Implement topic recency ordering and pagination

- Indexer maintains topicSummaries and topicRecency for topic messages and follows, idempotently, moving each room's recency record to its newest post. - DB adds the two index stores and serves GET /topics from them with limit/offset pagination, without iterating the rooms store. Adds util/room/backfill-topic-indexes.js to build the indexes idempotently. - Client topics page loads 50 per page with Previous/Next. By coder.
2026-09-21 16:52:01 -07:00 · 2026-09-17 07:24:45 -07:00
parent e2c0bbe523
commit e02bea5460
28 changed files with 1397 additions and 134 deletions
@@ -361,13 +361,18 @@ function makeMemoDb () {
    async getMuteState (muterAddr, muteeAddr) {
      return muteState[`${muterAddr}:${muteeAddr}`] || false
    },
-    async getTopics () {
+    async getTopics ({ limit = 50, offset = 0 } = {}) {
      const list = []
      for (const [room, postCount] of topicCounts.entries()) {
        list.push({ room, postCount })
      }
      list.sort((a, b) => a.room.localeCompare(b.room))
-      return { topics: list }
+      const total = list.length
+      const page = list.slice(offset, offset + limit)
+      return {
+        topics: page,
+        pagination: { limit, offset, total, hasMore: offset + page.length < total }
+      }
    },
    async getTopicPosts (room, { limit = 50, offset = 0, viewer = null } = {}) {
      let all = topicPosts[room] || []
@@ -2141,6 +2146,54 @@ const handlers = [
      world.currentPath = TopicDiscoveryPage.TOPICS_PATH
    }
  },
+  {
+    name: 'open topics page at offset',
+    pattern: /^I open the topics page at offset (<[A-Za-z0-9_]+>)$/,
+    async run (m, example, world) {
+      const offset = parseInt(resolveParam(m[1], example), 10)
+      await world.topicDiscoveryPage.load({ limit: 50, offset })
+      world.currentPath = TopicDiscoveryPage.TOPICS_PATH
+    }
+  },
+  {
+    name: 'API serves N topics',
+    pattern: /^the psf-memo-db API serves (<[A-Za-z0-9_]+>) topics$/,
+    run (m, example, world) {
+      const count = parseInt(resolveParam(m[1], example), 10)
+      for (let i = 0; i < count; i++) {
+        world.memoDb.addTopic(`topic-${String(i + 1).padStart(3, '0')}`, 1)
+      }
+    }
+  },
+  {
+    name: 'topics page shows N topics',
+    pattern: /^the topics page shows (<[A-Za-z0-9_]+>) topics$/,
+    run (m, example, world) {
+      const expected = parseInt(resolveParam(m[1], example), 10)
+      const actual = world.topicDiscoveryPage.topics.length
+      if (actual !== expected) {
+        throw new Error(`Expected ${expected} topics on the topics page, got ${actual}.`)
+      }
+    }
+  },
+  {
+    name: 'topics page can load more topics',
+    pattern: /^the topics page can load more topics$/,
+    run (m, example, world) {
+      if (!world.topicDiscoveryPage.canLoadMore()) {
+        throw new Error('Expected the topics page to have more topics, but pagination says there are none.')
+      }
+    }
+  },
+  {
+    name: 'topics page has no more topics',
+    pattern: /^the topics page has no more topics$/,
+    run (m, example, world) {
+      if (world.topicDiscoveryPage.canLoadMore()) {
+        throw new Error('Expected the topics page to have no more topics, but pagination says there are more.')
+      }
+    }
+  },
  {
    name: 'topics page shows topic count',
    pattern: /^the topics page shows the topic (<topic>) with (<count>) posts$/,
@@ -1,10 +1,10 @@
 /*
-  Display the list of Memo topics served by psf-memo-db.
+  Display a page of Memo topics served by psf-memo-db.
 */

 // Global npm libraries
 import React, { useState, useEffect } from 'react'
-import { Container, Row, Col, Spinner, ListGroup } from 'react-bootstrap'
+import { Container, Row, Col, Spinner, ListGroup, Button } from 'react-bootstrap'
 import { useNavigate } from 'react-router-dom'

 // Local libraries
@@ -12,11 +12,15 @@ import MemoDb from '../../../services/memo-db'
 import TopicDiscoveryPage from '../../../services/topic-discovery-page'
 import '../../../App.css'

+const PAGE_SIZE = 50
+
 function Topics (props) {
  const navigate = useNavigate()
  const [loading, setLoading] = useState(true)
  const [error, setError] = useState(null)
  const [topics, setTopics] = useState([])
+  const [pagination, setPagination] = useState(null)
+  const [offset, setOffset] = useState(0)

  useEffect(() => {
    const loadTopics = async () => {
@@ -26,23 +30,36 @@ function Topics (props) {
      try {
        const memoDb = new MemoDb()
        const page = new TopicDiscoveryPage({ memoDb, navigate })
-        const result = await page.load()
+        const result = await page.load({ limit: PAGE_SIZE, offset })
        setTopics(result.topics || [])
+        setPagination(result.pagination || null)
      } catch (err) {
        setError(err.message || 'Failed to load topics')
        setTopics([])
+        setPagination(null)
      }

      setLoading(false)
    }

    loadTopics()
-  }, [navigate])
+  }, [navigate, offset])

  const handleClick = (room) => {
    navigate(TopicDiscoveryPage.topicFeedPath(room))
  }

+  const canGoBack = offset > 0
+  const canGoNext = pagination?.hasMore ?? false
+
+  const handlePrevious = () => {
+    setOffset((prev) => Math.max(0, prev - PAGE_SIZE))
+  }
+
+  const handleNext = () => {
+    setOffset((prev) => prev + PAGE_SIZE)
+  }
+
  return (
    <Container className='topics-page'>
      <Row className='justify-content-center'>
@@ -50,6 +67,13 @@ function Topics (props) {
          <header className='topics-heading'>
            <h1>Topics</h1>
            <p>Discover Memo conversations organized by topic.</p>
+
+            {pagination && topics.length > 0 && (
+              <span className='topics-count'>
+                Showing {pagination.offset + 1}–
+                {pagination.offset + topics.length} of {pagination.total}
+              </span>
+            )}
          </header>

          {error && (
@@ -84,6 +108,26 @@ function Topics (props) {
              ))}
            </ListGroup>
          )}
+
+          {!loading && !error && (pagination || offset > 0) && (
+            <div className='topics-pagination mt-3'>
+              <Button
+                variant='outline-dark'
+                onClick={handlePrevious}
+                disabled={!canGoBack}
+              >
+                Previous
+              </Button>
+
+              <Button
+                variant='outline-dark'
+                onClick={handleNext}
+                disabled={!canGoNext}
+              >
+                Next
+              </Button>
+            </div>
+          )}
        </Col>
      </Row>
    </Container>
@@ -44,8 +44,8 @@ class MemoDb {
    return this._getList(`/mute/muted/${encodeURIComponent(muterAddr)}`, 'getMuted', 'muted')
  }

-  async getTopics () {
-    return this.getRecent('/topics', 'getTopics', {})
+  async getTopics ({ limit = 50, offset = 0 } = {}) {
+    return this.getRecent('/topics', 'getTopics', { limit, offset })
  }

  async getTopicPosts (room, { limit = 50, offset = 0, viewer = null } = {}) {
@@ -1,29 +1,23 @@
 /*
-  Topic Discovery Page behavior: load and display the list of Memo topics.
+  Topic Discovery Page behavior: load and display a page of Memo topics.

  This is the testable controller behind the React "Topics" page. It wraps
-  the MemoDb client and exposes the loaded topics so the view can render each
-  topic name and post count.
+  the MemoDb client and exposes the loaded topics and pagination so the view
+  can render each topic name and post count and move between pages.
 */

+const PaginatedPage = require('./paginated-page')
+
 const TOPICS_PATH = '/topics'

-class TopicDiscoveryPage {
+class TopicDiscoveryPage extends PaginatedPage {
  constructor (deps = {}) {
-    this.memoDb = deps.memoDb || null
+    super(deps, {
+      listField: 'topics',
+      loadMethod: 'getTopics',
+      errorMessage: 'Topic discovery page requires a memo db client.'
+    })
    this.navigate = deps.navigate || (() => {})
-    this.topics = []
-  }
-
-  async load () {
-    if (!this.memoDb) {
-      throw new Error('Topic discovery page requires a memo db client.')
-    }
-
-    const data = await this.memoDb.getTopics()
-    this.topics = data.topics || []
-
-    return { topics: this.topics }
  }

  getTopic (room) {
@@ -8,10 +8,13 @@ const test = require('node:test')
 const assert = require('node:assert/strict')
 const TopicDiscoveryPage = require('../../src/services/topic-discovery-page')

-function makeMemoDb (topics) {
+function makeMemoDb ({ topics = [], pagination = null } = {}) {
+  const calls = []
  return {
-    async getTopics () {
-      return { topics }
+    calls,
+    async getTopics (opts) {
+      calls.push(opts)
+      return { topics, pagination }
    }
  }
 }
@@ -21,13 +24,56 @@ test('load returns topics with post counts', async () => {
    { room: 'bitcoin', postCount: 2 },
    { room: 'cash', postCount: 1 }
  ]
-  const page = new TopicDiscoveryPage({ memoDb: makeMemoDb(topics) })
+  const page = new TopicDiscoveryPage({ memoDb: makeMemoDb({ topics }) })

  const result = await page.load()

  assert.deepEqual(result.topics, topics)
 })

+test('load defaults to a page of 50 from offset 0', async () => {
+  const memoDb = makeMemoDb({ topics: [] })
+  const page = new TopicDiscoveryPage({ memoDb })
+
+  await page.load()
+
+  assert.deepEqual(memoDb.calls[0], { limit: 50, offset: 0 })
+})
+
+test('load forwards limit and offset and stores pagination', async () => {
+  const memoDb = makeMemoDb({
+    topics: [{ room: 'bitcoin', postCount: 2 }],
+    pagination: { limit: 50, offset: 50, total: 60, hasMore: false }
+  })
+  const page = new TopicDiscoveryPage({ memoDb })
+
+  const result = await page.load({ limit: 50, offset: 50 })
+
+  assert.deepEqual(memoDb.calls[0], { limit: 50, offset: 50 })
+  assert.deepEqual(result.pagination, { limit: 50, offset: 50, total: 60, hasMore: false })
+  assert.equal(page.pagination.offset, 50)
+})
+
+test('canLoadMore reflects the pagination hasMore flag', async () => {
+  const more = new TopicDiscoveryPage({
+    memoDb: makeMemoDb({
+      topics: [{ room: 'a', postCount: 0 }],
+      pagination: { limit: 50, offset: 0, total: 60, hasMore: true }
+    })
+  })
+  await more.load()
+  assert.equal(more.canLoadMore(), true)
+
+  const last = new TopicDiscoveryPage({
+    memoDb: makeMemoDb({
+      topics: [{ room: 'a', postCount: 0 }],
+      pagination: { limit: 50, offset: 50, total: 60, hasMore: false }
+    })
+  })
+  await last.load()
+  assert.equal(last.canLoadMore(), false)
+})
+
 test('load throws when no memo db client is provided', async () => {
  const page = new TopicDiscoveryPage({})

@@ -39,7 +85,7 @@ test('load throws when no memo db client is provided', async () => {

 test('stores the provided navigate function', () => {
  const navigate = () => {}
-  const page = new TopicDiscoveryPage({ memoDb: makeMemoDb([]), navigate })
+  const page = new TopicDiscoveryPage({ memoDb: makeMemoDb({ topics: [] }), navigate })

  assert.equal(page.navigate, navigate)
 })
@@ -49,7 +95,7 @@ test('getTopic returns the matching topic', async () => {
    { room: 'bitcoin', postCount: 2 },
    { room: 'cash', postCount: 1 }
  ]
-  const page = new TopicDiscoveryPage({ memoDb: makeMemoDb(topics) })
+  const page = new TopicDiscoveryPage({ memoDb: makeMemoDb({ topics }) })

  await page.load()

@@ -57,7 +103,7 @@ test('getTopic returns the matching topic', async () => {
 })

 test('getTopic returns null when the topic is not loaded', async () => {
-  const page = new TopicDiscoveryPage({ memoDb: makeMemoDb([]) })
+  const page = new TopicDiscoveryPage({ memoDb: makeMemoDb({ topics: [] }) })

  await page.load()

@@ -28,6 +28,7 @@ import GetPoll from '../../src/use-cases/get-poll.js'
 import GetPollOptions from '../../src/use-cases/get-poll-options.js'
 import GetPollVotes from '../../src/use-cases/get-poll-votes.js'
 import { repairTxidEncoding } from '../../src/lib/repair-txid-encoding.js'
+import { backfillTopicIndexes, topicRecencyKey } from '../../src/lib/backfill-topic-indexes.js'

 const __dirname = path.dirname(fileURLToPath(import.meta.url))
 const tmpDir = path.resolve(__dirname, '..', '..', 'tmp', 'acceptance')
@@ -97,12 +98,16 @@ async function createWorld () {
  const postsGetCounter = { calls: 0 }
  const likesIteratorCounter = { calls: 0 }
  const postLikesIteratorCounter = { calls: 0 }
+  const roomsIteratorCounter = { calls: 0, entries: 0 }
+  const topicRecencyIteratorCounter = { calls: 0, entries: 0 }
  wrapIterator(adapters.level.postHeightsDb, postHeightsIteratorCounter)
  wrapIterator(adapters.level.addrPostHeightsDb, addrPostHeightsIteratorCounter)
  wrapIterator(adapters.level.postChildrenDb, postChildrenIteratorCounter)
  wrapGet(adapters.level.postsDb, postsGetCounter)
  wrapIterator(adapters.level.likesDb, likesIteratorCounter)
  wrapIterator(adapters.level.postLikesDb, postLikesIteratorCounter)
+  wrapIterator(adapters.level.roomsDb, roomsIteratorCounter)
+  wrapIterator(adapters.level.topicRecencyDb, topicRecencyIteratorCounter)

  const listRecentPosts = new ListRecentPosts({ adapters })
  const listPostsByAddr = new ListPostsByAddr({ adapters })
@@ -145,6 +150,8 @@ async function createWorld () {
    postsGetCounter,
    likesIteratorCounter,
    postLikesIteratorCounter,
+    roomsIteratorCounter,
+    topicRecencyIteratorCounter,
    getLastResponse: () => lastResponse,
    setLastResponse: (resp) => { lastResponse = resp },
    close: async () => {
@@ -195,6 +202,16 @@ async function loadFixture (world, name) {
    return
  }

+  if (name === 'topic-indexes') {
+    await loadTopicIndexes(world)
+    return
+  }
+
+  if (name === 'rooms-with-topics-and-follows') {
+    await loadRoomsWithTopicsAndFollows(world)
+    return
+  }
+
  if (name === 'topic-follows') {
    await loadTopicFollows(world)
    return
@@ -502,6 +519,56 @@ async function loadTopicsWithPosts (world) {
  for (const [txid, post] of Object.entries(posts)) {
    await world.adapters.level.postsDb.put(txid, post)
  }
+
+  // Derived topic indexes (see topic-read.feature).
+  const summaries = [
+    { room: 'bitcoin', postCount: 2, lastHeight: 300 },
+    { room: 'cash', postCount: 1, lastHeight: 250 },
+    { room: 'dev', postCount: 1, lastHeight: 400 },
+    { room: 'lone', postCount: 0, lastHeight: 0 }
+  ]
+  for (const summary of summaries) {
+    await world.adapters.level.topicSummariesDb.put(summary.room, summary)
+    await world.adapters.level.topicRecencyDb.put(
+      topicRecencyKey(summary.lastHeight, summary.room),
+      { room: summary.room, blockHeight: summary.lastHeight }
+    )
+  }
+}
+
+// Fixture "topic-indexes" from topic-pagination.feature: indexes only.
+async function loadTopicIndexes (world) {
+  const summaries = [
+    { room: 'memo', postCount: 5, lastHeight: 600500 },
+    { room: 'cash', postCount: 2, lastHeight: 600400 },
+    { room: 'dance', postCount: 3, lastHeight: 600400 },
+    { room: 'anime', postCount: 1, lastHeight: 600300 },
+    { room: 'lone', postCount: 0, lastHeight: 0 },
+    { room: 'quiet', postCount: 0, lastHeight: 0 }
+  ]
+
+  for (const summary of summaries) {
+    await world.adapters.level.topicSummariesDb.put(summary.room, summary)
+    await world.adapters.level.topicRecencyDb.put(
+      topicRecencyKey(summary.lastHeight, summary.room),
+      { room: summary.room, blockHeight: summary.lastHeight }
+    )
+  }
+}
+
+// Fixture "rooms-with-topics-and-follows" from backfill-topic-indexes.feature.
+async function loadRoomsWithTopicsAndFollows (world) {
+  const entries = [
+    { key: 'bitcoin:post-100', room: 'bitcoin', txid: 'post-100', type: 'post', blockHeight: 600100 },
+    { key: 'bitcoin:post-200', room: 'bitcoin', txid: 'post-200', type: 'post', blockHeight: 600200 },
+    { key: 'bitcoin:addr-f', room: 'bitcoin', addr: 'addr-f', type: 'follow', unfollow: false },
+    { key: 'cash:post-250', room: 'cash', txid: 'post-250', type: 'post', blockHeight: 600250 },
+    { key: 'lone:addr-f', room: 'lone', addr: 'addr-f', type: 'follow', unfollow: false }
+  ]
+
+  for (const entry of entries) {
+    await world.adapters.level.roomsDb.put(entry.key, entry)
+  }
 }

 async function loadTopicFollows (world) {
@@ -1119,7 +1186,7 @@ const handlers = [
  },
  {
    name: 'response lists topics in order',
-    pattern: /^the response lists topics in order (<expected_order>)$/,
+    pattern: /^the response lists topics in order (<[A-Za-z0-9_]+>)$/,
    run (m, example, world) {
      const expected = resolveParam(m[1], example).split(',').map((s) => s.trim())
      const actual = world.getLastResponse().topics.map((t) => t.room)
@@ -1163,6 +1230,157 @@ const handlers = [
      await loadFixture(world, m[1])
    }
  },
+  {
+    name: 'db instance with rooms, posts, and topic index stores',
+    pattern: /^a psf-memo-db instance with rooms, posts, topicSummaries, and topicRecency stores$/,
+    async run () {
+      // World is already created with all stores.
+    }
+  },
+  {
+    name: 'load fixture into rooms, posts, and topic index stores',
+    pattern: /^the fixture "(.+)" is loaded into the rooms, posts, topicSummaries, and topicRecency stores$/,
+    async run (m, example, world) {
+      await loadFixture(world, m[1])
+    }
+  },
+  {
+    name: 'db instance with topic index stores',
+    pattern: /^a psf-memo-db instance with topicSummaries and topicRecency stores$/,
+    async run () {
+      // World is already created with both topic index stores.
+    }
+  },
+  {
+    name: 'load fixture into topic index stores',
+    pattern: /^the fixture "(.+)" is loaded into the topic index stores$/,
+    async run (m, example, world) {
+      await loadFixture(world, m[1])
+    }
+  },
+  {
+    name: 'db instance with rooms and topic index stores',
+    pattern: /^a psf-memo-db instance with rooms, topicSummaries, and topicRecency stores$/,
+    async run () {
+      // World is already created with all stores.
+    }
+  },
+  {
+    name: 'request topics with limit and offset',
+    pattern: /^the client requests \/topics with limit (<limit>) and offset (<offset>)$/,
+    async run (m, example, world) {
+      const limit = parseInt(resolveParam(m[1], example), 10)
+      const offset = parseInt(resolveParam(m[2], example), 10)
+      const resp = await world.listTopics.execute({ limit, offset })
+      world.setLastResponse(resp)
+    }
+  },
+  {
+    name: 'response contains quoted topic with post count',
+    pattern: /^the response contains the topic "(<topic>)" with post count (<postCount>)$/,
+    run (m, example, world) {
+      const topic = resolveParam(m[1], example)
+      const expectedCount = parseInt(resolveParam(m[2], example), 10)
+      const found = world.getLastResponse().topics.find((t) => t.room === topic)
+      if (!found) {
+        throw new Error(`Topic ${topic} not found in response`)
+      }
+      if (found.postCount !== expectedCount) {
+        throw new Error(`Expected post count ${expectedCount} for ${topic}, got ${found.postCount}`)
+      }
+    }
+  },
+  {
+    name: 'response pagination shows total and hasMore',
+    pattern: /^the response pagination shows total (<total>) and hasMore (<hasMore>)$/,
+    run (m, example, world) {
+      const expectedTotal = parseInt(resolveParam(m[1], example), 10)
+      const expectedHasMore = resolveParam(m[2], example) === 'true'
+      const pagination = world.getLastResponse().pagination
+      if (!pagination) {
+        throw new Error('Response has no pagination metadata')
+      }
+      if (pagination.total !== expectedTotal) {
+        throw new Error(`Expected total ${expectedTotal}, got ${pagination.total}`)
+      }
+      if (pagination.hasMore !== expectedHasMore) {
+        throw new Error(`Expected hasMore ${expectedHasMore}, got ${pagination.hasMore}`)
+      }
+    }
+  },
+  {
+    name: 'topicRecency store read count',
+    pattern: /^the topicRecency store was read exactly (<read_count>) records$/,
+    run (m, example, world) {
+      const expected = parseInt(resolveParam(m[1], example), 10)
+      const actual = world.topicRecencyIteratorCounter.entries || 0
+      if (actual !== expected) {
+        throw new Error(`Expected topicRecency to be read ${expected} times, got ${actual}`)
+      }
+    }
+  },
+  {
+    name: 'rooms store was not iterated',
+    pattern: /^the rooms store was not iterated$/,
+    run (m, example, world) {
+      if (world.roomsIteratorCounter.calls !== 0) {
+        throw new Error(`Expected rooms store not to be iterated, got ${world.roomsIteratorCounter.calls} call(s)`)
+      }
+    }
+  },
+  {
+    name: 'run topic backfill utility',
+    pattern: /^the topic backfill utility is run$/,
+    async run (m, example, world) {
+      await backfillTopicIndexes({
+        roomsDb: world.adapters.level.roomsDb,
+        topicSummariesDb: world.adapters.level.topicSummariesDb,
+        topicRecencyDb: world.adapters.level.topicRecencyDb
+      })
+    }
+  },
+  {
+    name: 'run topic backfill utility again',
+    pattern: /^the topic backfill utility is run again$/,
+    async run (m, example, world) {
+      await backfillTopicIndexes({
+        roomsDb: world.adapters.level.roomsDb,
+        topicSummariesDb: world.adapters.level.topicSummariesDb,
+        topicRecencyDb: world.adapters.level.topicRecencyDb
+      })
+    }
+  },
+  {
+    name: 'topicSummaries contains room with postCount and lastHeight',
+    pattern: /^the topicSummaries store contains the room "(<room>)" with postCount (<postCount>) and lastHeight (<[A-Za-z0-9_]+>)$/,
+    async run (m, example, world) {
+      const room = resolveParam(m[1], example)
+      const postCount = parseInt(resolveParam(m[2], example), 10)
+      const lastHeight = parseInt(resolveParam(m[3], example), 10)
+      const record = await world.adapters.level.topicSummariesDb.get(room)
+      if (!record) {
+        throw new Error(`No topicSummaries record for ${room}`)
+      }
+      if (record.room !== room || record.postCount !== postCount || record.lastHeight !== lastHeight) {
+        throw new Error(`Expected ${room} postCount ${postCount} lastHeight ${lastHeight}, got ${JSON.stringify(record)}`)
+      }
+    }
+  },
+  {
+    name: 'topicRecency records room at height',
+    pattern: /^the topicRecency store records the room "(<room>)" at block height (<height>)$/,
+    async run (m, example, world) {
+      const room = resolveParam(m[1], example)
+      const height = parseInt(resolveParam(m[2], example), 10)
+      const record = await world.adapters.level.topicRecencyDb.get(topicRecencyKey(height, room))
+      if (!record) {
+        throw new Error(`No topicRecency record for ${room} at ${height}`)
+      }
+      if (record.room !== room || record.blockHeight !== height) {
+        throw new Error(`Expected topicRecency ${room} at ${height}, got ${JSON.stringify(record)}`)
+      }
+    }
+  },
  {
    name: 'request topic follow state',
    pattern: /^the client requests the topic follow state for room (<[A-Za-z0-9_]+>) and address (<[A-Za-z0-9_]+>)$/,
@@ -50,6 +50,8 @@ class Adapters {
    this.topicQuery = new TopicQuery({
      roomsDb: level.roomsDb,
      postsDb: level.postsDb,
+      topicSummariesDb: level.topicSummariesDb,
+      topicRecencyDb: level.topicRecencyDb,
      muteQuery: this.muteQuery
    })
    this.pollQuery = new PollQuery({
@@ -24,6 +24,8 @@ const DB_NAMES = [
  'follows',
  'mutes',
  'rooms',
+  'topicSummaries',
+  'topicRecency',
  'processErrors',
  'ptxs',
  'polls',
@@ -5,8 +5,16 @@
    - Topic posts are keyed `${room}:${txid}` with type 'post'.
    - Topic follows are keyed `${room}:${addr}` with type 'follow'.

+  Topic listing is served from two indexes so it can order and paginate
+  without iterating the rooms store:
+    - topicSummaries: one record per room keyed by room, value
+      { room, postCount, lastHeight }.
+    - topicRecency: one record per room keyed `${invertedHeight}:${room}`, value
+      { room, blockHeight }. Follow-only rooms live at height 0. The height is
+      inverted so an ascending scan yields newest-first.
+
  This adapter exposes:
-    - listTopics()           - distinct rooms with post counts
+    - listTopics()           - ordered, paginated distinct rooms with counts
    - getTopicPostTxids()    - paginated txids for a room sorted by block height
 */

@@ -14,15 +22,23 @@ import { loadMutedAddrs, isMutedPost } from './lib/muted-posts.js'

 class TopicQuery {
  constructor (localConfig = {}) {
-    const { roomsDb, postsDb, muteQuery } = localConfig
+    const { roomsDb, postsDb, topicSummariesDb, topicRecencyDb, muteQuery } = localConfig
    if (!roomsDb) {
      throw new Error('roomsDb required when instantiating TopicQuery adapter.')
    }
    if (!postsDb) {
      throw new Error('postsDb required when instantiating TopicQuery adapter.')
    }
+    if (!topicSummariesDb) {
+      throw new Error('topicSummariesDb required when instantiating TopicQuery adapter.')
+    }
+    if (!topicRecencyDb) {
+      throw new Error('topicRecencyDb required when instantiating TopicQuery adapter.')
+    }
    this.roomsDb = roomsDb
    this.postsDb = postsDb
+    this.topicSummariesDb = topicSummariesDb
+    this.topicRecencyDb = topicRecencyDb
    this.muteQuery = muteQuery || null

    this.listTopics = this.listTopics.bind(this)
@@ -44,35 +60,52 @@ class TopicQuery {
    return parts[parts.length - 1]
  }

-  async listTopics () {
-    const topics = new Map()
+  // The topicSummaries key is the room name; fall back to the key when the
+  // stored value omits it.
+  summaryRoom (key, value) {
+    if (value && typeof value.room === 'string') return value.room
+    return String(key)
+  }

-    for await (const [key, value] of this.roomsDb.iterator()) {
-      const room = this.roomFromKey(key, value)
-      if (!topics.has(room)) {
-        topics.set(room, { postCount: 0, lastHeight: 0 })
-      }
-      const topic = topics.get(room)
-      if (value?.type === 'post') {
-        topic.postCount++
-        const height = value?.blockHeight ?? 0
-        if (height > topic.lastHeight) {
-          topic.lastHeight = height
-        }
-      }
+  // The topicRecency key is `${invertedHeight}:${room}`; the stored value
+  // carries the room name, but fall back to the key for robustness.
+  recencyRoom (key, value) {
+    if (value && typeof value.room === 'string') return value.room
+    const parts = String(key).split(':')
+    return parts.slice(1).join(':')
+  }
+
+  // Return a page of topics ordered by their most recent post, descending,
+  // with rooms at the same height ordered by name ascending. Counts and the
+  // total come from topicSummaries; ordering and pagination come from
+  // topicRecency, which is read only through offset + limit records.
+  async listTopics ({ limit = 100, offset = 0 } = {}) {
+    const postCounts = new Map()
+    for await (const [key, value] of this.topicSummariesDb.iterator()) {
+      postCounts.set(this.summaryRoom(key, value), value?.postCount ?? 0)
+    }
+    const total = postCounts.size
+
+    const recencyRooms = []
+    for await (const [key, value] of this.topicRecencyDb.iterator({ limit: offset + limit })) {
+      recencyRooms.push(this.recencyRoom(key, value))
    }

-    return Array.from(topics.entries())
-      .map(([room, { postCount, lastHeight }]) => ({ room, postCount, lastHeight }))
-      .sort((a, b) => {
-        if (b.lastHeight !== a.lastHeight) {
-          return b.lastHeight - a.lastHeight
-        }
-        return a.room.localeCompare(b.room)
-      })
-      // lastHeight is an internal ordering key; keep it out of the public
-      // API contract so the response exposes only room and postCount.
-      .map(({ room, postCount }) => ({ room, postCount }))
+    const pageRooms = recencyRooms.slice(offset, offset + limit)
+    const topics = pageRooms.map((room) => ({
+      room,
+      postCount: postCounts.get(room) ?? 0
+    }))
+
+    return {
+      topics,
+      pagination: {
+        limit,
+        offset,
+        total,
+        hasMore: offset + topics.length < total
+      }
+    }
  }

  async getTopicPostTxids (room, { limit, offset, viewerAddr = null }) {
@@ -47,6 +47,8 @@ export const ENTITY_CONFIG = [
  { route: 'profilepic', dbProp: 'profilePicsDb', keyParam: 'addr', bodyIdField: 'addr', bodyDataField: 'profilePicData' },
  { route: 'follow', dbProp: 'followsDb', keyParam: 'key', bodyIdField: 'key', bodyDataField: 'followData' },
  { route: 'room', dbProp: 'roomsDb', keyParam: 'key', bodyIdField: 'key', bodyDataField: 'roomData' },
+  { route: 'topicsummary', dbProp: 'topicSummariesDb', keyParam: 'key', bodyIdField: 'key', bodyDataField: 'topicSummaryData' },
+  { route: 'topicrecency', dbProp: 'topicRecencyDb', keyParam: 'key', bodyIdField: 'key', bodyDataField: 'topicRecencyData' },
  { route: 'processerror', dbProp: 'processErrorsDb', keyParam: 'txid', bodyIdField: 'txid', bodyDataField: 'errorData' },
  { route: 'ptx', dbProp: 'ptxsDb', keyParam: 'txid', bodyIdField: 'txid', bodyDataField: 'ptxData' },
  { route: 'poll', dbProp: 'pollsDb', keyParam: 'txid', bodyIdField: 'txid', bodyDataField: 'pollData' },
@@ -32,7 +32,11 @@ class TopicsRESTControllerLib {
   * @apiName GetTopics
   * @apiGroup REST Topics
   *
-   * @apiDescription Returns all distinct Memo topics with their post counts.
+   * @apiDescription Returns a page of distinct Memo topics ordered by their
+   * most recent post, newest first. Supports limit and offset.
+   *
+   * @apiQuery {Number} [limit=100] Page size (max 100)
+   * @apiQuery {Number} [offset=0] Number of topics to skip after sorting
   *
   * @apiExample Example usage:
   * curl -X GET "localhost:5021/topics"
@@ -40,10 +44,12 @@ class TopicsRESTControllerLib {
   * @apiSuccess {Object[]} topics Array of topic objects
   * @apiSuccess {String} topics.room Topic name
   * @apiSuccess {Number} topics.postCount Number of posts in the topic
+   * @apiSuccess {Object} pagination Pagination metadata
   */
  async getTopics (ctx) {
    try {
-      ctx.body = await this.useCases.listTopics.execute()
+      const { limit, offset } = ctx.query
+      ctx.body = await this.useCases.listTopics.execute({ limit, offset })
    } catch (err) {
      this.handleError(ctx, err)
    }
@@ -0,0 +1,76 @@
+/*
+  Library to build the topicSummaries and topicRecency indexes from an
+  existing rooms store.
+
+  The read side serves GET /topics from these two indexes:
+    - topicSummaries: one record per room keyed by room, value
+      { room, postCount, lastHeight }.
+    - topicRecency: one record per room keyed `${invertedHeight}:${room}`,
+      value { room, blockHeight }. Follow-only rooms live at height 0. The
+      height is inverted so an ascending scan yields newest-first.
+
+  The backfill recomputes both indexes from the rooms store, so running it
+  more than once is idempotent. Stale index records for rooms that no longer
+  have any room entry, or that moved to a new height, are removed.
+
+  The LevelDB handles are injected so the logic stays testable and free of
+  file-system concerns; the CLI wrapper in util/room opens the real stores.
+*/
+
+const HEIGHT_PAD = 12
+// Heights are inverted in the recency key so a plain ascending iterator yields
+// rooms from most recent to least recent. Rooms at the same height still sort
+// by room name ascending.
+const MAX_HEIGHT = 999999999999
+
+export function topicRecencyKey (blockHeight, room) {
+  const inverted = MAX_HEIGHT - (blockHeight ?? 0)
+  return `${String(inverted).padStart(HEIGHT_PAD, '0')}:${room}`
+}
+
+async function collectSummaries (roomsDb) {
+  const summaries = new Map()
+
+  for await (const [key, value] of roomsDb.iterator()) {
+    const room = (value && typeof value.room === 'string') ? value.room : String(key).split(':')[0]
+    if (!summaries.has(room)) {
+      summaries.set(room, { room, postCount: 0, lastHeight: 0 })
+    }
+
+    if (value?.type === 'post') {
+      const summary = summaries.get(room)
+      summary.postCount++
+      const height = value.blockHeight ?? 0
+      if (height > summary.lastHeight) summary.lastHeight = height
+    }
+  }
+
+  return summaries
+}
+
+// Remove index records that no longer have a desired key.
+async function removeStale (db, desiredKeys) {
+  for await (const [key] of db.iterator()) {
+    if (!desiredKeys.has(key)) {
+      await db.del(key)
+    }
+  }
+}
+
+export async function backfillTopicIndexes ({ roomsDb, topicSummariesDb, topicRecencyDb }) {
+  const summaries = await collectSummaries(roomsDb)
+  const desiredRecencyKeys = new Set()
+
+  for (const summary of summaries.values()) {
+    await topicSummariesDb.put(summary.room, summary)
+
+    const key = topicRecencyKey(summary.lastHeight, summary.room)
+    desiredRecencyKeys.add(key)
+    await topicRecencyDb.put(key, { room: summary.room, blockHeight: summary.lastHeight })
+  }
+
+  await removeStale(topicRecencyDb, desiredRecencyKeys)
+  await removeStale(topicSummariesDb, new Set(summaries.keys()))
+
+  return { rooms: summaries.size }
+}
@@ -1,8 +1,9 @@
 /*
-  Use case: list all distinct Memo topics with their post counts.
+  Use case: list a page of distinct Memo topics ordered by most recent post.
 */

 import { ListUseCase } from './lib/use-case.js'
+import { parseLimit, parseOffset } from './lib/pagination.js'

 class ListTopics extends ListUseCase {
  constructor (localConfig = {}) {
@@ -10,8 +11,9 @@ class ListTopics extends ListUseCase {
  }

  async execute (inObj = {}) {
-    const topics = await this.adapters.topicQuery.listTopics()
-    return { topics }
+    const limit = parseLimit(inObj.limit)
+    const offset = parseOffset(inObj.offset)
+    return this.adapters.topicQuery.listTopics({ limit, offset })
  }
 }

@@ -47,10 +47,19 @@ function makeRoomsDb (entries) {
  }
 }

+// Empty index store; this suite exercises the follow read side, not listTopics.
+function makeEmptyIndexDb () {
+  return {
+    async * iterator () {}
+  }
+}
+
 function makeQuery (entries) {
  return new TopicQuery({
    roomsDb: makeRoomsDb(entries),
-    postsDb: {}
+    postsDb: {},
+    topicSummariesDb: makeEmptyIndexDb(),
+    topicRecencyDb: makeEmptyIndexDb()
  })
 }

@@ -19,6 +19,7 @@ import test from 'node:test'

 import { seededRandom, forAll, intGen, txidGen } from './harness.js'
 import TopicQuery from '../../src/adapters/topic-query.js'
+import { topicRecencyKey } from '../../src/lib/backfill-topic-indexes.js'

 const rng = seededRandom(20260828)

@@ -45,10 +46,55 @@ function makeRoomsDb (entries) {
 function makeQuery (entries) {
  return new TopicQuery({
    roomsDb: makeRoomsDb(entries),
-    postsDb: {}
+    postsDb: {},
+    topicSummariesDb: makeIndexDb([]),
+    topicRecencyDb: makeIndexDb([])
  })
 }

+// In-memory index store honoring the LevelDB `limit` option used by listTopics.
+function makeIndexDb (records) {
+  const store = new Map(records)
+  return {
+    async * iterator (opts = {}) {
+      const keys = Array.from(store.keys()).sort()
+      const limit = opts.limit === undefined ? keys.length : opts.limit
+      for (const key of keys.slice(0, limit)) {
+        yield [key, store.get(key)]
+      }
+    }
+  }
+}
+
+// Derive topicSummaries and topicRecency records from rooms-store entries the
+// way the indexer and backfill do.
+function indexDbsFromEntries (entries) {
+  const summaries = new Map()
+  for (const e of entries) {
+    const room = e.value.room
+    if (!summaries.has(room)) summaries.set(room, { room, postCount: 0, lastHeight: 0 })
+    if (e.value.type === 'post') {
+      const summary = summaries.get(room)
+      summary.postCount++
+      const height = e.value.blockHeight ?? 0
+      if (height > summary.lastHeight) summary.lastHeight = height
+    }
+  }
+
+  const recency = []
+  for (const summary of summaries.values()) {
+    recency.push([
+      topicRecencyKey(summary.lastHeight, summary.room),
+      { room: summary.room, blockHeight: summary.lastHeight }
+    ])
+  }
+
+  return {
+    topicSummariesDb: makeIndexDb(Array.from(summaries.entries())),
+    topicRecencyDb: makeIndexDb(recency)
+  }
+}
+
 // In-memory posts store exposing the LevelDB `get` contract used by the mute
 // post lookup. Returns the post record or throws a LEVEL_NOT_FOUND-style error.
 function makePostsDb (entries) {
@@ -112,14 +158,15 @@ function fixtureGen () {
 test('listTopics conserves post counts and returns rooms sorted by most recent post', async () => {
  await forAll(
    fixtureGen(),
-    async ({ entries, rooms }) => {
-      const query = makeQuery(entries)
-      const topics = await query.listTopics()
+    async ({ entries, limit, offset }) => {
+      const query = new TopicQuery({
+        roomsDb: makeRoomsDb(entries),
+        postsDb: {},
+        ...indexDbsFromEntries(entries)
+      })
+      const { topics, pagination } = await query.listTopics({ limit, offset })

      const postEntries = entries.filter((e) => e.value.type === 'post')
-      const totalPosts = topics.reduce((sum, t) => sum + t.postCount, 0)
-      if (totalPosts !== postEntries.length) return false
-
      const expectedTopics = [...new Set(entries.map((e) => e.value.room))]
        .map((room) => {
          const heights = entries
@@ -131,7 +178,12 @@ test('listTopics conserves post counts and returns rooms sorted by most recent p
          if (b.lastHeight !== a.lastHeight) return b.lastHeight - a.lastHeight
          return a.room.localeCompare(b.room)
        })
-      if (JSON.stringify(topics.map((t) => t.room)) !== JSON.stringify(expectedTopics.map((t) => t.room))) return false
+
+      if (pagination.total !== expectedTopics.length) return false
+
+      const expectedPage = expectedTopics.slice(offset, offset + limit)
+      if (JSON.stringify(topics.map((t) => t.room)) !== JSON.stringify(expectedPage.map((t) => t.room))) return false
+      if (pagination.hasMore !== (offset + topics.length < pagination.total)) return false

      for (const topic of topics) {
        const roomPosts = postEntries.filter((e) => e.value.room === topic.room).length
@@ -139,7 +191,7 @@ test('listTopics conserves post counts and returns rooms sorted by most recent p
      }
      return true
    },
-    { label: 'listTopics conservation and ordering' }
+    { label: 'listTopics conservation, ordering, and pagination' }
  )
 })

@@ -204,6 +256,8 @@ test('getTopicPostTxids excludes muted addresses and conserves total and paginat
      const query = new TopicQuery({
        roomsDb: makeRoomsDb(entries),
        postsDb: makePostsDb(entries),
+        topicSummariesDb: makeIndexDb([]),
+        topicRecencyDb: makeIndexDb([]),
        muteQuery
      })

@@ -1,6 +1,7 @@
 import { assert } from 'chai'
 import sinon from 'sinon'
 import TopicQuery from '../../../src/adapters/topic-query.js'
+import { topicRecencyKey } from '../../../src/lib/backfill-topic-indexes.js'

 function makeRoomsDb (records = {}) {
  const store = new Map(Object.entries(records))
@@ -37,11 +38,38 @@ function makeRoomsDb (records = {}) {
  }
 }

+// In-memory index store whose iterator honors the LevelDB `limit` option, so
+// the read path's bounded recency reads can be asserted.
+function makeIteratorDb (records = []) {
+  const store = new Map(records)
+  return {
+    async get (key) {
+      if (!store.has(key)) {
+        const err = new Error('not found')
+        err.notFound = true
+        throw err
+      }
+      return store.get(key)
+    },
+    async * iterator (opts = {}) {
+      let keys = Array.from(store.keys()).sort()
+      if (opts.gte !== undefined) keys = keys.filter((k) => k >= opts.gte)
+      if (opts.lte !== undefined) keys = keys.filter((k) => k <= opts.lte)
+      const limit = opts.limit === undefined ? keys.length : opts.limit
+      for (const key of keys.slice(0, limit)) {
+        yield [key, store.get(key)]
+      }
+    }
+  }
+}
+
 describe('#TopicQuery', () => {
  let uut
  let sandbox
  let roomsDb
  let postsDb
+  let topicSummariesDb
+  let topicRecencyDb

  beforeEach(() => {
    sandbox = sinon.createSandbox()
@@ -52,8 +80,10 @@ describe('#TopicQuery', () => {
    postsDb = {
      get: sandbox.stub()
    }
+    topicSummariesDb = makeIteratorDb()
+    topicRecencyDb = makeIteratorDb()

-    uut = new TopicQuery({ roomsDb, postsDb })
+    uut = new TopicQuery({ roomsDb, postsDb, topicSummariesDb, topicRecencyDb })
  })

  afterEach(() => sandbox.restore())
@@ -71,13 +101,33 @@ describe('#TopicQuery', () => {
  it('should throw when postsDb is missing', () => {
    try {
      // eslint-disable-next-line no-new
-      new TopicQuery({ roomsDb })
+      new TopicQuery({ roomsDb, topicSummariesDb, topicRecencyDb })
      assert.fail('Expected error')
    } catch (err) {
      assert.include(err.message, 'postsDb required')
    }
  })

+  it('should throw when topicSummariesDb is missing', () => {
+    try {
+      // eslint-disable-next-line no-new
+      new TopicQuery({ roomsDb, postsDb, topicRecencyDb })
+      assert.fail('Expected error')
+    } catch (err) {
+      assert.include(err.message, 'topicSummariesDb required')
+    }
+  })
+
+  it('should throw when topicRecencyDb is missing', () => {
+    try {
+      // eslint-disable-next-line no-new
+      new TopicQuery({ roomsDb, postsDb, topicSummariesDb })
+      assert.fail('Expected error')
+    } catch (err) {
+      assert.include(err.message, 'topicRecencyDb required')
+    }
+  })
+
  describe('#roomFromKey', () => {
    it('should return the room from the value when present', () => {
      assert.equal(uut.roomFromKey('ignored:post-1', { room: 'bitcoin' }), 'bitcoin')
@@ -95,60 +145,85 @@ describe('#TopicQuery', () => {
  })

  describe('#listTopics', () => {
-    it('should return distinct topics with post counts', async () => {
-      async function * mockRooms () {
-        yield ['bitcoin:post-300', { room: 'bitcoin', txid: 'post-300', type: 'post', blockHeight: 300 }]
-        yield ['bitcoin:post-200', { room: 'bitcoin', txid: 'post-200', type: 'post', blockHeight: 200 }]
-        yield ['cash:post-250', { room: 'cash', txid: 'post-250', type: 'post', blockHeight: 250 }]
-        yield ['dev:post-100', { room: 'dev', txid: 'post-100', type: 'post', blockHeight: 100 }]
-        yield ['lone:addr-f', { room: 'lone', addr: 'addr-f', type: 'follow', unfollow: false }]
-      }
-      roomsDb.iterator.returns(mockRooms())
+    const summaries = [
+      ['memo', { room: 'memo', postCount: 5, lastHeight: 600500 }],
+      ['cash', { room: 'cash', postCount: 2, lastHeight: 600400 }],
+      ['dance', { room: 'dance', postCount: 3, lastHeight: 600400 }],
+      ['anime', { room: 'anime', postCount: 1, lastHeight: 600300 }],
+      ['lone', { room: 'lone', postCount: 0, lastHeight: 0 }],
+      ['quiet', { room: 'quiet', postCount: 0, lastHeight: 0 }]
+    ]
+    const recency = [
+      [topicRecencyKey(600500, 'memo'), { room: 'memo', blockHeight: 600500 }],
+      [topicRecencyKey(600400, 'cash'), { room: 'cash', blockHeight: 600400 }],
+      [topicRecencyKey(600400, 'dance'), { room: 'dance', blockHeight: 600400 }],
+      [topicRecencyKey(600300, 'anime'), { room: 'anime', blockHeight: 600300 }],
+      [topicRecencyKey(0, 'lone'), { room: 'lone', blockHeight: 0 }],
+      [topicRecencyKey(0, 'quiet'), { room: 'quiet', blockHeight: 0 }]
+    ]

-      const result = await uut.listTopics()
+    it('should return topics ordered by recency with counts from summaries', async () => {
+      uut = new TopicQuery({
+        roomsDb,
+        postsDb,
+        topicSummariesDb: makeIteratorDb(summaries),
+        topicRecencyDb: makeIteratorDb(recency)
+      })

-      assert.deepEqual(result, [
-        { room: 'bitcoin', postCount: 2 },
-        { room: 'cash', postCount: 1 },
-        { room: 'dev', postCount: 1 },
-        { room: 'lone', postCount: 0 }
+      const result = await uut.listTopics({ limit: 100, offset: 0 })
+
+      assert.deepEqual(result.topics, [
+        { room: 'memo', postCount: 5 },
+        { room: 'cash', postCount: 2 },
+        { room: 'dance', postCount: 3 },
+        { room: 'anime', postCount: 1 },
+        { room: 'lone', postCount: 0 },
+        { room: 'quiet', postCount: 0 }
      ])
+      assert.deepEqual(result.pagination, { limit: 100, offset: 0, total: 6, hasMore: false })
    })

-    it('should sort topics by most recent post descending', async () => {
-      async function * mockRooms () {
-        yield ['zoo:post-1', { room: 'zoo', txid: 'post-1', type: 'post', blockHeight: 1 }]
-        yield ['alpha:post-1', { room: 'alpha', txid: 'post-1', type: 'post', blockHeight: 2 }]
-      }
-      roomsDb.iterator.returns(mockRooms())
+    it('should paginate using recency order and report total and hasMore', async () => {
+      uut = new TopicQuery({
+        roomsDb,
+        postsDb,
+        topicSummariesDb: makeIteratorDb(summaries),
+        topicRecencyDb: makeIteratorDb(recency)
+      })

-      const result = await uut.listTopics()
+      const result = await uut.listTopics({ limit: 2, offset: 2 })

-      assert.deepEqual(result.map((t) => t.room), ['alpha', 'zoo'])
+      assert.deepEqual(result.topics.map((t) => t.room), ['dance', 'anime'])
+      assert.equal(result.pagination.total, 6)
+      assert.equal(result.pagination.hasMore, true)
    })

-    it('should treat a post at block height 0 as the least recent', async () => {
-      async function * mockRooms () {
-        yield ['a:post-1', { room: 'a', txid: 'post-1', type: 'post', blockHeight: 0 }]
-        yield ['b:post-1', { room: 'b', txid: 'post-1', type: 'post', blockHeight: 1 }]
-      }
-      roomsDb.iterator.returns(mockRooms())
+    it('should report hasMore false on the last page', async () => {
+      uut = new TopicQuery({
+        roomsDb,
+        postsDb,
+        topicSummariesDb: makeIteratorDb(summaries),
+        topicRecencyDb: makeIteratorDb(recency)
+      })

-      const result = await uut.listTopics()
+      const result = await uut.listTopics({ limit: 2, offset: 4 })

-      assert.deepEqual(result.map((t) => t.room), ['b', 'a'])
+      assert.deepEqual(result.topics.map((t) => t.room), ['lone', 'quiet'])
+      assert.equal(result.pagination.total, 6)
+      assert.equal(result.pagination.hasMore, false)
    })

-    it('should treat a post with no block height as height 0', async () => {
-      async function * mockRooms () {
-        yield ['a:post-1', { room: 'a', txid: 'post-1', type: 'post' }]
-        yield ['b:post-1', { room: 'b', txid: 'post-1', type: 'post', blockHeight: 1 }]
-      }
-      roomsDb.iterator.returns(mockRooms())
+    it('should read the recency index without iterating the rooms store', async () => {
+      uut = new TopicQuery({
+        roomsDb,
+        postsDb,
+        topicSummariesDb: makeIteratorDb(summaries),
+        topicRecencyDb: makeIteratorDb(recency)
+      })

-      const result = await uut.listTopics()
+      await uut.listTopics({ limit: 2, offset: 2 })

-      assert.deepEqual(result.map((t) => t.room), ['b', 'a'])
+      assert.equal(roomsDb.iterator.callCount, 0)
    })
  })

@@ -258,7 +333,7 @@ describe('#TopicQuery', () => {
      const muteQuery = {
        listMuted: sandbox.stub().resolves(['muted-addr'])
      }
-      uut = new TopicQuery({ roomsDb, postsDb, muteQuery })
+      uut = new TopicQuery({ roomsDb, postsDb, topicSummariesDb, topicRecencyDb, muteQuery })

      const result = await uut.getTopicPostTxids('bitcoin', { limit: 100, offset: 0, viewerAddr: 'viewer-addr' })

@@ -320,7 +395,9 @@ describe('#TopicQuery', () => {
          'bitcoin:addr-c': { room: 'bitcoin', addr: 'addr-c', type: 'follow', unfollow: true },
          'cash:addr-a': { room: 'cash', addr: 'addr-a', type: 'follow', unfollow: false }
        }),
-        postsDb
+        postsDb,
+        topicSummariesDb,
+        topicRecencyDb
      })

      const result = await query.listRoomFollowers('bitcoin')
@@ -332,7 +409,9 @@ describe('#TopicQuery', () => {
        roomsDb: makeRoomsDb({
          'bitcoin:addr-a': { room: 'bitcoin', type: 'follow', unfollow: false }
        }),
-        postsDb
+        postsDb,
+        topicSummariesDb,
+        topicRecencyDb
      })

      const result = await query.listRoomFollowers('bitcoin')
@@ -342,7 +421,9 @@ describe('#TopicQuery', () => {
    it('should return an empty array for a room with no followers', async () => {
      const query = new TopicQuery({
        roomsDb: makeRoomsDb({}),
-        postsDb
+        postsDb,
+        topicSummariesDb,
+        topicRecencyDb
      })

      const result = await query.listRoomFollowers('lone')
@@ -45,10 +45,11 @@ describe('#TopicsRESTController', () => {
  afterEach(() => sandbox.restore())

  it('should return topics from use case', async () => {
-    const ctx = { body: null, throw: sandbox.stub() }
+    const ctx = { query: { limit: '2', offset: '4' }, body: null, throw: sandbox.stub() }
    await uut.getTopics(ctx)

    assert.equal(uut.useCases.listTopics.execute.callCount, 1)
+    assert.deepEqual(uut.useCases.listTopics.execute.firstCall.args[0], { limit: '2', offset: '4' })
    assert.equal(ctx.body.topics.length, 2)
    assert.equal(ctx.body.topics[0].room, 'bitcoin')
  })
@@ -106,7 +107,7 @@ describe('#TopicsRESTController', () => {

  it('should throw a 500 when the use case fails without a status', async () => {
    uut.useCases.listTopics.execute = sandbox.stub().rejects(new Error('boom'))
-    const ctx = { body: null, throw: sandbox.stub() }
+    const ctx = { query: {}, body: null, throw: sandbox.stub() }

    await uut.getTopics(ctx)

@@ -0,0 +1,89 @@
+import { assert } from 'chai'
+import { backfillTopicIndexes, topicRecencyKey } from '../../../src/lib/backfill-topic-indexes.js'
+
+function makeDb (records = []) {
+  const store = new Map(records)
+  return {
+    store,
+    async get (key) {
+      if (!store.has(key)) {
+        const err = new Error('not found')
+        err.notFound = true
+        throw err
+      }
+      return store.get(key)
+    },
+    async put (key, value) {
+      store.set(key, value)
+    },
+    async del (key) {
+      store.delete(key)
+    },
+    async * iterator () {
+      for (const key of Array.from(store.keys()).sort()) {
+        yield [key, store.get(key)]
+      }
+    }
+  }
+}
+
+describe('#backfillTopicIndexes', () => {
+  it('should summarize rooms with posts and follow-only rooms', async () => {
+    const roomsDb = makeDb([
+      ['bitcoin:post-100', { room: 'bitcoin', txid: 'post-100', type: 'post', blockHeight: 600100 }],
+      ['bitcoin:post-200', { room: 'bitcoin', txid: 'post-200', type: 'post', blockHeight: 600200 }],
+      ['bitcoin:addr-f', { room: 'bitcoin', addr: 'addr-f', type: 'follow', unfollow: false }],
+      ['cash:post-250', { room: 'cash', txid: 'post-250', type: 'post', blockHeight: 600250 }],
+      ['lone:addr-f', { room: 'lone', addr: 'addr-f', type: 'follow', unfollow: false }]
+    ])
+    const topicSummariesDb = makeDb()
+    const topicRecencyDb = makeDb()
+
+    const result = await backfillTopicIndexes({ roomsDb, topicSummariesDb, topicRecencyDb })
+
+    assert.equal(result.rooms, 3)
+    assert.deepEqual(topicSummariesDb.store.get('bitcoin'), { room: 'bitcoin', postCount: 2, lastHeight: 600200 })
+    assert.deepEqual(topicSummariesDb.store.get('cash'), { room: 'cash', postCount: 1, lastHeight: 600250 })
+    assert.deepEqual(topicSummariesDb.store.get('lone'), { room: 'lone', postCount: 0, lastHeight: 0 })
+
+    assert.deepEqual(topicRecencyDb.store.get(topicRecencyKey(600200, 'bitcoin')), { room: 'bitcoin', blockHeight: 600200 })
+    assert.deepEqual(topicRecencyDb.store.get(topicRecencyKey(600250, 'cash')), { room: 'cash', blockHeight: 600250 })
+    assert.deepEqual(topicRecencyDb.store.get(topicRecencyKey(0, 'lone')), { room: 'lone', blockHeight: 0 })
+    assert.equal(topicRecencyDb.store.size, 3)
+  })
+
+  it('should be idempotent across repeated runs', async () => {
+    const roomsDb = makeDb([
+      ['bitcoin:post-200', { room: 'bitcoin', txid: 'post-200', type: 'post', blockHeight: 600200 }],
+      ['lone:addr-f', { room: 'lone', addr: 'addr-f', type: 'follow', unfollow: false }]
+    ])
+    const topicSummariesDb = makeDb()
+    const topicRecencyDb = makeDb()
+
+    await backfillTopicIndexes({ roomsDb, topicSummariesDb, topicRecencyDb })
+    const firstSummaries = new Map(topicSummariesDb.store)
+    const firstRecency = new Map(topicRecencyDb.store)
+
+    await backfillTopicIndexes({ roomsDb, topicSummariesDb, topicRecencyDb })
+
+    assert.deepEqual(topicSummariesDb.store, firstSummaries)
+    assert.deepEqual(topicRecencyDb.store, firstRecency)
+  })
+
+  it('should remove stale recency records from a previous run', async () => {
+    const roomsDb = makeDb([
+      ['bitcoin:post-200', { room: 'bitcoin', txid: 'post-200', type: 'post', blockHeight: 600200 }]
+    ])
+    const topicSummariesDb = makeDb()
+    const topicRecencyDb = makeDb([
+      [topicRecencyKey(600100, 'bitcoin'), { room: 'bitcoin', blockHeight: 600100 }],
+      [topicRecencyKey(0, 'stale'), { room: 'stale', blockHeight: 0 }]
+    ])
+
+    await backfillTopicIndexes({ roomsDb, topicSummariesDb, topicRecencyDb })
+
+    assert.isFalse(topicRecencyDb.store.has(topicRecencyKey(600100, 'bitcoin')))
+    assert.isFalse(topicRecencyDb.store.has(topicRecencyKey(0, 'stale')))
+    assert.isTrue(topicRecencyDb.store.has(topicRecencyKey(600200, 'bitcoin')))
+  })
+})
@@ -7,13 +7,18 @@ describe('#ListTopics', () => {
  let sandbox
  let topicQuery

+  const adapterResult = {
+    topics: [
+      { room: 'bitcoin', postCount: 2 },
+      { room: 'cash', postCount: 1 }
+    ],
+    pagination: { limit: 100, offset: 0, total: 2, hasMore: false }
+  }
+
  beforeEach(() => {
    sandbox = sinon.createSandbox()
    topicQuery = {
-      listTopics: sandbox.stub().resolves([
-        { room: 'bitcoin', postCount: 2 },
-        { room: 'cash', postCount: 1 }
-      ])
+      listTopics: sandbox.stub().resolves(adapterResult)
    }
    uut = new ListTopics({
      adapters: { topicQuery }
@@ -42,12 +47,36 @@ describe('#ListTopics', () => {
    }
  })

-  it('should return topics from the adapter', async () => {
+  it('should return the adapter page with default limit and offset', async () => {
    const result = await uut.execute()

-    assert.deepEqual(result.topics, [
-      { room: 'bitcoin', postCount: 2 },
-      { room: 'cash', postCount: 1 }
-    ])
+    assert.deepEqual(result, adapterResult)
+    assert.deepEqual(topicQuery.listTopics.firstCall.args[0], { limit: 100, offset: 0 })
+  })
+
+  it('should forward limit and offset to the adapter', async () => {
+    await uut.execute({ limit: 2, offset: 4 })
+
+    assert.deepEqual(topicQuery.listTopics.firstCall.args[0], { limit: 2, offset: 4 })
+  })
+
+  it('should reject a non-positive limit', async () => {
+    try {
+      await uut.execute({ limit: 0 })
+      assert.fail('Expected error')
+    } catch (err) {
+      assert.include(err.message, 'limit must be a positive integer')
+      assert.equal(err.status, 400)
+    }
+  })
+
+  it('should reject a negative offset', async () => {
+    try {
+      await uut.execute({ offset: -1 })
+      assert.fail('Expected error')
+    } catch (err) {
+      assert.include(err.message, 'offset must be a non-negative integer')
+      assert.equal(err.status, 400)
+    }
  })
 })
@@ -0,0 +1,71 @@
+/*
+  Utility: build the topicSummaries and topicRecency indexes for an existing
+  psf-memo-db.
+
+  New deployments write these indexes while indexing live topic messages, but
+  existing databases that were populated before the topic recency feature need
+  a one-time backfill.
+
+  Run from the psf-memo-db repo root on the host that owns the LevelDB files:
+
+    node util/room/backfill-topic-indexes.js
+
+  The script is idempotent: re-running it produces the same indexes. Progress
+  and a summary are printed to stderr.
+
+  WARNING:
+  - This script opens the LevelDB files directly. psf-memo-db must NOT be
+    running, or another process must not hold the database locks.
+  - Make a backup of leveldb/current before running on a production server:
+      cp -r leveldb/current leveldb/current-pre-topic-index-backup
+*/
+
+import level from 'level'
+import * as fs from 'fs'
+import * as path from 'path'
+import * as url from 'url'
+import { backfillTopicIndexes } from '../../src/lib/backfill-topic-indexes.js'
+
+const __dirname = url.fileURLToPath(new URL('.', import.meta.url))
+
+const DATA_DIR = process.env.PSF_MEMO_DB_DATA_DIR
+  ? path.resolve(process.env.PSF_MEMO_DB_DATA_DIR)
+  : path.resolve(__dirname, '../../leveldb/current')
+
+function requiredStorePath (dir, name) {
+  const storePath = path.join(dir, name)
+  if (!fs.existsSync(storePath)) {
+    throw new Error(`Required LevelDB store not found: ${storePath}. Set PSF_MEMO_DB_DATA_DIR to the directory containing the rooms store.`)
+  }
+  return storePath
+}
+
+async function main () {
+  console.error(`Using LevelDB data directory: ${DATA_DIR}`)
+
+  const roomsPath = requiredStorePath(DATA_DIR, 'rooms')
+  const topicSummariesPath = path.join(DATA_DIR, 'topicSummaries')
+  const topicRecencyPath = path.join(DATA_DIR, 'topicRecency')
+
+  console.error('Opening LevelDB stores...')
+  const roomsDb = level(roomsPath, { valueEncoding: 'json' })
+  const topicSummariesDb = level(topicSummariesPath, { valueEncoding: 'json', createIfMissing: true })
+  const topicRecencyDb = level(topicRecencyPath, { valueEncoding: 'json', createIfMissing: true })
+
+  try {
+    console.error('Backfilling topicSummaries and topicRecency from rooms...')
+    const summary = await backfillTopicIndexes({ roomsDb, topicSummariesDb, topicRecencyDb })
+
+    console.error('\nTopic index backfill complete.')
+    console.error(`  rooms summarized: ${summary.rooms}`)
+  } catch (err) {
+    console.error('\nTopic index backfill failed:', err.message)
+    process.exitCode = 1
+  } finally {
+    await topicRecencyDb.close().catch(() => {})
+    await topicSummariesDb.close().catch(() => {})
+    await roomsDb.close().catch(() => {})
+  }
+}
+
+main()
@@ -14,6 +14,9 @@ import { handleCreatePoll } from '../../src/use-cases/action-types/poll-create.j
 import { handleAddPollOption } from '../../src/use-cases/action-types/poll-option.js'
 import { handlePollVote } from '../../src/use-cases/action-types/poll-vote.js'
 import { handleMute } from '../../src/use-cases/action-types/mute.js'
+import { handleTopicMessage } from '../../src/use-cases/action-types/topic-message.js'
+import { handleTopicFollow } from '../../src/use-cases/action-types/topic-follow.js'
+import { topicRecencyKey } from '../../src/use-cases/action-types/helpers.js'
 import BackupDb from '../../src/use-cases/backup-db.js'

 function makeInMemoryDb () {
@@ -85,6 +88,9 @@ async function createWorld () {
  const pollOptionDb = makeInMemoryDb()
  const pollVoteDb = makeInMemoryDb()
  const muteDb = makeInMemoryDb()
+  const roomDb = makeInMemoryDb()
+  const topicSummaryDb = makeInMemoryDb()
+  const topicRecencyDb = makeInMemoryDb()

  const adapters = {
    postDb: postsDb,
@@ -98,6 +104,9 @@ async function createWorld () {
    pollOptionDb,
    pollVoteDb,
    muteDb,
+    roomDb,
+    topicSummaryDb,
+    topicRecencyDb,
    processErrorDb: makeInMemoryDb(),
    dbCtrl: {
      backupDb: async (height, epoch) => {
@@ -121,6 +130,9 @@ async function createWorld () {
    pollOptionsDb: pollOptionDb,
    pollVotesDb: pollVoteDb,
    mutesDb: muteDb,
+    roomsDb: roomDb,
+    topicSummariesDb: topicSummaryDb,
+    topicRecencyDb,
    txidMap: new Map(),
    lastTxid: null,
    lastHeight: null,
@@ -697,6 +709,112 @@ const muteHandlers = [
        throw new Error(`Expected no mute document for txid ${txid}, but one was stored`)
      }
    }
+  },
+  {
+    name: 'db instance with rooms and topic index stores',
+    pattern: /^a psf-memo-db instance with rooms, topicSummaries, and topicRecency stores$/,
+    async run () {
+      // World is already created with the room and topic index stores.
+    }
+  },
+  {
+    name: 'process a Memo topic message',
+    pattern: /^the indexer processes a Memo topic message (.+) in room "(.+)" from (.+) at block height (.+) with text "(.+)"$/,
+    async run (m, example, world) {
+      const txid = resolveTxid(m[1], example, world)
+      const room = m[2]
+      const addr = resolveParam(m[3], example)
+      const height = parseInt(resolveParam(m[4], example), 10)
+      const text = m[5]
+
+      world.lastTxid = txid
+      world.lastHeight = height
+      world.lastAddr = addr
+
+      const prefix = Buffer.from('6d0c', 'hex')
+      const ctx = {
+        adapters: world.adapters,
+        txid,
+        signerAddr: addr,
+        seen: Date.now(),
+        blockHeight: height,
+        decoded: {
+          action: 'topicMessage',
+          prefix,
+          pushDatas: [prefix, Buffer.from(room, 'utf8'), Buffer.from(text, 'utf8')]
+        }
+      }
+      world.lastTopicMessage = ctx
+
+      await handleTopicMessage(ctx)
+    }
+  },
+  {
+    name: 'reprocess the last Memo topic message',
+    pattern: /^the indexer processes the same Memo topic message (.+) again$/,
+    async run (m, example, world) {
+      if (!world.lastTopicMessage) {
+        throw new Error('No previous topic message to reprocess')
+      }
+      await handleTopicMessage(world.lastTopicMessage)
+    }
+  },
+  {
+    name: 'process a Memo topic follow',
+    pattern: /^the indexer processes a Memo topic follow for room "(.+)" from (.+)$/,
+    async run (m, example, world) {
+      const room = m[1]
+      const addr = resolveParam(m[2], example)
+      const txid = deriveTxid(`topic-follow-${room}-${addr}`)
+
+      world.lastTxid = txid
+      world.lastAddr = addr
+
+      const prefix = Buffer.from('6d0d', 'hex')
+      await handleTopicFollow({
+        adapters: world.adapters,
+        txid,
+        signerAddr: addr,
+        seen: Date.now(),
+        blockHeight: 600000,
+        decoded: {
+          action: 'topicFollow',
+          prefix,
+          pushDatas: [prefix, Buffer.from(room, 'utf8')]
+        }
+      })
+    }
+  },
+  {
+    name: 'topicSummaries contains room with postCount and lastHeight',
+    pattern: /^the topicSummaries store contains the room "(.+)" with postCount (.+) and lastHeight (.+)$/,
+    async run (m, example, world) {
+      const room = m[1]
+      const postCount = parseInt(resolveParam(m[2], example), 10)
+      const lastHeight = parseInt(resolveParam(m[3], example), 10)
+      const record = await world.topicSummariesDb.get(room)
+      if (!record) {
+        throw new Error(`No topicSummaries record for ${room}`)
+      }
+      if (record.room !== room || record.postCount !== postCount || record.lastHeight !== lastHeight) {
+        throw new Error(`Expected ${room} postCount ${postCount} lastHeight ${lastHeight}, got ${JSON.stringify(record)}`)
+      }
+    }
+  },
+  {
+    name: 'topicRecency records room at height',
+    pattern: /^the topicRecency store records the room "(.+)" at block height (.+)$/,
+    async run (m, example, world) {
+      const room = m[1]
+      const height = parseInt(resolveParam(m[2], example), 10)
+      const record = await world.topicRecencyDb.get(topicRecencyKey(height, room))
+      if (!record) {
+        throw new Error(`No topicRecency record for ${room} at ${height}`)
+      }
+      if (record.room !== room || record.blockHeight !== height) {
+        throw new Error(`Expected topicRecency ${room} at ${height}, got ${JSON.stringify(record)}`)
+      }
+    }
  }
 ]

@@ -32,6 +32,8 @@ class Adapters {
    this.followDb = createEntityDb('follow', 'key', 'followData')
    this.muteDb = createEntityDb('mute', 'key', 'muteData')
    this.roomDb = createEntityDb('room', 'key', 'roomData')
+    this.topicSummaryDb = createEntityDb('topicsummary', 'key', 'topicSummaryData')
+    this.topicRecencyDb = createEntityDb('topicrecency', 'key', 'topicRecencyData')
    this.pollDb = createEntityDb('poll', 'txid', 'pollData')
    this.pollOptionDb = createEntityDb('polloption', 'txid', 'optionData')
    this.pollVoteDb = createEntityDb('pollvote', 'txid', 'voteData')
@@ -62,6 +62,30 @@ export function roomKey (roomName, txid) {
  return `${roomName}:${txid}`
 }

+// The topicRecency store is keyed with an inverted height so a plain iterator
+// yields rooms ordered by most recent post height and, within a height, by
+// room name ascending.
+export function topicRecencyKey (blockHeight, roomName) {
+  const inverted = 999999999999 - (blockHeight ?? 0)
+  return `${String(inverted).padStart(12, '0')}:${roomName}`
+}
+
+// True for the LevelDB not-found error, its in-memory equivalent, and the
+// 404 surfaced by the psf-memo-db entity routes.
+export function isNotFound (err) {
+  return Boolean(err && (err.notFound || err.code === 'LEVEL_NOT_FOUND' || err.response?.status === 404))
+}
+
+// Read a record, or null when it does not exist. Rethrows real errors.
+export async function getIfPresent (db, key) {
+  try {
+    return await db.get(key)
+  } catch (err) {
+    if (isNotFound(err)) return null
+    throw err
+  }
+}
+
 export function postHeightKey (blockHeight, txid) {
  const padded = String(blockHeight).padStart(12, '0')
  return `${padded}:${txid}`
@@ -1,5 +1,6 @@
 import { utf8FromPush, logProcessError, roomKey } from './helpers.js'
 import { PREFIX_TOPIC_UNFOLLOW } from '../../lib/memo-codes.js'
+import { ensureTopicRoom } from './topic-indexing.js'

 export async function handleTopicFollow (ctx) {
  const { adapters, txid, signerAddr, decoded, seen, blockHeight } = ctx
@@ -22,4 +23,8 @@ export async function handleTopicFollow (ctx) {
    type: 'follow',
    blockHeight
  })
+
+  if (!unfollow) {
+    await ensureTopicRoom(adapters, room)
+  }
 }
@@ -0,0 +1,50 @@
+/*
+  Maintain the topicSummaries and topicRecency indexes for Memo topic activity.
+
+  topicSummaries: one record per room keyed by room, value
+    { room, postCount, lastHeight }.
+  topicRecency: one record per room keyed by `${invertedHeight}:${room}`, value
+    { room, blockHeight }. Follow-only rooms live at height 0. The height is
+    inverted so an ascending scan yields newest-first.
+
+  Both indexes are written with the generic entity `update`, which upserts, so
+  re-running an action is safe.
+*/
+
+import { getIfPresent, topicRecencyKey } from './helpers.js'
+
+// Add a post to the room's summary and move its recency record to the newest
+// height. A room that is reprocessed is filtered out before this runs, so the
+// post count is not double-counted.
+export async function recordTopicPost (adapters, room, blockHeight) {
+  const height = blockHeight ?? 0
+  const summary = await getIfPresent(adapters.topicSummaryDb, room)
+  const lastHeight = Math.max(summary?.lastHeight ?? 0, height)
+
+  await adapters.topicSummaryDb.update(room, {
+    room,
+    postCount: (summary?.postCount ?? 0) + 1,
+    lastHeight
+  })
+
+  // When the room moved to a newer height, delete the stale recency record so
+  // each room has exactly one recency entry.
+  if (summary && (summary.lastHeight ?? 0) !== lastHeight) {
+    await adapters.topicRecencyDb.delete(topicRecencyKey(summary.lastHeight ?? 0, room))
+  }
+
+  await adapters.topicRecencyDb.update(topicRecencyKey(lastHeight, room), {
+    room,
+    blockHeight: lastHeight
+  })
+}
+
+// Give a follow-only room a zero-post summary and recency record. A room that
+// already has a summary (because it has posts) is left unchanged.
+export async function ensureTopicRoom (adapters, room) {
+  const summary = await getIfPresent(adapters.topicSummaryDb, room)
+  if (summary) return
+
+  await adapters.topicSummaryDb.update(room, { room, postCount: 0, lastHeight: 0 })
+  await adapters.topicRecencyDb.update(topicRecencyKey(0, room), { room, blockHeight: 0 })
+}
@@ -1,6 +1,7 @@
-import { utf8FromPush, logProcessError, roomKey } from './helpers.js'
+import { utf8FromPush, logProcessError, roomKey, getIfPresent } from './helpers.js'
 import { MAX_POST_SIZE } from '../../lib/memo-codes.js'
 import { handlePost } from './post.js'
+import { recordTopicPost } from './topic-indexing.js'

 export async function handleTopicMessage (ctx) {
  const { adapters, txid, decoded, seen, blockHeight } = ctx
@@ -18,10 +19,20 @@ export async function handleTopicMessage (ctx) {
    return
  }

+  // The room record is the marker that this topic message was already
+  // indexed. Check it before writing so reprocessing does not double-count the
+  // room's post summary.
+  const key = roomKey(room, txid)
+  const alreadyIndexed = (await getIfPresent(adapters.roomDb, key)) !== null
+
  await handlePost({
    ...ctx,
    decoded: { ...decoded, pushDatas: [pushDatas[0], pushDatas[2]] }
  })

-  await adapters.roomDb.create(roomKey(room, txid), { room, txid, seen, type: 'post', blockHeight })
+  await adapters.roomDb.create(key, { room, txid, seen, type: 'post', blockHeight })
+
+  if (!alreadyIndexed) {
+    await recordTopicPost(adapters, room, blockHeight)
+  }
 }
@@ -0,0 +1,106 @@
+import { assert } from 'chai'
+import { handleTopicFollow } from '../../../../src/use-cases/action-types/topic-follow.js'
+import { PREFIX_TOPIC_FOLLOW, PREFIX_TOPIC_UNFOLLOW } from '../../../../src/lib/memo-codes.js'
+import { topicRecencyKey } from '../../../../src/use-cases/action-types/helpers.js'
+
+function makeDb () {
+  const store = new Map()
+  return {
+    store,
+    async get (key) {
+      if (!store.has(key)) {
+        const err = new Error('not found')
+        err.notFound = true
+        throw err
+      }
+      return store.get(key)
+    },
+    async create (key, value) {
+      store.set(key, value)
+      return { success: true }
+    },
+    async update (key, value) {
+      store.set(key, value)
+      return { success: true }
+    },
+    async delete (key) {
+      store.delete(key)
+      return { success: true }
+    }
+  }
+}
+
+function makeAdapters () {
+  return {
+    roomDb: makeDb(),
+    topicSummaryDb: makeDb(),
+    topicRecencyDb: makeDb(),
+    processErrorDb: makeDb()
+  }
+}
+
+async function processFollow (adapters, { txid, room, addr, prefix = PREFIX_TOPIC_FOLLOW }) {
+  await handleTopicFollow({
+    adapters,
+    txid,
+    signerAddr: addr,
+    seen: 1,
+    blockHeight: 600000,
+    decoded: {
+      action: 'topic-follow',
+      prefix,
+      pushDatas: [prefix, Buffer.from(room, 'utf8')]
+    }
+  })
+}
+
+describe('#handleTopicFollow topic indexes', () => {
+  it('should record a zero-post room for a follow-only room', async () => {
+    const adapters = makeAdapters()
+
+    await processFollow(adapters, { txid: 'follow-1', room: 'lone', addr: 'bitcoincash:qaddr-a' })
+
+    assert.deepEqual(adapters.topicSummaryDb.store.get('lone'), {
+      room: 'lone',
+      postCount: 0,
+      lastHeight: 0
+    })
+    assert.deepEqual(adapters.topicRecencyDb.store.get(topicRecencyKey(0, 'lone')), {
+      room: 'lone',
+      blockHeight: 0
+    })
+  })
+
+  it('should leave an existing room summary unchanged', async () => {
+    const adapters = makeAdapters()
+    adapters.topicSummaryDb.store.set('bitcoin', { room: 'bitcoin', postCount: 1, lastHeight: 600400 })
+    adapters.topicRecencyDb.store.set(topicRecencyKey(600400, 'bitcoin'), { room: 'bitcoin', blockHeight: 600400 })
+
+    await processFollow(adapters, { txid: 'follow-2', room: 'bitcoin', addr: 'bitcoincash:qaddr-a' })
+
+    assert.deepEqual(adapters.topicSummaryDb.store.get('bitcoin'), {
+      room: 'bitcoin',
+      postCount: 1,
+      lastHeight: 600400
+    })
+    assert.deepEqual(adapters.topicRecencyDb.store.get(topicRecencyKey(600400, 'bitcoin')), {
+      room: 'bitcoin',
+      blockHeight: 600400
+    })
+    assert.isFalse(adapters.topicRecencyDb.store.has(topicRecencyKey(0, 'bitcoin')))
+  })
+
+  it('should not create a zero-post room for an unfollow', async () => {
+    const adapters = makeAdapters()
+
+    await processFollow(adapters, {
+      txid: 'unfollow-1',
+      room: 'lone',
+      addr: 'bitcoincash:qaddr-a',
+      prefix: PREFIX_TOPIC_UNFOLLOW
+    })
+
+    assert.equal(adapters.topicSummaryDb.store.size, 0)
+    assert.equal(adapters.topicRecencyDb.store.size, 0)
+  })
+})
@@ -0,0 +1,135 @@
+import { assert } from 'chai'
+import { handleTopicMessage } from '../../../../src/use-cases/action-types/topic-message.js'
+import { topicRecencyKey } from '../../../../src/use-cases/action-types/helpers.js'
+
+function makeDb () {
+  const store = new Map()
+  return {
+    store,
+    async get (key) {
+      if (!store.has(key)) {
+        const err = new Error('not found')
+        err.notFound = true
+        throw err
+      }
+      return store.get(key)
+    },
+    async create (key, value) {
+      store.set(key, value)
+      return { success: true }
+    },
+    async update (key, value) {
+      store.set(key, value)
+      return { success: true }
+    },
+    async delete (key) {
+      store.delete(key)
+      return { success: true }
+    },
+    entries () {
+      return Array.from(store.entries())
+    }
+  }
+}
+
+function makeAdapters () {
+  return {
+    postDb: makeDb(),
+    postHeightDb: makeDb(),
+    addrPostHeightDb: makeDb(),
+    roomDb: makeDb(),
+    topicSummaryDb: makeDb(),
+    topicRecencyDb: makeDb(),
+    processErrorDb: makeDb()
+  }
+}
+
+async function processTopicMessage (adapters, { txid, room, addr, height, text }) {
+  const prefix = Buffer.from('6d0c', 'hex')
+  await handleTopicMessage({
+    adapters,
+    txid,
+    signerAddr: addr,
+    seen: 1,
+    blockHeight: height,
+    decoded: {
+      action: 'topic-message',
+      prefix,
+      pushDatas: [prefix, Buffer.from(room, 'utf8'), Buffer.from(text, 'utf8')]
+    }
+  })
+}
+
+describe('#handleTopicMessage topic indexes', () => {
+  it('should record a room summary and a recency record', async () => {
+    const adapters = makeAdapters()
+
+    await processTopicMessage(adapters, {
+      txid: 'topic-a1',
+      room: 'bitcoin',
+      addr: 'bitcoincash:qaddr-a',
+      height: 600100,
+      text: 'hello'
+    })
+
+    assert.deepEqual(adapters.topicSummaryDb.store.get('bitcoin'), {
+      room: 'bitcoin',
+      postCount: 1,
+      lastHeight: 600100
+    })
+    assert.deepEqual(adapters.topicRecencyDb.store.get(topicRecencyKey(600100, 'bitcoin')), {
+      room: 'bitcoin',
+      blockHeight: 600100
+    })
+  })
+
+  it('should accumulate postCount and keep the newest height', async () => {
+    const adapters = makeAdapters()
+
+    await processTopicMessage(adapters, { txid: 'topic-a1', room: 'bitcoin', addr: 'bitcoincash:qaddr-a', height: 600100, text: 'hello' })
+    await processTopicMessage(adapters, { txid: 'topic-a2', room: 'bitcoin', addr: 'bitcoincash:qaddr-a', height: 600200, text: 'again' })
+
+    assert.deepEqual(adapters.topicSummaryDb.store.get('bitcoin'), {
+      room: 'bitcoin',
+      postCount: 2,
+      lastHeight: 600200
+    })
+    assert.deepEqual(adapters.topicRecencyDb.store.get(topicRecencyKey(600200, 'bitcoin')), {
+      room: 'bitcoin',
+      blockHeight: 600200
+    })
+    assert.equal(adapters.topicRecencyDb.store.size, 1)
+  })
+
+  it('should keep the newest height when a later message has an earlier height', async () => {
+    const adapters = makeAdapters()
+
+    await processTopicMessage(adapters, { txid: 'topic-a3', room: 'bitcoin', addr: 'bitcoincash:qaddr-a', height: 600200, text: 'later' })
+    await processTopicMessage(adapters, { txid: 'topic-a4', room: 'bitcoin', addr: 'bitcoincash:qaddr-a', height: 600100, text: 'earlier' })
+
+    assert.deepEqual(adapters.topicSummaryDb.store.get('bitcoin'), {
+      room: 'bitcoin',
+      postCount: 2,
+      lastHeight: 600200
+    })
+    assert.deepEqual(adapters.topicRecencyDb.store.get(topicRecencyKey(600200, 'bitcoin')), {
+      room: 'bitcoin',
+      blockHeight: 600200
+    })
+    assert.equal(adapters.topicRecencyDb.store.size, 1)
+  })
+
+  it('should not double-count a reprocessed topic message', async () => {
+    const adapters = makeAdapters()
+
+    await processTopicMessage(adapters, { txid: 'topic-c1', room: 'bitcoin', addr: 'bitcoincash:qaddr-c', height: 600300, text: 'repeated' })
+    await processTopicMessage(adapters, { txid: 'topic-c1', room: 'bitcoin', addr: 'bitcoincash:qaddr-c', height: 600300, text: 'repeated' })
+
+    assert.deepEqual(adapters.topicSummaryDb.store.get('bitcoin'), {
+      room: 'bitcoin',
+      postCount: 1,
+      lastHeight: 600300
+    })
+    assert.equal(adapters.topicRecencyDb.store.size, 1)
+  })
+})