[#1039] Implement ChecksBeforeSyncAction

Closes #1039

Sources/ZcashLightClientKit/Block/Actions/ChecksBeforeSyncAction.swift

@@ -8,7 +8,37 @@
 import Foundation
 
 class ChecksBeforeSyncAction {
-    init(container: DIContainer) { }
+    let internalSyncProgress: InternalSyncProgress
+    let storage: CompactBlockRepository
+    init(container: DIContainer) {
+        internalSyncProgress = container.resolve(InternalSyncProgress.self)
+        storage = container.resolve(CompactBlockRepository.self)
+    }
+
+    /// Tells whether the state represented by these sync ranges evidence some sort of
+    /// outdated state on the cache or the internal state of the compact block processor.
+    ///
+    /// - Note: this can mean that the processor has synced over the height that the internal
+    /// state knows of because the sync process was interrupted before it could reflect
+    /// it in the internal state storage. This could happen because of many factors, the
+    /// most feasible being OS shutting down a background process or the user abruptly
+    /// exiting the app.
+    /// - Returns: an ``Optional<BlockHeight>`` where Some represents what's the
+    /// new state the internal state should reflect and indicating that the cache should be cleared
+    /// as well. c`None` means that no action is required.
+    func shouldClearBlockCacheAndUpdateInternalState(syncRange: SyncRanges) -> BlockHeight? {
+        guard syncRange.downloadedButUnscannedRange != nil else {
+            return nil
+        }
+
+        guard
+            let latestScannedHeight = syncRange.latestScannedHeight,
+            let latestDownloadedHeight = syncRange.latestDownloadedBlockHeight,
+            latestScannedHeight > latestDownloadedHeight
+        else { return nil }
+
+        return latestScannedHeight
+    }
 }
 
 extension ChecksBeforeSyncAction: Action {
@@ -19,12 +49,12 @@ extension ChecksBeforeSyncAction: Action {
         // this checks if there was a sync in progress that was
         // interrupted abruptly and cache was not able to be cleared
         // properly and internal state set to the appropriate value
-//        if let newLatestDownloadedHeight = ranges.shouldClearBlockCacheAndUpdateInternalState() {
-//            try await storage.clear()
-//            await internalSyncProgress.set(newLatestDownloadedHeight, .latestDownloadedBlockHeight)
-//        } else {
-//            try await storage.create()
-//        }
+        if let newLatestDownloadedHeight = shouldClearBlockCacheAndUpdateInternalState(syncRange: await context.syncRanges) {
+            try await storage.clear()
+            await internalSyncProgress.set(newLatestDownloadedHeight, .latestDownloadedBlockHeight)
+        } else {
+            try await storage.create()
+        }
 
         await context.update(state: .fetchUTXO)
         return context