feat: implement sync conflict resolver with timestamp-based merging

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Co-Authored-By: Claude <noreply@anthropic.com>

__tests__/lib/sync-conflict-resolver.test.ts

@@ -0,0 +1,75 @@
+import { resolveConflict } from '@/lib/sync-conflict-resolver'
+import { BibleHighlight } from '@/types'
+
+describe('SyncConflictResolver', () => {
+  it('should prefer server version if newer', () => {
+    const clientVersion: BibleHighlight = {
+      id: 'h-1',
+      verseId: 'v-1',
+      color: 'yellow',
+      createdAt: 1000,
+      updatedAt: 1000,
+      syncStatus: 'pending'
+    }
+
+    const serverVersion: BibleHighlight = {
+      id: 'h-1',
+      verseId: 'v-1',
+      color: 'blue',
+      createdAt: 1000,
+      updatedAt: 2000, // newer
+      syncStatus: 'synced'
+    }
+
+    const result = resolveConflict(clientVersion, serverVersion)
+    expect(result.color).toBe('blue')
+    expect(result.updatedAt).toBe(2000)
+  })
+
+  it('should prefer client version if newer', () => {
+    const clientVersion: BibleHighlight = {
+      id: 'h-1',
+      verseId: 'v-1',
+      color: 'blue',
+      createdAt: 1000,
+      updatedAt: 3000, // newer
+      syncStatus: 'pending'
+    }
+
+    const serverVersion: BibleHighlight = {
+      id: 'h-1',
+      verseId: 'v-1',
+      color: 'yellow',
+      createdAt: 1000,
+      updatedAt: 2000,
+      syncStatus: 'synced'
+    }
+
+    const result = resolveConflict(clientVersion, serverVersion)
+    expect(result.color).toBe('blue')
+    expect(result.updatedAt).toBe(3000)
+  })
+
+  it('should mark as synced after resolution', () => {
+    const clientVersion: BibleHighlight = {
+      id: 'h-1',
+      verseId: 'v-1',
+      color: 'yellow',
+      createdAt: 1000,
+      updatedAt: 2000,
+      syncStatus: 'pending'
+    }
+
+    const serverVersion: BibleHighlight = {
+      id: 'h-1',
+      verseId: 'v-1',
+      color: 'yellow',
+      createdAt: 1000,
+      updatedAt: 2000,
+      syncStatus: 'synced'
+    }
+
+    const result = resolveConflict(clientVersion, serverVersion)
+    expect(result.syncStatus).toBe('synced')
+  })
+})

lib/sync-conflict-resolver.ts

@@ -0,0 +1,58 @@
+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())
+}