dchain/relay/feed_mailbox.go

package relay

// FeedMailbox — BadgerDB-backed storage for social-feed post bodies.
//
// Posts are PUBLIC (plaintext) — unlike the E2E inbox envelopes, feed posts
// have no recipient key. They live keyed by post ID and can be read by
// anyone via GET /feed/post/{id}.
//
// Storage layout (keys):
//
//	post:<postID>                  → FeedPost JSON (body + metadata)
//	post-by-author:<author>:<ts>   → postID (chrono index for GET /feed/author)
//	post-views:<postID>            → uint64 big-endian (view counter)
//	post-hashtag:<tag>:<ts>:<id>   → postID (inverted index for #tag search)
//	post-trending:<score>:<id>     → postID (ranked index; score = likes × 2 + views)
//
// View counts are off-chain because on-chain would mean one tx per view —
// financially and architecturally unreasonable. Likes stay on-chain
// (provable authorship + anti-Sybil via fee).
//
// Anti-spam:
//   - MaxPostBodySize is enforced at Store time.
//   - Per-sender rate limiting happens at the HTTP layer (withSubmitTxGuards).

import (
	"encoding/binary"
	"encoding/json"
	"errors"
	"fmt"
	"regexp"
	"sort"
	"strings"
	"time"

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

const (
	feedPostPrefix       = "feedpost:"
	feedAuthorIdxPrefix  = "feedauthor:"    // feedauthor:<author>:<ts_20d>:<postID>
	feedViewPrefix       = "feedview:"      // feedview:<postID> → uint64
	feedHashtagPrefix    = "feedtag:"       // feedtag:<tag>:<ts_20d>:<postID>
	feedTrendingPrefix   = "feedtrend:"     // feedtrend:<score_20d_inv>:<postID>

	// MaxPostBodySize is the hard cap on a post's on-wire size. Matches
	// blockchain.MaxPostSize so the on-chain fee estimate is always
	// enforceable (no "I claimed 10 KiB but sent 50 KiB" trick).
	MaxPostBodySize = 256 * 1024 // 256 KiB

	// FeedPostDefaultTTLDays is how long a post body lives before BadgerDB
	// auto-evicts it. On-chain metadata stays forever, so a reader hitting
	// a stale post sees the record with a "body unavailable" indicator.
	// Configurable via the env var DCHAIN_FEED_TTL_DAYS (handled in main.go).
	FeedPostDefaultTTLDays = 30

	// maxHashtagsPerPost caps how many distinct hashtags we'll index per
	// post. Prevents a spammer from polluting every tag namespace with one
	// mega-post.
	maxHashtagsPerPost = 8

	// trendingHalfLifeSeconds controls how quickly a post's score decays.
	// Used when computing "trending": recent engagement weighs more than old.
	trendingHalfLifeSeconds = 12 * 3600 // 12 hours
)

// FeedPost is the off-chain body. On-chain we keep the metadata in
// blockchain.PostRecord — here we store the readable payload.
//
// Why not just put the body on-chain? Size — a 256 KiB post × thousands
// per day would bloat the block history. Keeping it in a relay DB with a
// TTL gives us ephemerality while still letting on-chain records serve as
// the permanent proof of authorship.
type FeedPost struct {
	// Identity (matches on-chain PostRecord.PostID).
	PostID string `json:"post_id"`
	Author string `json:"author"` // Ed25519 hex

	// Payload. Content is always plaintext (posts are public). Attachment is
	// a pre-compressed blob — client is expected to have minimised size
	// before publish. If empty, the post is text-only.
	Content       string   `json:"content"`
	ContentType   string   `json:"content_type,omitempty"` // "text/plain" | "text/markdown" | ...
	Attachment    []byte   `json:"attachment,omitempty"`
	AttachmentMIME string  `json:"attachment_mime,omitempty"`
	Hashtags      []string `json:"hashtags,omitempty"` // lowercased, without leading #

	// CreatedAt matches the on-chain tx timestamp — we stamp it server-side
	// at Store() so senders can't back-date.
	CreatedAt int64 `json:"created_at"`

	// ReplyTo / QuoteOf mirror the on-chain PostRecord fields, included
	// here so the client can thread without a second RPC.
	ReplyTo string `json:"reply_to,omitempty"`
	QuoteOf string `json:"quote_of,omitempty"`
}

// ErrPostTooLarge is returned by Store when the post body exceeds MaxPostBodySize.
var ErrPostTooLarge = errors.New("post body exceeds maximum allowed size")

// FeedMailbox stores feed post bodies.
type FeedMailbox struct {
	db  *badger.DB
	ttl time.Duration
}

// NewFeedMailbox wraps an already-open Badger DB. TTL controls how long
// post bodies live before auto-eviction (on-chain metadata persists
// forever independently).
func NewFeedMailbox(db *badger.DB, ttl time.Duration) *FeedMailbox {
	if ttl <= 0 {
		ttl = time.Duration(FeedPostDefaultTTLDays) * 24 * time.Hour
	}
	return &FeedMailbox{db: db, ttl: ttl}
}

// OpenFeedMailbox opens (or creates) a dedicated BadgerDB at dbPath.
func OpenFeedMailbox(dbPath string, ttl time.Duration) (*FeedMailbox, error) {
	opts := badger.DefaultOptions(dbPath).
		WithLogger(nil).
		WithValueLogFileSize(128 << 20).
		WithNumVersionsToKeep(1).
		WithCompactL0OnClose(true)
	db, err := badger.Open(opts)
	if err != nil {
		return nil, fmt.Errorf("open feed mailbox db: %w", err)
	}
	return NewFeedMailbox(db, ttl), nil
}

// Close releases the underlying Badger handle.
func (fm *FeedMailbox) Close() error { return fm.db.Close() }

// Store persists a post body and updates all indices. `createdAt` is the
// canonical timestamp (usually from the chain tx) and becomes the
// server's view of when the post happened — clients' wall-clock values
// are ignored.
//
// Returns the set of hashtags actually indexed (after dedup + cap).
func (fm *FeedMailbox) Store(post *FeedPost, createdAt int64) ([]string, error) {
	size := estimatePostSize(post)
	if size > MaxPostBodySize {
		return nil, ErrPostTooLarge
	}

	post.CreatedAt = createdAt
	// Normalise hashtags — the client may or may not have supplied them;
	// we derive from Content as the authoritative source, then dedup.
	tags := extractHashtags(post.Content)
	if len(tags) > maxHashtagsPerPost {
		tags = tags[:maxHashtagsPerPost]
	}
	post.Hashtags = tags

	val, err := json.Marshal(post)
	if err != nil {
		return nil, fmt.Errorf("marshal post: %w", err)
	}

	err = fm.db.Update(func(txn *badger.Txn) error {
		// Idempotent on postID — second Store is a no-op.
		key := []byte(feedPostPrefix + post.PostID)
		if _, err := txn.Get(key); err == nil {
			return nil
		}
		entry := badger.NewEntry(key, val).WithTTL(fm.ttl)
		if err := txn.SetEntry(entry); err != nil {
			return err
		}

		// Author chrono index.
		authorKey := fmt.Sprintf("%s%s:%020d:%s", feedAuthorIdxPrefix, post.Author, createdAt, post.PostID)
		if err := txn.SetEntry(
			badger.NewEntry([]byte(authorKey), []byte(post.PostID)).WithTTL(fm.ttl),
		); err != nil {
			return err
		}

		// Hashtag inverted index.
		for _, tag := range tags {
			tagKey := fmt.Sprintf("%s%s:%020d:%s", feedHashtagPrefix, tag, createdAt, post.PostID)
			if err := txn.SetEntry(
				badger.NewEntry([]byte(tagKey), []byte(post.PostID)).WithTTL(fm.ttl),
			); err != nil {
				return err
			}
		}
		return nil
	})
	if err != nil {
		return nil, err
	}
	return tags, nil
}

// Get returns the full post body, or nil if not found / evicted.
func (fm *FeedMailbox) Get(postID string) (*FeedPost, error) {
	var p FeedPost
	err := fm.db.View(func(txn *badger.Txn) error {
		item, err := txn.Get([]byte(feedPostPrefix + postID))
		if err != nil {
			return err
		}
		return item.Value(func(val []byte) error {
			return json.Unmarshal(val, &p)
		})
	})
	if errors.Is(err, badger.ErrKeyNotFound) {
		return nil, nil
	}
	if err != nil {
		return nil, err
	}
	return &p, nil
}

// Delete removes a post body and its indices. On-chain soft-delete stays;
// this just frees storage. Called by DELETE_POST event handler hook.
func (fm *FeedMailbox) Delete(postID string) error {
	// We need author and createdAt to build index keys — fetch first.
	post, err := fm.Get(postID)
	if err != nil {
		return err
	}
	if post == nil {
		return nil
	}
	return fm.db.Update(func(txn *badger.Txn) error {
		if err := txn.Delete([]byte(feedPostPrefix + postID)); err != nil {
			return err
		}
		authorKey := fmt.Sprintf("%s%s:%020d:%s",
			feedAuthorIdxPrefix, post.Author, post.CreatedAt, postID)
		if err := txn.Delete([]byte(authorKey)); err != nil {
			return err
		}
		for _, tag := range post.Hashtags {
			tagKey := fmt.Sprintf("%s%s:%020d:%s",
				feedHashtagPrefix, tag, post.CreatedAt, postID)
			if err := txn.Delete([]byte(tagKey)); err != nil {
				return err
			}
		}
		return nil
	})
}

// IncrementView bumps the view counter for a post. No-op on missing post.
// Returns the new count. Views are ephemeral (tied to the post TTL) —
// a fresh relay that gossip-loads an old post starts from 0, which is
// acceptable for a non-authoritative metric.
func (fm *FeedMailbox) IncrementView(postID string) (uint64, error) {
	var next uint64
	err := fm.db.Update(func(txn *badger.Txn) error {
		key := []byte(feedViewPrefix + postID)
		var cur uint64
		if item, err := txn.Get(key); err == nil {
			_ = item.Value(func(val []byte) error {
				if len(val) == 8 {
					cur = binary.BigEndian.Uint64(val)
				}
				return nil
			})
		} else if !errors.Is(err, badger.ErrKeyNotFound) {
			return err
		}
		next = cur + 1
		var buf [8]byte
		binary.BigEndian.PutUint64(buf[:], next)
		return txn.SetEntry(badger.NewEntry(key, buf[:]).WithTTL(fm.ttl))
	})
	return next, err
}

// ViewCount returns the current (off-chain) view count for a post.
func (fm *FeedMailbox) ViewCount(postID string) (uint64, error) {
	var n uint64
	err := fm.db.View(func(txn *badger.Txn) error {
		item, err := txn.Get([]byte(feedViewPrefix + postID))
		if errors.Is(err, badger.ErrKeyNotFound) {
			return nil
		}
		if err != nil {
			return err
		}
		return item.Value(func(val []byte) error {
			if len(val) == 8 {
				n = binary.BigEndian.Uint64(val)
			}
			return nil
		})
	})
	return n, err
}

// PostsByAuthor lists the N most recent post IDs by an author, newest first.
// Pure ID listing — callers fetch bodies via Get.
func (fm *FeedMailbox) PostsByAuthor(authorPub string, limit int) ([]string, error) {
	if limit <= 0 || limit > 200 {
		limit = 50
	}
	prefix := []byte(feedAuthorIdxPrefix + authorPub + ":")
	return fm.reverseIDScan(prefix, limit)
}

// PostsByHashtag lists the N most recent posts tagged with tag (lowercased).
func (fm *FeedMailbox) PostsByHashtag(tag string, limit int) ([]string, error) {
	tag = strings.ToLower(strings.TrimPrefix(tag, "#"))
	if tag == "" {
		return nil, nil
	}
	if limit <= 0 || limit > 200 {
		limit = 50
	}
	prefix := []byte(feedHashtagPrefix + tag + ":")
	return fm.reverseIDScan(prefix, limit)
}

// reverseIDScan walks prefix in reverse lex order and returns the value
// (postID) of each entry up to limit. Used for newest-first indices.
func (fm *FeedMailbox) reverseIDScan(prefix []byte, limit int) ([]string, error) {
	out := make([]string, 0, limit)
	err := fm.db.View(func(txn *badger.Txn) error {
		opts := badger.DefaultIteratorOptions
		opts.Prefix = prefix
		opts.Reverse = true
		seek := append([]byte{}, prefix...)
		seek = append(seek, 0xff)
		it := txn.NewIterator(opts)
		defer it.Close()
		for it.Seek(seek); it.ValidForPrefix(prefix) && len(out) < limit; it.Next() {
			item := it.Item()
			_ = item.Value(func(val []byte) error {
				out = append(out, string(val))
				return nil
			})
		}
		return nil
	})
	return out, err
}

// RecentPostIDs enumerates the most recent posts stored by this relay
// across ALL authors. Used by the trending / recommendations endpoints to
// seed the candidate pool. maxAgeSeconds bounds the walk (0 = no bound).
func (fm *FeedMailbox) RecentPostIDs(maxAgeSeconds int64, limit int) ([]string, error) {
	if limit <= 0 || limit > 500 {
		limit = 100
	}
	// Can't reuse chrono indices because they're per-author. We scan post:*
	// and collect, sorted by CreatedAt from the decoded body. This is O(M)
	// where M = #posts in DB — fine for MVP since TTL-bounded M is small
	// (~5k posts × 30d TTL on a busy node).
	type candidate struct {
		id string
		ts int64
	}
	cutoff := int64(0)
	if maxAgeSeconds > 0 {
		cutoff = time.Now().Unix() - maxAgeSeconds
	}
	var candidates []candidate
	prefix := []byte(feedPostPrefix)
	err := fm.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(); it.Next() {
			item := it.Item()
			err := item.Value(func(val []byte) error {
				var p FeedPost
				if err := json.Unmarshal(val, &p); err != nil {
					return nil // skip corrupt
				}
				if p.CreatedAt < cutoff {
					return nil
				}
				candidates = append(candidates, candidate{id: p.PostID, ts: p.CreatedAt})
				return nil
			})
			if err != nil {
				return err
			}
		}
		return nil
	})
	if err != nil {
		return nil, err
	}
	sort.Slice(candidates, func(i, j int) bool { return candidates[i].ts > candidates[j].ts })
	if len(candidates) > limit {
		candidates = candidates[:limit]
	}
	out := make([]string, len(candidates))
	for i, c := range candidates {
		out[i] = c.id
	}
	return out, nil
}

// extractHashtags finds #word tokens in text, lowercases, dedups, preserves
// first-seen order. Word = [A-Za-z0-9_] chars, length 1..40.
func extractHashtags(text string) []string {
	re := hashtagRegex
	matches := re.FindAllString(text, -1)
	seen := make(map[string]struct{}, len(matches))
	out := make([]string, 0, len(matches))
	for _, m := range matches {
		tag := strings.ToLower(strings.TrimPrefix(m, "#"))
		if len(tag) == 0 || len(tag) > 40 {
			continue
		}
		if _, ok := seen[tag]; ok {
			continue
		}
		seen[tag] = struct{}{}
		out = append(out, tag)
	}
	return out
}

var hashtagRegex = regexp.MustCompile(`#[A-Za-z0-9_\p{L}]{1,40}`)

// estimatePostSize returns the on-disk size used for fee calculation.
// Matches the client's pre-publish size estimate so fees are predictable.
func estimatePostSize(post *FeedPost) uint64 {
	n := uint64(len(post.Content)) + uint64(len(post.Attachment))
	// Small overhead for metadata (~120 bytes of JSON scaffolding).
	n += 128
	return n
}