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// Mobile BFF: Network-Aware Response Strategy
 
interface NetworkContext {
  connectionType: 'wifi' | '4g' | '3g' | '2g' | 'slow-2g' | 'offline';
  effectiveBandwidthMbps: number;
  roundTripTimeMs: number;
  dataSaverEnabled: boolean;
}
 
class NetworkAdaptiveBFF {
  
  async getHomeScreen(
    userId: string, 
    network: NetworkContext
  ): Promise<HomeScreenResponse> {
    
    // Determine quality tier based on network conditions
    const qualityTier = this.determineQualityTier(network);
    
    // Adjust data fetching strategy
    const fetchStrategy = this.selectFetchStrategy(qualityTier);
    
    // Execute with appropriate concurrency and timeouts
    const data = await this.orchestrate(userId, fetchStrategy);
    
    // Transform with network-appropriate image sizes and data depth
    return this.transform(data, qualityTier);
  }
 
  private determineQualityTier(network: NetworkContext): QualityTier {
    // Data saver always gets minimal tier
    if (network.dataSaverEnabled) {
      return QualityTier.MINIMAL;
    }
 
    // Network type based degradation
    switch (network.connectionType) {
      case 'wifi':
        return QualityTier.FULL;
      case '4g':
        return network.effectiveBandwidthMbps > 10 
          ? QualityTier.STANDARD 
          : QualityTier.REDUCED;
      case '3g':
        return QualityTier.REDUCED;
      case '2g':
      case 'slow-2g':
        return QualityTier.MINIMAL;
      case 'offline':
        throw new OfflineError('No network connectivity');
    }
  }
 
  private selectFetchStrategy(tier: QualityTier): FetchStrategy {
    return {
      // Number of items to fetch
      itemLimits: {
        [QualityTier.FULL]: { recommendations: 30, continueWatching: 20 },
        [QualityTier.STANDARD]: { recommendations: 15, continueWatching: 10 },
        [QualityTier.REDUCED]: { recommendations: 8, continueWatching: 5 },
        [QualityTier.MINIMAL]: { recommendations: 4, continueWatching: 3 },
      }[tier],
      
      // Timeout configuration
      timeouts: {
        [QualityTier.FULL]: { primary: 3000, secondary: 2000 },
        [QualityTier.STANDARD]: { primary: 5000, secondary: 3000 },
        [QualityTier.REDUCED]: { primary: 8000, secondary: 5000 },
        [QualityTier.MINIMAL]: { primary: 12000, secondary: 8000 },
      }[tier],
      
      // Which optional data to fetch
      optionalData: {
        [QualityTier.FULL]: ['personalization', 'socialActivity', 'trending'],
        [QualityTier.STANDARD]: ['personalization', 'trending'],
        [QualityTier.REDUCED]: ['trending'],
        [QualityTier.MINIMAL]: [],
      }[tier],
      
      // Image quality selection
      imageQuality: {
        [QualityTier.FULL]: 'high',
        [QualityTier.STANDARD]: 'medium',
        [QualityTier.REDUCED]: 'low',
        [QualityTier.MINIMAL]: 'placeholder',
      }[tier],
    };
  }
}
 
enum QualityTier {
  FULL = 'full',
  STANDARD = 'standard', 
  REDUCED = 'reduced',
  MINIMAL = 'minimal'
}

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// Mobile BFF: Speculative data inclusion
 
interface HomeScreenRequest {
  includeNavigation?: 'none' | 'likely' | 'all';
  prefetchScreens?: string[];
}
 
async function getHomeScreenWithPrefetch(
  userId: string,
  request: HomeScreenRequest
): Promise<HomeScreenWithPrefetch> {
  
  const homeData = await this.getHomeScreen(userId);
  
  // Determine what the user is likely to tap next
  const prefetchTargets = request.prefetchScreens || 
    this.predictNextScreens(homeData, userId);
  
  // Fetch those screens' data speculatively
  const prefetchedData = await Promise.all(
    prefetchTargets.map(screen => 
      this.getScreenData(userId, screen).catch(() => null)
    )
  );
  
  return {
    ...homeData,
    
    // Embed prefetched data in response
    _prefetch: Object.fromEntries(
      prefetchTargets
        .map((screen, i) => [screen, prefetchedData[i]])
        .filter(([, data]) => data !== null)
    )
  };
}
 
function predictNextScreens(
  homeData: HomeScreenData, 
  userId: string
): string[] {
  // Use ML model or simple heuristics
  const predictions: string[] = [];
  
  // Most users check their "continue watching" first
  if (homeData.continueWatching.length > 0) {
    predictions.push(`/content/${homeData.continueWatching[0].id}`);
  }
  
  // Top recommendation is commonly tapped
  if (homeData.recommendations.length > 0) {
    predictions.push(`/content/${homeData.recommendations[0].id}`);
  }
  
  // Search is frequently accessed from home
  predictions.push('/search/trending');
  
  return predictions.slice(0, 3); // Limit prefetch size
}

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// Mobile BFF: Sync-aware endpoint design
 
interface SyncRequest {
  lastSyncTimestamp: number;        // Unix millis
  localChanges: LocalChange[];       // Pending offline changes
  deviceId: string;
}
 
interface SyncResponse {
  serverTimestamp: number;
  
  // Items modified since lastSyncTimestamp
  modified: {
    users: UserDelta[];
    content: ContentDelta[];
    watchHistory: WatchHistoryDelta[];
  };
  
  // Items deleted since lastSyncTimestamp
  deleted: {
    contentIds: string[];
    watchHistoryIds: string[];
  };
  
  // Resolution of locally-made changes
  changeResults: ChangeResult[];
  
  // For pagination of large sync sets
  hasMore: boolean;
  continuationToken?: string;
}
 
interface ChangeResult {
  localId: string;
  status: 'applied' | 'conflict' | 'rejected';
  serverId?: string;            // If new entity was created
  conflictDetails?: {
    serverVersion: any;         // For client-side resolution UI
    conflictType: 'concurrent_edit' | 'deleted' | 'permission';
  };
}
 
async function handleSync(request: SyncRequest): Promise<SyncResponse> {
  const serverTime = Date.now();
  
  // 1. Apply local changes first (with conflict detection)
  const changeResults = await this.applyLocalChanges(
    request.localChanges,
    request.deviceId
  );
  
  // 2. Calculate deltas since last sync
  const deltas = await this.calculateDeltas(
    request.lastSyncTimestamp,
    serverTime
  );
  
  // 3. Check if delta set is too large
  const { data, hasMore, token } = this.paginate(deltas, {
    maxItems: 500,
    maxBytes: 256 * 1024  // 256KB max per sync response
  });
  
  return {
    serverTimestamp: serverTime,
    modified: data.modified,
    deleted: data.deleted,
    changeResults,
    hasMore,
    continuationToken: hasMore ? token : undefined
  };
}

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// Web BFF: SSR-optimized endpoint
 
interface SSRPageRequest {
  path: string;
  query: Record<string, string>;
  cookies: Record<string, string>;
  userAgent: string;
}
 
interface SSRPageResponse {
  // Page-specific data
  data: any;
  
  // SEO metadata for <head>
  seo: {
    title: string;
    description: string;
    canonicalUrl: string;
    ogImage?: string;
    structuredData?: object;  // JSON-LD
  };
  
  // Cache control directives
  cache: {
    maxAge: number;
    staleWhileRevalidate?: number;
    private: boolean;
    varyHeaders: string[];
  };
  
  // Preload hints for browsers
  preloadHints: Array<{
    href: string;
    as: 'image' | 'script' | 'style' | 'font';
    crossorigin?: 'anonymous' | 'use-credentials';
  }>;
}
 
async function getPageForSSR(request: SSRPageRequest): Promise<SSRPageResponse> {
  const route = this.router.match(request.path);
  const userId = this.auth.extractUserId(request.cookies);
  
  // Fetch all data needed for complete page render
  const pageData = await route.handler.getFullPageData(userId, request.query);
  
  // Generate SEO metadata
  const seo = route.handler.generateSEO(pageData, request);
  
  // Determine cache strategy based on auth state
  const cache = userId 
    ? { maxAge: 0, private: true, varyHeaders: ['Cookie'] }
    : { 
        maxAge: 300, 
        staleWhileRevalidate: 3600, 
        private: false,
        varyHeaders: ['Accept-Encoding']
      };
  
  // Compute critical resource preloads
  const preloadHints = this.computePreloadHints(pageData, request);
  
  return { data: pageData, seo, cache, preloadHints };
}

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// Mobile BFF: Device registration (no web equivalent needed)
 
interface DeviceRegistration {
  deviceId: string;           // Persistent device identifier
  platform: 'ios' | 'android';
  osVersion: string;
  appVersion: string;
  pushToken?: string;         // For push notifications
  capabilities: {
    biometric: boolean;
    nfc: boolean;
    camera: boolean;
  };
}
 
interface DeviceSession {
  sessionToken: string;
  refreshToken: string;
  expiresAt: number;
  deviceTrustLevel: 'new' | 'known' | 'trusted';
}
 
async function registerDevice(
  userId: string,
  device: DeviceRegistration
): Promise<DeviceSession> {
  // Check if device is known
  const existingDevice = await this.deviceStore.find(
    userId, 
    device.deviceId
  );
  
  let trustLevel: DeviceTrustLevel;
  if (!existingDevice) {
    trustLevel = 'new';
    await this.notifyNewDevice(userId, device);
  } else if (existingDevice.lastSeen > Date.now() - 30 * 24 * 60 * 60 * 1000) {
    trustLevel = 'trusted';  // Used within last 30 days
  } else {
    trustLevel = 'known';    // Known but stale
  }
  
  // Update device record
  await this.deviceStore.upsert(userId, device);
  
  // Issue tokens with trust-appropriate expiry
  const tokenExpiry = {
    new: 15 * 60 * 1000,      // 15 min for new devices
    known: 60 * 60 * 1000,    // 1 hour for known
    trusted: 7 * 24 * 60 * 60 * 1000  // 7 days for trusted
  }[trustLevel];
  
  return this.issueSession(userId, device.deviceId, tokenExpiry, trustLevel);
}

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// Mobile BFF: Version-aware response shaping
 
interface ClientVersion {
  major: number;
  minor: number;
  patch: number;
}
 
function parseClientVersion(header: string): ClientVersion {
  const [major, minor, patch] = header.split('.').map(Number);
  return { major, minor, patch };
}
 
function compareVersions(a: ClientVersion, b: ClientVersion): number {
  if (a.major !== b.major) return a.major - b.major;
  if (a.minor !== b.minor) return a.minor - b.minor;
  return a.patch - b.patch;
}
 
class VersionAwareBFF {
  async getContent(contentId: string, clientVersion: ClientVersion) {
    const content = await this.contentService.get(contentId);
    
    // Version 2.5.0 introduced new content types
    if (compareVersions(clientVersion, { major: 2, minor: 5, patch: 0 }) < 0) {
      // Older clients don't understand 'interactive' content
      if (content.type === 'interactive') {
        return this.transformToLegacyFormat(content);
      }
    }
    
    // Version 3.0.0 changed pricing display
    if (compareVersions(clientVersion, { major: 3, minor: 0, patch: 0 }) < 0) {
      content.pricing = this.convertToLegacyPricing(content.pricing);
    }
    
    // Version 3.2.0 added new fields - older clients ignore them
    // Safe to include for all versions due to forward compatibility
    
    return content;
  }
 
  private transformToLegacyFormat(content: InteractiveContent): LegacyContent {
    // Convert interactive content to static format for old clients
    return {
      type: 'article',
      title: content.title,
      body: this.renderInteractiveToStatic(content),
      // Lose interactivity but remain functional
    };
  }
}
 
// Minimum supported version configuration
const MINIMUM_SUPPORTED_VERSIONS = {
  ios: { major: 2, minor: 0, patch: 0 },
  android: { major: 2, minor: 1, patch: 0 },
};
 
function isVersionSupported(
  platform: 'ios' | 'android', 
  version: ClientVersion
): boolean {
  return compareVersions(version, MINIMUM_SUPPORTED_VERSIONS[platform]) >= 0;
}