package relay

import (
	"encoding/json"
	"errors"
	"fmt"
	"strings"
	"time"

	badger "github.com/dgraph-io/badger/v4"
)

const (
	// mailboxTTL is how long undelivered envelopes are retained.
	mailboxTTL = 7 * 24 * time.Hour

	// mailboxPrefix is the BadgerDB key prefix for stored envelopes.
	// Key format: mail:<recipientX25519Hex>:<sentAt_20d>:<envelopeID>
	mailboxPrefix = "mail:"

	// MailboxMaxLimit caps the number of envelopes returned per single query.
	MailboxMaxLimit = 200

	// MailboxPerRecipientCap is the maximum number of envelopes stored per
	// recipient key. When the cap is reached, the oldest envelope is evicted
	// before the new one is written (sliding window, FIFO).
	// At 64 KB max per envelope, one recipient occupies at most ~32 MB.
	MailboxPerRecipientCap = 500

	// MailboxMaxEnvelopeSize is the maximum allowed ciphertext length in bytes.
	// Rejects oversized envelopes before writing to disk.
	MailboxMaxEnvelopeSize = 64 * 1024 // 64 KB
)

// ErrEnvelopeTooLarge is returned by Store when the envelope exceeds the size limit.
var ErrEnvelopeTooLarge = errors.New("envelope ciphertext exceeds maximum allowed size")

// ErrMailboxFull is never returned externally (oldest entry is evicted instead),
// but kept as a sentinel for internal logic.
var errMailboxFull = errors.New("recipient mailbox is at capacity")

// Mailbox is a BadgerDB-backed store for relay envelopes awaiting pickup.
// Every received envelope is stored with a 7-day TTL regardless of whether
// the recipient is currently online. Recipients poll GET /relay/inbox to fetch
// and DELETE /relay/inbox/{id} to acknowledge delivery.
//
// Anti-spam guarantees:
//   - Envelopes larger than MailboxMaxEnvelopeSize (64 KB) are rejected.
//   - At most MailboxPerRecipientCap (500) envelopes per recipient are stored;
//     when the cap is hit the oldest entry is silently evicted (FIFO).
//   - All entries expire automatically after 7 days (BadgerDB TTL).
//
// Messages are stored encrypted — the relay cannot read their contents.
type Mailbox struct {
	db *badger.DB

	// onStore, if set, is invoked after every successful Store. Used by the
	// node to push a WebSocket `inbox` event to subscribers of the
	// recipient's x25519 pubkey so the mobile client stops polling
	// /relay/inbox every 3 seconds.
	//
	// The callback MUST NOT block — it runs on the writer goroutine. Long
	// work should be fanned out to a goroutine by the callback itself.
	onStore func(*Envelope)
}

// SetOnStore registers a post-Store hook. Pass nil to clear. Safe to call
// before accepting traffic (wired once at node startup in main.go).
func (m *Mailbox) SetOnStore(cb func(*Envelope)) {
	m.onStore = cb
}

// NewMailbox creates a Mailbox backed by the given BadgerDB instance.
func NewMailbox(db *badger.DB) *Mailbox {
	return &Mailbox{db: db}
}

// OpenMailbox opens (or creates) a dedicated BadgerDB at dbPath for the mailbox.
//
// Storage tuning matches blockchain/chain.NewChain — 64 MiB vlog files
// (instead of 1 GiB default) so GC can actually shrink the DB, and single
// version retention since envelopes are either present or deleted.
func OpenMailbox(dbPath string) (*Mailbox, error) {
	opts := badger.DefaultOptions(dbPath).
		WithLogger(nil).
		WithValueLogFileSize(64 << 20).
		WithNumVersionsToKeep(1).
		WithCompactL0OnClose(true)
	db, err := badger.Open(opts)
	if err != nil {
		return nil, fmt.Errorf("open mailbox db: %w", err)
	}
	return &Mailbox{db: db}, nil
}

// Close closes the underlying database.
func (m *Mailbox) Close() error { return m.db.Close() }

// Store persists an envelope with a 7-day TTL.
//
// Anti-spam checks (in order):
//  1. Ciphertext > MailboxMaxEnvelopeSize → returns ErrEnvelopeTooLarge.
//  2. Duplicate envelope ID → silently overwritten (idempotent).
//  3. Recipient already has MailboxPerRecipientCap entries → oldest evicted first.
func (m *Mailbox) Store(env *Envelope) error {
	if len(env.Ciphertext) > MailboxMaxEnvelopeSize {
		return ErrEnvelopeTooLarge
	}

	key := mailboxKey(env.RecipientPub, env.SentAt, env.ID)
	val, err := json.Marshal(env)
	if err != nil {
		return fmt.Errorf("marshal envelope: %w", err)
	}

	// Track whether this was a fresh insert (vs. duplicate) so we can skip
	// firing the WS hook for idempotent resubmits — otherwise a misbehaving
	// sender could amplify events by spamming the same envelope ID.
	fresh := false
	err = m.db.Update(func(txn *badger.Txn) error {
		// Check if this exact envelope is already stored (idempotent).
		if _, err := txn.Get([]byte(key)); err == nil {
			return nil // already present, no-op
		}

		// Count existing envelopes for this recipient and collect the oldest key.
		prefix := []byte(fmt.Sprintf("%s%s:", mailboxPrefix, env.RecipientPub))
		opts := badger.DefaultIteratorOptions
		opts.PrefetchValues = false
		opts.Prefix = prefix

		var count int
		var oldestKey []byte
		it := txn.NewIterator(opts)
		for it.Rewind(); it.Valid(); it.Next() {
			if count == 0 {
				oldestKey = it.Item().KeyCopy(nil) // first = oldest (sorted by sentAt)
			}
			count++
		}
		it.Close()

		// Evict the oldest envelope if cap is reached.
		if count >= MailboxPerRecipientCap && oldestKey != nil {
			if err := txn.Delete(oldestKey); err != nil {
				return fmt.Errorf("evict oldest envelope: %w", err)
			}
		}

		e := badger.NewEntry([]byte(key), val).WithTTL(mailboxTTL)
		if err := txn.SetEntry(e); err != nil {
			return err
		}
		fresh = true
		return nil
	})
	if err == nil && fresh && m.onStore != nil {
		m.onStore(env)
	}
	return err
}

// List returns up to limit envelopes for recipientPubHex, ordered oldest-first.
// Pass since > 0 to skip envelopes with SentAt < since (unix timestamp).
func (m *Mailbox) List(recipientPubHex string, since int64, limit int) ([]*Envelope, error) {
	if limit <= 0 || limit > MailboxMaxLimit {
		limit = MailboxMaxLimit
	}
	prefix := []byte(fmt.Sprintf("%s%s:", mailboxPrefix, recipientPubHex))
	var out []*Envelope

	err := m.db.View(func(txn *badger.Txn) error {
		opts := badger.DefaultIteratorOptions
		opts.Prefix = prefix
		it := txn.NewIterator(opts)
		defer it.Close()

		for it.Rewind(); it.Valid() && len(out) < limit; it.Next() {
			if err := it.Item().Value(func(val []byte) error {
				var env Envelope
				if err := json.Unmarshal(val, &env); err != nil {
					return nil // skip corrupt entries
				}
				if since > 0 && env.SentAt < since {
					return nil
				}
				out = append(out, &env)
				return nil
			}); err != nil {
				return err
			}
		}
		return nil
	})
	return out, err
}

// Delete removes an envelope by ID.
// It scans the recipient's prefix to locate the full key (sentAt is not known by caller).
// Returns nil if the envelope is not found (already expired or never stored).
func (m *Mailbox) Delete(recipientPubHex, envelopeID string) error {
	prefix := []byte(fmt.Sprintf("%s%s:", mailboxPrefix, recipientPubHex))
	var found []byte

	err := m.db.View(func(txn *badger.Txn) error {
		opts := badger.DefaultIteratorOptions
		opts.PrefetchValues = false
		opts.Prefix = prefix
		it := txn.NewIterator(opts)
		defer it.Close()

		suffix := ":" + envelopeID
		for it.Rewind(); it.Valid(); it.Next() {
			key := it.Item().KeyCopy(nil)
			if strings.HasSuffix(string(key), suffix) {
				found = key
				return nil
			}
		}
		return nil
	})
	if err != nil || found == nil {
		return err
	}

	return m.db.Update(func(txn *badger.Txn) error {
		return txn.Delete(found)
	})
}

// Count returns the number of stored envelopes for a recipient.
func (m *Mailbox) Count(recipientPubHex string) (int, error) {
	prefix := []byte(fmt.Sprintf("%s%s:", mailboxPrefix, recipientPubHex))
	count := 0
	err := m.db.View(func(txn *badger.Txn) error {
		opts := badger.DefaultIteratorOptions
		opts.PrefetchValues = false
		opts.Prefix = prefix
		it := txn.NewIterator(opts)
		defer it.Close()
		for it.Rewind(); it.Valid(); it.Next() {
			count++
		}
		return nil
	})
	return count, err
}

// RunGC periodically runs BadgerDB value log garbage collection.
// Call in a goroutine — blocks until cancelled via channel close or process exit.
func (m *Mailbox) RunGC() {
	ticker := time.NewTicker(5 * time.Minute)
	defer ticker.Stop()
	for range ticker.C {
		for m.db.RunValueLogGC(0.5) == nil {
			// drain until nothing left to collect
		}
	}
}

func mailboxKey(recipientPubHex string, sentAt int64, envelopeID string) string {
	// Zero-padded sentAt keeps lexicographic order == chronological order.
	// Oldest entry = first key in iterator — used for FIFO eviction.
	return fmt.Sprintf("%s%s:%020d:%s", mailboxPrefix, recipientPubHex, sentAt, envelopeID)
}