#41

Reviewed-on: #359
Co-authored-by: miteruzo <miteruzo@naver.com>
Co-committed-by: miteruzo <miteruzo@naver.com>

AGENTS.md

@@ -126,12 +126,16 @@ npm run preview
 - In TypeScript and TSX only, replace every leading run of 8 spaces with a tab.
 - Tabs are only for leading indentation, never for spaces after non-space text.
 - Do not add production dependencies without explicit approval.
+- Do not create, modify, or run tests unless the user explicitly asks for
+  test work. When the user asks for tests, keep working and rerun them until
+  they pass or the remaining failure is clearly blocked.
 
 ## Backend rules
 
 - Inspect existing routes, controllers, models, services, and specs before
   editing backend behavior.
-- For API behavior changes, add or update request specs under `backend/spec/requests`.
+- For API behavior changes, add or update request specs under
+  `backend/spec/requests` only when the user explicitly asks for tests.
 - Prefer RSpec for new backend tests; existing minitest files under
   `backend/test` do not make minitest the default for new coverage.
 - Do not weaken authentication, BAN user checks, or IP BAN checks.
@@ -211,10 +215,11 @@ function PostFormTagsArea ({ tags, setTags }: Props) {
   `node_modules`, `dist`, `tmp`, `log`, and `storage` unless explicitly needed.
 - Before touching wiki, tag, versioning, BAN, IP BAN, or authentication
   behavior, inspect the related request specs and service objects.
-- If frontend code changes, run the existing frontend verification commands
-  that apply: `npm run build`, `npm run lint`, and `npm run test:run`.
-- If backend code changes, run the relevant RSpec command; for broad backend
-  changes, run `bundle exec rspec`.
+- If frontend code changes, run only non-test verification commands that
+  apply, such as `npm run build` and `npm run lint`. Run `npm run test:run`
+  only when the user explicitly asks for tests.
+- If backend code changes, do not run RSpec unless the user explicitly asks
+  for tests.
 - If a verification command cannot be run or fails, report the exact command and failure.
 
 ## Completion criteria
@@ -222,7 +227,8 @@ function PostFormTagsArea ({ tags, setTags }: Props) {
 A task is complete only when:
 
 - implementation is complete,
-- relevant verification commands pass, or failures are clearly explained,
+- relevant non-test verification commands pass, or failures are clearly
+  explained,
 - unrelated files are not changed,
 - migrations and schema are consistent when schema changes are made,
 - user-facing behavior is documented when needed.

backend/AGENTS.md

@@ -47,6 +47,10 @@ bundle exec rspec
 
 If a command cannot be run or fails, report the exact command and failure.
 
+Do not create, modify, or run tests unless the user explicitly asks for test
+work. When the user asks for tests, keep working and rerun them until they
+pass or the remaining failure is clearly blocked.
+
 ## Rails structure
 
 - `app/controllers`: API controllers.
@@ -116,7 +120,8 @@ service, representation, and spec.
 - `User#banned?` and `IpAddress#banned?` check `banned_at.present?`.
 - Do not weaken BAN or IP BAN behavior.
 - If changing request authentication or controller before actions, add or
-  update request specs covering banned users and banned IP addresses.
+  update request specs covering banned users and banned IP addresses only when
+  the user explicitly asks for tests.
 
 ## RSpec
 
@@ -130,8 +135,9 @@ service, representation, and spec.
 - `AuthHelper#sign_in_as(user)` stubs
   `ApplicationController#current_user`; use it when matching existing
   request spec style.
-- Add or update request specs for API behavior changes, especially status
-  codes, permissions, response shape, and version conflict behavior.
+- Add or update request specs for API behavior changes only when the user
+  explicitly asks for tests, especially status codes, permissions, response
+  shape, and version conflict behavior.
 
 ## Migrations
 
@@ -164,7 +170,8 @@ service, representation, and spec.
   the record `version_no`.
 - Do not update versioned records without considering whether a version snapshot must be created.
 - For optimistic concurrency paths, preserve `base_version_no`, `force`, and
-  `merge` semantics and cover conflicts in request specs.
+  `merge` semantics. Cover conflicts in request specs only when the user
+  explicitly asks for tests.
 
 ## Domain cautions
 

backend/app/controllers/gekanator_games_controller.rb

@@ -0,0 +1,23 @@
+class GekanatorGamesController < ApplicationController
+  def create
+    return head :not_found unless current_user&.admin?
+
+    guessed_post_id = params.require(:guessed_post_id)
+    correct_post_id = params[:correct_post_id].presence
+    answers = params.require(:answers).as_json
+
+    game = GekanatorGame.new(
+        user: current_user,
+        guessed_post_id:,
+        correct_post_id:,
+        won: correct_post_id.present? && guessed_post_id.to_i == correct_post_id.to_i,
+        question_count: answers.length,
+        answers:)
+
+    if game.save
+      render json: { id: game.id }, status: :created
+    else
+      render json: { errors: game.errors.full_messages }, status: :unprocessable_entity
+    end
+  end
+end

backend/app/models/gekanator_game.rb

@@ -0,0 +1,9 @@
+class GekanatorGame < ApplicationRecord
+  belongs_to :user
+  belongs_to :guessed_post, class_name: 'Post'
+  belongs_to :correct_post, class_name: 'Post'
+
+  validates :answers, presence: true
+  validates :question_count, numericality: { greater_than_or_equal_to: 0 }
+  validates :won, inclusion: { in: [true, false] }
+end

backend/app/models/post.rb

@@ -11,6 +11,16 @@ class Post < ApplicationRecord
   has_many :user_post_views, dependent: :delete_all
   has_many :post_similarities, dependent: :delete_all
   has_many :post_versions
+  has_many :gekanator_guessed_games,
+           class_name: 'GekanatorGame',
+           foreign_key: :guessed_post_id,
+           dependent: :delete_all,
+           inverse_of: :guessed_post
+  has_many :gekanator_correct_games,
+           class_name: 'GekanatorGame',
+           foreign_key: :correct_post_id,
+           dependent: :nullify,
+           inverse_of: :correct_post
 
   has_many :parent_post_implications,
            class_name: 'PostImplication',

backend/config/routes.rb

@@ -63,6 +63,10 @@ Rails.application.routes.draw do
     end
   end
 
+  namespace :gekanator do
+    resources :games, only: [:create], controller: '/gekanator_games'
+  end
+
   resources :users, only: [:create, :update] do
     collection do
       post :verify

backend/db/migrate/20260607000000_create_gekanator_games.rb

@@ -0,0 +1,18 @@
+class CreateGekanatorGames < ActiveRecord::Migration[8.0]
+  def change
+    create_table :gekanator_games do |t|
+      t.references :user, null: false, foreign_key: true
+      t.references :guessed_post, null: false, foreign_key: { to_table: :posts }
+      t.references :correct_post, null: false, foreign_key: { to_table: :posts }
+      t.boolean :won, null: false
+      t.integer :question_count, null: false
+      t.json :answers, null: false
+
+      t.timestamps
+    end
+
+    add_check_constraint :gekanator_games,
+                         'question_count >= 0',
+                         name: 'chk_gekanator_games_question_count_nonnegative'
+  end
+end

backend/db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2026_06_06_000000) do
+ActiveRecord::Schema[8.0].define(version: 2026_06_07_000000) do
   create_table "active_storage_attachments", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "name", null: false
     t.string "record_type", null: false
@@ -48,6 +48,21 @@ ActiveRecord::Schema[8.0].define(version: 2026_06_06_000000) do
     t.index ["tag_id"], name: "index_deerjikists_on_tag_id"
   end
 
+  create_table "gekanator_games", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.bigint "user_id", null: false
+    t.bigint "guessed_post_id", null: false
+    t.bigint "correct_post_id", null: false
+    t.boolean "won", null: false
+    t.integer "question_count", null: false
+    t.json "answers", null: false
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["correct_post_id"], name: "index_gekanator_games_on_correct_post_id"
+    t.index ["guessed_post_id"], name: "index_gekanator_games_on_guessed_post_id"
+    t.index ["user_id"], name: "index_gekanator_games_on_user_id"
+    t.check_constraint "`question_count` >= 0", name: "chk_gekanator_games_question_count_nonnegative"
+  end
+
   create_table "ip_addresses", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.binary "ip_address", limit: 16, null: false
     t.datetime "banned_at"
@@ -478,6 +493,9 @@ ActiveRecord::Schema[8.0].define(version: 2026_06_06_000000) do
 
   add_foreign_key "active_storage_attachments", "active_storage_blobs", column: "blob_id"
   add_foreign_key "active_storage_variant_records", "active_storage_blobs", column: "blob_id"
+  add_foreign_key "gekanator_games", "posts", column: "correct_post_id"
+  add_foreign_key "gekanator_games", "posts", column: "guessed_post_id"
+  add_foreign_key "gekanator_games", "users"
   add_foreign_key "material_versions", "materials"
   add_foreign_key "material_versions", "materials", column: "parent_id"
   add_foreign_key "material_versions", "tags"

backend/spec/requests/gekanator_games_spec.rb

@@ -0,0 +1,75 @@
+require 'rails_helper'
+
+RSpec.describe 'Gekanator games API', type: :request do
+  let!(:admin) { create_admin_user! }
+  let!(:user) { create_member_user! }
+  let!(:guessed_post) { Post.create!(title: 'guess', url: 'https://example.com/guess') }
+  let!(:correct_post) { Post.create!(title: 'correct', url: 'https://example.com/correct') }
+
+  describe 'POST /gekanator/games' do
+    it 'stores a won game' do
+      sign_in_as admin
+
+      post '/gekanator/games', params: {
+          guessed_post_id: guessed_post.id,
+          correct_post_id: guessed_post.id,
+          answers: [{ question_id: 'tag:1', answer: 'yes' }] }
+
+      expect(response).to have_http_status(:created)
+      game = GekanatorGame.find(json['id'])
+      expect(game.user).to eq(admin)
+      expect(game.guessed_post).to eq(guessed_post)
+      expect(game.correct_post).to eq(guessed_post)
+      expect(game.won).to eq(true)
+      expect(game.question_count).to eq(1)
+      expect(game.answers).to eq([{ 'question_id' => 'tag:1', 'answer' => 'yes' }])
+    end
+
+    it 'stores a lost game with the correct post' do
+      sign_in_as admin
+
+      post '/gekanator/games', params: {
+          guessed_post_id: guessed_post.id,
+          correct_post_id: correct_post.id,
+          question_count: 4,
+          answers: [{ question_id: 'tag:1', answer: 'no' }] }
+
+      expect(response).to have_http_status(:created)
+      game = GekanatorGame.find(json['id'])
+      expect(game.correct_post).to eq(correct_post)
+      expect(game.won).to eq(false)
+      expect(game.question_count).to eq(1)
+    end
+
+    it 'rejects a game without the correct post' do
+      sign_in_as admin
+
+      post '/gekanator/games', params: {
+          guessed_post_id: guessed_post.id,
+          question_count: 4,
+          answers: [{ question_id: 'tag:1', answer: 'no' }] }
+
+      expect(response).to have_http_status(:unprocessable_entity)
+    end
+
+    it 'returns not found without an admin user' do
+      post '/gekanator/games', params: {
+          guessed_post_id: guessed_post.id,
+          correct_post_id: guessed_post.id,
+          answers: [] }
+
+      expect(response).to have_http_status(:not_found)
+    end
+
+    it 'returns not found for a non-admin user' do
+      sign_in_as user
+
+      post '/gekanator/games', params: {
+          guessed_post_id: guessed_post.id,
+          correct_post_id: guessed_post.id,
+          answers: [{ question_id: 'tag:1', answer: 'yes' }] }
+
+      expect(response).to have_http_status(:not_found)
+    end
+  end
+end

frontend/AGENTS.md

@@ -33,6 +33,10 @@ npm run lint
 
 If either command cannot be run or fails, report the exact command and failure.
 
+Do not create, modify, or run tests unless the user explicitly asks for test
+work. When the user asks for tests, keep working and rerun them until they
+pass or the remaining failure is clearly blocked.
+
 ## TypeScript
 
 - TypeScript is strict. `tsconfig.app.json` enables `strict`,

frontend/src/App.tsx

@@ -18,6 +18,7 @@ import MaterialListPage from '@/pages/materials/MaterialListPage'
 import MaterialNewPage from '@/pages/materials/MaterialNewPage'
 // import MaterialSearchPage from '@/pages/materials/MaterialSearchPage'
 import MorePage from '@/pages/MorePage'
+import GekanatorPage from '@/pages/GekanatorPage'
 import NicoTagListPage from '@/pages/tags/NicoTagListPage'
 import NotFound from '@/pages/NotFound'
 import TOSPage from '@/pages/TOSPage.mdx'
@@ -39,7 +40,7 @@ import WikiHistoryPage from '@/pages/wiki/WikiHistoryPage'
 import WikiNewPage from '@/pages/wiki/WikiNewPage'
 import WikiSearchPage from '@/pages/wiki/WikiSearchPage'
 
-import type { Dispatch, FC, SetStateAction } from 'react'
+import type { Dispatch, FC, ReactNode, SetStateAction } from 'react'
 
 import type { User } from '@/types'
 
@@ -80,6 +81,10 @@ const RouteTransitionWrapper = ({ user, setUser }: {
 	  <Route path="/users/settings" element={<SettingPage user={user} setUser={setUser}/>}/>
 	  <Route path="/settings" element={<Navigate to="/users/settings" replace/>}/>
 	  <Route path="/tos" element={<TOSPage/>}/>
+	  <Route path="/gekanator" element={
+	      <AdminOnly user={user}>
+		<GekanatorPage/>
+	      </AdminOnly>}/>
 	  <Route path="/more" element={<MorePage/>}/>
 	  <Route path="*" element={<NotFound/>}/>
 	</Routes>
@@ -87,6 +92,16 @@ const RouteTransitionWrapper = ({ user, setUser }: {
 }
 
 
+const AdminOnly = ({ user, children }: {
+    user:      User | null
+    children:  ReactNode }) => {
+  if (user?.role !== 'admin')
+    return <NotFound/>
+
+  return <>{children}</>
+}
+
+
 const PostDetailRoute = ({ user }: { user: User | null }) => {
   const location = useLocation ()
   const key = location.pathname

frontend/src/components/NicoViewer.test.tsx

@@ -0,0 +1,83 @@
+import { act, fireEvent, render } from '@testing-library/react'
+import { afterEach, describe, expect, it, vi } from 'vitest'
+import { createRef } from 'react'
+
+import NicoViewer from '@/components/NicoViewer'
+
+import type { NiconicoViewerHandle } from '@/types'
+
+
+describe ('NicoViewer', () => {
+  afterEach (() => {
+    vi.useRealTimers ()
+  })
+
+  it ('does not time out after metadata reports a playable duration', () => {
+    vi.useFakeTimers ()
+
+    const onError = vi.fn ()
+    const onMetadataChange = vi.fn ()
+    const { container } = render (
+        <NicoViewer
+            id="sm12345"
+            width={640}
+            height={360}
+            onMetadataChange={onMetadataChange}
+            onError={onError}/>,
+    )
+    const iframe = container.querySelector ('iframe')
+    expect (iframe).not.toBeNull ()
+
+    fireEvent.load (iframe!)
+    act (() => {
+      window.dispatchEvent (new MessageEvent ('message', {
+        origin: 'https://embed.nicovideo.jp',
+        source: iframe!.contentWindow,
+        data:   {
+          eventName: 'playerMetadataChange',
+          data:      {
+            currentTime:           7,
+            duration:              120,
+            isVideoMetaDataLoaded: true,
+            maximumBuffered:       30,
+            muted:                 false,
+            showComment:           true,
+            volume:                1,
+          },
+        },
+      }))
+    })
+
+    act (() => {
+      vi.advanceTimersByTime (8_000)
+    })
+
+    expect (onMetadataChange).toHaveBeenCalled ()
+    expect (onError).not.toHaveBeenCalled ()
+  })
+
+  it ('seeks with milliseconds', () => {
+    const ref = createRef<NiconicoViewerHandle> ()
+    const { container } = render (
+        <NicoViewer
+            ref={ref}
+            id="sm12345"
+            width={640}
+            height={360}/>,
+    )
+    const iframe = container.querySelector ('iframe')!
+    const postMessage = vi.spyOn (iframe.contentWindow!, 'postMessage')
+
+    act (() => {
+      ref.current!.seek (7_000)
+    })
+
+    expect (postMessage).toHaveBeenCalledWith (
+        expect.objectContaining ({
+          eventName: 'seek',
+          data:      { time: 7_000 },
+        }),
+        'https://embed.nicovideo.jp',
+    )
+  })
+})

frontend/src/components/NicoViewer.tsx

@@ -129,7 +129,9 @@ export default forwardRef ((props: Props, ref: ForwardedRef<NiconicoViewerHandle
   }, [playerId, postToPlayer])
 
   const seek = useCallback ((time: number) => {
-    postToPlayer ({ eventName: 'seek', sourceConnectorType: 1, playerId, data: { time } })
+    postToPlayer (
+	{ eventName: 'seek', sourceConnectorType: 1, playerId,
+	  data: { time } })
   }, [playerId, postToPlayer])
 
   const mute = useCallback (() => {
@@ -202,6 +204,9 @@ export default forwardRef ((props: Props, ref: ForwardedRef<NiconicoViewerHandle
 
       if (data.eventName === 'playerMetadataChange')
 	{
+	  if (Number.isFinite (data.data.duration) && data.data.duration > 0)
+	    clearLoadCompleteTimer ()
+
 	  onMetadataChange?.(data.data)
 	  return
 	}

frontend/src/components/PostEmbed.test.tsx

@@ -8,12 +8,19 @@ const dialogue = vi.hoisted (() => ({
   confirm: vi.fn (),
 }))
 
+const nicoViewer = vi.hoisted (() => ({
+  props: vi.fn (),
+}))
+
 vi.mock ('@/components/dialogues/DialogueProvider', () => ({
   useDialogue: () => dialogue,
 }))
 
 vi.mock ('@/components/NicoViewer', () => ({
-  default: ({ id }: { id: string }) => <div>Nico:{id}</div>,
+  default: (props: { id: string }) => {
+    nicoViewer.props (props)
+    return <div>Nico:{props.id}</div>
+  },
 }))
 
 vi.mock ('react-youtube', () => ({
@@ -31,6 +38,64 @@ describe ('PostEmbed', () => {
     expect (screen.getByText ('Nico:sm12345')).toBeInTheDocument ()
   })
 
+  it ('reports niconico metadata as milliseconds', () => {
+    const onVideoReady = vi.fn ()
+    const onPlaybackChange = vi.fn ()
+    render (
+        <PostEmbed
+            post={buildPost ({ url: 'https://www.nicovideo.jp/watch/sm12345' })}
+            onVideoReady={onVideoReady}
+            onPlaybackChange={onPlaybackChange}/>,
+    )
+
+    nicoViewer.props.mock.calls[0][0].onMetadataChange ({
+      currentTime:            7_000,
+      duration:               120_000,
+      isVideoMetaDataLoaded:  true,
+      maximumBuffered:        30,
+      muted:                  false,
+      showComment:            true,
+      volume:                 1,
+    })
+
+    expect (onVideoReady).toHaveBeenCalledWith (120_000)
+    expect (onPlaybackChange).toHaveBeenCalledWith (7_000)
+  })
+
+  it ('reports niconico video readiness only once', () => {
+    const onVideoReady = vi.fn ()
+    render (
+        <PostEmbed
+            post={buildPost ({ url: 'https://www.nicovideo.jp/watch/sm12345' })}
+            onVideoReady={onVideoReady}/>,
+    )
+
+    nicoViewer.props.mock.calls[0][0].onLoadComplete ({
+      title:           '動画',
+      videoId:         'sm12345',
+      lengthInSeconds: 120,
+      thumbnailUrl:    'https://example.com/thumb.jpg',
+      description:     '',
+      viewCount:       1,
+      commentCount:    2,
+      mylistCount:     3,
+      postedAt:        '2026-01-02T03:04:05.000Z',
+      watchId:         12345,
+    })
+    nicoViewer.props.mock.calls[0][0].onMetadataChange ({
+      currentTime:            7_000,
+      duration:               120_000,
+      isVideoMetaDataLoaded:  true,
+      maximumBuffered:        30,
+      muted:                  false,
+      showComment:            true,
+      volume:                 1,
+    })
+
+    expect (onVideoReady).toHaveBeenCalledTimes (1)
+    expect (onVideoReady).toHaveBeenCalledWith (120_000)
+  })
+
   it ('embeds x/twitter status URLs', () => {
     render (<PostEmbed post={buildPost ({ url: 'https://x.com/someone/status/12345' })}/>)
 

frontend/src/components/PostEmbed.tsx

@@ -1,4 +1,4 @@
-import { useCallback, useEffect, useState } from 'react'
+import { useCallback, useEffect, useRef, useState } from 'react'
 import YoutubeEmbed from 'react-youtube'
 
 import NicoViewer from '@/components/NicoViewer'
@@ -36,6 +36,17 @@ const PostEmbed: FC<Props> = ({
   const dialogue = useDialogue ()
   const [framed, setFramed] = useState (false)
   const [youtubePlayer, setYoutubePlayer] = useState<YouTubePlayer | null> (null)
+  const niconicoVideoReadyRef = useRef (false)
+
+  const notifyNiconicoVideoReady = useCallback ((durationMs: number) => {
+    if (niconicoVideoReadyRef.current
+	|| !(Number.isFinite (durationMs))
+	|| durationMs <= 0)
+      return
+
+    niconicoVideoReadyRef.current = true
+    onVideoReady?.(durationMs)
+  }, [onVideoReady])
 
   const reportYoutubePlayback = useCallback (async (player: YouTubePlayer) => {
     const currentTime = await player.getCurrentTime ()
@@ -80,15 +91,20 @@ const PostEmbed: FC<Props> = ({
   }
 
   const handleNiconicoLoadComplete = (info: NiconicoVideoInfo) => {
-    onVideoReady?.(info.lengthInSeconds * 1_000)
+    notifyNiconicoVideoReady (info.lengthInSeconds * 1_000)
     onLoadComplete?.(info)
   }
 
   const handleNiconicoMetadataChange = (meta: NiconicoMetadata) => {
+    notifyNiconicoVideoReady (meta.duration)
     onPlaybackChange?.(meta.currentTime)
     onMetadataChange?.(meta)
   }
 
+  useEffect (() => {
+    niconicoVideoReadyRef.current = false
+  }, [post.url])
+
   useEffect (() => {
     if (!(youtubePlayer) || !(onPlaybackChange))
       return

frontend/src/lib/gekanator.ts

@@ -0,0 +1,214 @@
+import { apiPost } from '@/lib/api'
+import { fetchPosts } from '@/lib/posts'
+
+import type { Post } from '@/types'
+
+export type GekanatorAnswerValue =
+    | 'yes'
+    | 'no'
+    | 'partial'
+    | 'probably_no'
+    | 'unknown'
+
+export type GekanatorAnswerLog = {
+    questionId:    string
+    questionText:  string
+    answer:        GekanatorAnswerValue }
+
+export type GekanatorQuestionKind =
+    | 'tag'
+    | 'source'
+    | 'title'
+    | 'original_date'
+
+export type GekanatorQuestion = {
+    id:    string
+    text:  string
+    kind:  GekanatorQuestionKind
+    test:  (post: Post) => boolean }
+
+const countBy = <T extends string | number> (values: T[]): Map<T, number> => {
+  const counts = new Map<T, number> ()
+  values.forEach (value => counts.set (value, (counts.get (value) ?? 0) + 1))
+  return counts
+}
+
+
+const median = (values: number[]): number => {
+  const sorted = [...values].sort ((a, b) => a - b)
+  return sorted[Math.floor (sorted.length / 2)] ?? 0
+}
+
+
+const hostOf = (post: Post): string | null => {
+  try
+  {
+    return new URL (post.url).hostname.replace (/^www\./, '')
+  }
+  catch
+  {
+    return null
+  }
+}
+
+
+const originalYearOf = (post: Post): number | null => {
+  const value = post.originalCreatedFrom || post.originalCreatedBefore
+  if (!(value))
+    return null
+
+  const date = new Date (value)
+  if (Number.isNaN (date.getTime ()))
+    return null
+
+  return date.getFullYear ()
+}
+
+
+const tagQuestionKey = ({ category, name }: { category: string; name: string }): string =>
+    `${ category }:${ name }`
+
+
+const tagFromQuestionKey = (key: string): { category: string; name: string } => {
+  const [category, ...rest] = key.split (':')
+  return { category: category ?? '', name: rest.join (':') }
+}
+
+
+const nicoTagLabel = (name: string): string => name.replace (/^nico:/, '')
+
+
+const questionableTag = (post: Post, key: string): boolean => {
+  const { category, name } = tagFromQuestionKey (key)
+
+  return (
+    post.tags.some (tag =>
+	tag.name === name
+	&& tag.category === category
+	&& !(tag.category === 'meta')
+	&& !(tag.name.includes ('タグ希望'))
+	&& !(tag.name.includes ('bot操作'))))
+}
+
+
+export const fetchGekanatorPosts = async (): Promise<Post[]> => {
+  const limit = 200
+  const first = await fetchPosts ({
+      url: '', title: '', tags: '', match: 'all',
+      originalCreatedFrom: '', originalCreatedTo: '',
+      createdFrom: '', createdTo: '', updatedFrom: '', updatedTo: '',
+      page: 1, limit, order: 'original_created_at:desc' })
+  const posts = [...first.posts]
+  const totalPages = Math.ceil (first.count / limit)
+
+  for (let page = 2; page <= totalPages; page++)
+    {
+      const data = await fetchPosts ({
+	  url: '', title: '', tags: '', match: 'all',
+	  originalCreatedFrom: '', originalCreatedTo: '',
+	  createdFrom: '', createdTo: '', updatedFrom: '', updatedTo: '',
+	  page, limit, order: 'original_created_at:desc' })
+      posts.push (...data.posts)
+    }
+
+  return posts
+}
+
+
+export const buildGekanatorQuestions = (posts: Post[]): GekanatorQuestion[] => {
+  const tagCounts = countBy (posts.flatMap (post =>
+      post.tags
+      .filter (tag =>
+	  !(tag.category === 'meta')
+	  && !(tag.name.includes ('タグ希望'))
+	  && !(tag.name.includes ('bot操作')))
+      .map (tag => tagQuestionKey (tag))))
+  const hosts = countBy (posts.map (hostOf).filter ((host): host is string => Boolean (host)))
+  const originalYears = countBy (
+      posts
+      .map (originalYearOf)
+      .filter ((year): year is number => year !== null))
+  const titleLengthMedian = median (posts.map (post => post.title?.length ?? 0))
+
+  const usefulEntries = <T extends string | number> (counts: Map<T, number>) =>
+      [...counts.entries ()]
+      .filter (([, count]) => count > 0 && count < posts.length)
+      .sort ((a, b) => Math.abs (posts.length / 2 - a[1])
+	       - Math.abs (posts.length / 2 - b[1]))
+      .slice (0, 80)
+
+  const tagQuestions = usefulEntries (tagCounts)
+      .filter (([, count]) => count >= 2 && count <= Math.max (2, posts.length * .7))
+      .slice (0, 80)
+      .map (([key]) => {
+	const { category, name } = tagFromQuestionKey (String (key))
+	const label = category === 'nico' ? nicoTagLabel (name) : name
+
+	return {
+	    id:   `tag:${ key }`,
+	    text: category === 'nico'
+		? `ニコニコに「${ label }」といふタグが付いてゐる?`
+		: `内容として「${ label }」に関係しさう?`,
+	    kind: 'tag' as const,
+	    test: (post: Post) => questionableTag (post, String (key)) }
+      })
+
+  const sourceQuestions = usefulEntries (hosts)
+      .filter (([, count]) => count >= 2 && count <= Math.max (2, posts.length * .7))
+      .slice (0, 20)
+      .map (([host]) => ({
+	  id:   `source:${ host }`,
+	  text: `${ host } の投稿を思ひ浮かべてゐる?`,
+	  kind: 'source' as const,
+	  test: (post: Post) => hostOf (post) === host }))
+
+  const originalYearQuestions = usefulEntries (originalYears)
+      .filter (([, count]) => count >= 2 && count <= Math.max (2, posts.length * .7))
+      .slice (0, 20)
+      .map (([year]) => ({
+	  id:   `original-year:${ year }`,
+	  text: `オリジナルの投稿年は ${ year } 年?`,
+	  kind: 'original_date' as const,
+	  test: (post: Post) => originalYearOf (post) === year }))
+
+  const titleQuestions = [
+      {
+	  id:   'title:long',
+	  text: '題名が長めの投稿?',
+	  kind: 'title' as const,
+	  test: (post: Post) => (post.title?.length ?? 0) > titleLengthMedian },
+      {
+	  id:   'title:ascii',
+	  text: '題名に英数字が混じってゐる?',
+	  kind: 'title' as const,
+	  test: (post: Post) => /[A-Za-z0-9]/.test (post.title ?? '') }]
+      .filter (question => {
+	const yes = posts.filter (post => question.test (post)).length
+	const no = posts.length - yes
+	return yes >= 2 && no >= 2 && yes <= posts.length * .7 && no <= posts.length * .7
+      })
+
+  return [
+      ...sourceQuestions,
+      ...originalYearQuestions,
+      ...titleQuestions,
+      ...tagQuestions]
+}
+
+
+export const saveGekanatorGame = async ({
+    guessedPostId,
+    correctPostId,
+    answers,
+}: {
+    guessedPostId:  number
+    correctPostId:  number
+    answers:        GekanatorAnswerLog[]
+}): Promise<{ id: number }> =>
+    await apiPost ('/gekanator/games', {
+	guessed_post_id: guessedPostId,
+	correct_post_id: correctPostId,
+	answers: answers.map (answer => ({
+	    question_id: answer.questionId,
+	    question_text: answer.questionText,
+	    answer: answer.answer })) })

frontend/src/lib/queryKeys.ts

@@ -8,6 +8,10 @@ export const postsKeys = {
     changes:  (p: { post?: string; tag?: string; page: number; limit: number }) =>
 		  ['posts', 'changes', p] as const }
 
+export const gekanatorKeys = {
+    root:   ['gekanator'] as const,
+    posts:  () => ['gekanator', 'posts'] as const }
+
 export const tagsKeys = {
     root:         ['tags'] as const,
     index:        (p: FetchTagsParams) => ['tags', 'index', p] as const,

frontend/src/pages/theatres/TheatreDetailPage.test.tsx

@@ -1,4 +1,4 @@
-import { fireEvent, screen, waitFor } from '@testing-library/react'
+import { act, fireEvent, screen, waitFor } from '@testing-library/react'
 import { Route, Routes } from 'react-router-dom'
 import { beforeEach, describe, expect, it, vi } from 'vitest'
 
@@ -31,15 +31,31 @@ const dialogue = vi.hoisted (() => ({
   confirm: vi.fn (),
 }))
 
+const postEmbed = vi.hoisted (() => ({
+  props: vi.fn (),
+  play:  vi.fn (),
+  seek:  vi.fn (),
+}))
+
 vi.mock ('@/lib/api', () => api)
 vi.mock ('@/lib/posts', () => postsApi)
 vi.mock ('@/components/dialogues/DialogueProvider', () => ({
   useDialogue: () => dialogue,
 }))
 vi.mock ('@/components/PostEmbed', () => ({
-  default: ({ post }: {
+  default: (props: {
+    ref?: { current: unknown }
     post: { title: string | null; url: string }
-  }) => <div>Embed:{post.title || post.url}</div>,
+  }) => {
+    postEmbed.props (props)
+    if (props.ref)
+      props.ref.current = {
+        play: postEmbed.play,
+        seek: postEmbed.seek,
+      }
+
+    return <div>Embed:{props.post.title || props.post.url}</div>
+  },
 }))
 vi.mock ('@/components/PostEditForm', () => ({
   default: () => <div>Post edit form</div>,
@@ -153,6 +169,7 @@ const mockDefaultApi = () => {
 
 describe ('TheatreDetailPage', () => {
   beforeEach (() => {
+    vi.useRealTimers ()
     vi.clearAllMocks ()
     mockDefaultApi ()
   })
@@ -188,6 +205,83 @@ describe ('TheatreDetailPage', () => {
       .toBeInTheDocument ()
   })
 
+  it ('does not seek to zero while applying video length from the player', async () => {
+    api.apiPut.mockImplementation ((path: string) => {
+      switch (path)
+      {
+      case '/theatres/7/watching':
+        return Promise.resolve (buildTheatreInfo ({
+          hostFlg:       true,
+          postId:        currentPost.id,
+          postStartedAt: '2026-01-02T03:04:05.000Z',
+          postElapsedMs: 7_000,
+          watchingUsers: [{ id: 1, name: 'tester' }],
+          skipVote:      {
+            votesCount:         0,
+            requiredCount:      2,
+            watchingUsersCount: 1,
+            voted:              false,
+          },
+        }))
+
+      default:
+        return Promise.reject (new Error (`Unexpected PUT ${ path }`))
+      }
+    })
+
+    renderPage ()
+    await screen.findByText ('Embed:上映中の投稿')
+
+    const props = postEmbed.props.mock.calls.at (-1)![0]
+
+    act (() => {
+      props.onVideoReady (120_000)
+      props.onPlaybackChange (0)
+    })
+
+    const seekMs = postEmbed.seek.mock.calls[0][0]
+    expect (seekMs).toBeGreaterThanOrEqual (7_000)
+    expect (seekMs).toBeLessThan (10_000)
+    expect (postEmbed.seek).not.toHaveBeenCalledWith (0)
+  })
+
+  it ('does not advance host post while video length is unknown', async () => {
+    api.apiPut.mockImplementation ((path: string) => {
+      switch (path)
+      {
+      case '/theatres/7/watching':
+        return Promise.resolve (buildTheatreInfo ({
+          hostFlg:       true,
+          postId:        currentPost.id,
+          postStartedAt: '2026-01-02T03:04:05.000Z',
+          postElapsedMs: 4_000,
+          watchingUsers: [{ id: 1, name: 'tester' }],
+          skipVote:      {
+            votesCount:         0,
+            requiredCount:      2,
+            watchingUsersCount: 1,
+            voted:              false,
+          },
+        }))
+
+      default:
+        return Promise.reject (new Error (`Unexpected PUT ${ path }`))
+      }
+    })
+
+    renderPage ()
+
+    await screen.findByText ('Embed:上映中の投稿')
+    await waitFor (() => {
+      expect (api.apiPut).toHaveBeenCalledWith ('/theatres/7/watching')
+    })
+
+    await waitFor (() => {
+      expect (api.apiPut).toHaveBeenCalledTimes (2)
+    }, { timeout: 2_500 })
+    expect (api.apiPatch).not.toHaveBeenCalledWith ('/theatres/7/next_post')
+  })
+
   it ('deletes an owned comment after confirmation', async () => {
     renderPage ()
 

frontend/src/pages/theatres/TheatreDetailPage.tsx

@@ -443,9 +443,13 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
     if (!(theatreInfo.postStartedAt))
       return
 
+    const currentVideoLength = videoLengthRef.current
+    if (currentVideoLength <= 0)
+      return
+
     const targetTime = Math.min (
 	currentPostElapsedMs (theatreInfo),
-	videoLength)
+	currentVideoLength)
 
     const drift = Math.abs (currentTimeMs - targetTime)
 
@@ -477,6 +481,7 @@ const TheatreDetailPage: FC<Props> = ({ user }: Props) => {
 	: 0
 
     setVideoLength (playableDurationMs)
+    videoLengthRef.current = playableDurationMs
 
     if (playableDurationMs <= 0)
       {