
		}
		if e.Seq != o.seq+1 || e.Event.Source != o.source {
			return fmt.Errorf("outbox line %d has inconsistent sequence or source", line)
		}
		o.seq = e.Seq
		o.committed[e.Event.Type]++
		good += int64(len(raw))
		if e.Seq == o.cursor.Seq && good != o.cursor.Offset {
			return errors.New("drain cursor does not match outbox")
		}
		o.noteHeartbeat(e.Event)
	}
	if o.cursor.Seq > o.seq {
		return errors.New("drain cursor is ahead of outbox")
	}
	return nil
}

// boundedLine bounds startup and delivery allocations independently of log size.
// Its 2MiB limit includes the record newline.
func boundedLine(reader *bufio.Reader) ([]byte, error) {
	var line []byte
	for {
		part, err := reader.ReadSlice('\n')
		if len(line)+len(part) > MaxEntryBytes {
			return nil, ErrCapacity
		}
		line = append(line, part...)
		if err == bufio.ErrBufferFull {
			continue
		}
		return line, err
	}
}

// SetClock overrides the commit clock. Tests use it.
func (o *Outbox) SetClock(clock func() time.Time) {
	o.mu.Lock()
	defer o.mu.Unlock()
	o.clock = clock
}

// Close closes the outbox file.
func (o *Outbox) Close() error {
	// Drain holds drainMu through HTTP and cursor commit. Do not release writer
	// exclusion while an old delivery can still mutate the cursor.
	o.drainMu.Lock()
	defer o.drainMu.Unlock()
	o.mu.Lock()
	defer o.mu.Unlock()
	if o.closed {
		return nil
	}
	o.closed = true
	return errors.Join(o.file.Close(), o.writerLock.Close())
}

// Append commits one decision; it satisfies decision.DecisionRecorder.
func (o *Outbox) Append(decision api.DecisionEnvelope) error {
	return o.AppendBatch([]api.DecisionEnvelope{decision})
}

// AppendBatch commits decisions with one sync, so a batch check pays the
// durable-write cost once rather than per resource.
func (o *Outbox) AppendBatch(decisions []api.DecisionEnvelope) error {
	o.mu.Lock()
	defer o.mu.Unlock()
	now := o.clock().UTC().Format(time.RFC3339Nano)
	var events []Event
	for _, decision := range decisions {
		id, err := eventID()
		if err != nil {
			return err
		}
		events = append(events, o.event(id, now, decision))
	}
	return o.appendEvents(now, events)
}

// appendEvents requires mu; heartbeats use the same durable commit as decisions.
func (o *Outbox) appendEvents(now string, events []Event) error {
	if o.closed {
		return os.ErrClosed
	}
	if o.writeFault {
		return errors.New("outbox rollback unconfirmed; writer fenced")
	}
	var buf []byte
	seq := o.seq
	for _, event := range events {
		seq++
		line, err := json.Marshal(entry{Seq: seq, CommittedAt: now, Event: event})
		if err != nil {
			return fmt.Errorf("marshal outbox entry: %w", err)
		}
		if len(line)+1 > MaxEntryBytes || len(buf)+len(line)+1 > MaxBatchBytes || int64(len(buf))+int64(len(line))+1 > o.maxBytes-o.size {
			return ErrCapacity
		}
		buf = append(append(buf, line...), '\n')
	}
	if _, err := o.file.Write(buf); err != nil {
		return o.rollback(fmt.Errorf("write outbox: %w", err))
	}
	if err := o.file.Sync(); err != nil {
		return o.rollback(fmt.Errorf("sync outbox: %w", err))
	}
	o.seq = seq
	o.size += int64(len(buf))
	for _, event := range events {
		o.committed[event.Type]++
		o.noteHeartbeat(event)
	}
	return nil
}

// rollback cuts a failed commit back to the last committed size, so a partial
// line never sits in front of the next commit. The commit error is returned
// either way; a failed cut is reported alongside it.
func (o *Outbox) rollback(cause error) error {
	err := o.file.Truncate(o.size)
	if err == nil {
		err = o.file.Sync()
	}
	if err != nil {
		o.writeFault = true
		return errors.Join(cause, fmt.Errorf("outbox rollback unconfirmed; writer fenced: %w", err))
	}
	return cause
}

func syncOutboxDirectory(dir string) error {
	file, err := os.Open(dir)
	if err != nil {
		return err
	}
	defer file.Close()
	return file.Sync()
}

// NoteReleasedUncommitted counts a restriction released after its commit
// failed. Every non-zero count is a finding.
func (o *Outbox) NoteReleasedUncommitted(effect api.DecisionEffect) {
	o.mu.Lock()
	defer o.mu.Unlock()
	o.uncommitted[Class(effect)]++
}

// Status reports committed and released-uncommitted counts per class.
func (o *Outbox) Status() Status {
	o.mu.Lock()
	defer o.mu.Unlock()
	status := Status{
		Source:   o.source,
		MaxBytes: o.maxBytes, CommittedBytes: o.size, RemainingBytes: o.maxBytes - o.size, CapacityExhausted: o.size == o.maxBytes, WriterFenced: o.writeFault,
		Committed:           map[string]uint64{},
		ReleasedUncommitted: map[string]uint64{},
		Means: "committed counts this outbox holds since it was created; " +
			"released_uncommitted is decisions released without a record " +
			"since this process started, and any non-zero value is a finding",
	}
	status.Pending = o.seq - o.cursor.Seq
	if status.Pending > 0 {
		if e, _, err := o.nextEntry(); err == nil {
			status.OldestPendingAt = e.CommittedAt
			at, _ := time.Parse(time.RFC3339Nano, e.CommittedAt)
			status.LagExceeded = status.Pending > 100 || o.clock().Sub(at) > time.Hour
		} else {
			status.LagExceeded = true
		}
	}
	for _, effect := range api.DecisionEffects() {
		class := Class(effect)
		status.Committed[class] = o.committed[class]
		status.ReleasedUncommitted[class] = o.uncommitted[class]
	}
	return status
}

func (o *Outbox) event(id, at string, decision api.DecisionEnvelope) Event {
	tenant := decision.Resource.Tenant
	if decision.Binding != nil && decision.Binding.Tenant != "" {
		tenant = decision.Binding.Tenant
	}
	if tenant == "" {
		tenant = PlatformTenant
	}
	return Event{
		ID:            id,
		Type:          Class(decision.Effect),
		Source:        o.source,
		Subject:       resourceSubject(decision.Resource),
		Tenant:        tenant,
		CorrelationID: decision.ID,
		OccurredAt:    at,
		Data:          decision,
	}
}

func resourceSubject(resource api.ResourceRef) string {
	subject := resource.ID
	if resource.Type != "" {
		subject = resource.Type + "/" + subject
	}
	if resource.System != "" {
		subject = resource.System + "/" + subject
	}
	return subject
}

// eventID is random, not derived from the decision: decision ids are content
// digests, so two identical checks share one, and audit-core's idempotency key
// would merge two real decisions into one.
func eventID() (string, error) {
	var b [16]byte
	if _, err := rand.Read(b[:]); err != nil {
		return "", fmt.Errorf("event id: %w", err)
	}
	return "flex-auth-evt:" + hex.EncodeToString(b[:]), nil
}