biblical-guide.com/lib/sync-conflict-resolver.ts

import { BibleHighlight } from '@/types'

/**
 * Resolves conflicts between client and server versions of a highlight.
 * Uses timestamp-based "last write wins" strategy.
 */
export function resolveConflict(
  clientVersion: BibleHighlight,
  serverVersion: BibleHighlight
): BibleHighlight {
  // Use timestamp to determine which version is newer
  const isServerNewer = serverVersion.updatedAt > clientVersion.updatedAt

  // Take the newer version and mark as synced
  const resolvedVersion = isServerNewer ? serverVersion : clientVersion

  return {
    ...resolvedVersion,
    syncStatus: 'synced' as const
  }
}

/**
 * Merges server highlights with client highlights.
 * - Adds new highlights from server
 * - Updates existing highlights if server version is newer
 * - Keeps client highlights if client version is newer
 */
export function mergeHighlights(
  clientHighlights: BibleHighlight[],
  serverHighlights: BibleHighlight[]
): BibleHighlight[] {
  const clientMap = new Map(clientHighlights.map(h => [h.id, h]))
  const serverMap = new Map(serverHighlights.map(h => [h.id, h]))

  const merged = new Map<string, BibleHighlight>()

  // Add all client highlights, resolving conflicts with server
  for (const [id, clientH] of clientMap) {
    const serverH = serverMap.get(id)
    if (serverH) {
      // Conflict: both have this highlight
      merged.set(id, resolveConflict(clientH, serverH))
    } else {
      // No conflict: only client has it
      merged.set(id, clientH)
    }
  }

  // Add any server highlights not in client
  for (const [id, serverH] of serverMap) {
    if (!clientMap.has(id)) {
      merged.set(id, { ...serverH, syncStatus: 'synced' as const })
    }
  }

  return Array.from(merged.values())
}