fix crc32 calculation

card10badge/Bluetooth/DataTransferManager.swift

@@ -51,6 +51,7 @@ class DataTransferManager: NSObject {
     var lastPackageSentAt: TimeInterval?
     var dataToSend: Data?
     var currentChunkToSend: Data?
+    var currentChunkToSendCrc32: UInt32 = 0
     var retryCount: Int = 0
 
     init(_ delegateToUse: DataTransferManagerDelegate) {
@@ -78,7 +79,7 @@ class DataTransferManager: NSObject {
         case .CHUNK_ACK?:
             self.transferState = .CHUNK_SENT
             // calculate CRC and send new chunk
-            let range: Range<Data.Index> = 0..<1
+            let range: Range<Data.Index> = 1..<5
             let crc = package.subdata(in: range)
             updateProgress()
             if checkChunkWithResponse(crc) {
@@ -156,6 +157,8 @@ class DataTransferManager: NSObject {
             var int = UInt32(bigEndian: UInt32(offset))
             payloadToSend?.append(Data(bytes: &int, count: MemoryLayout.size(ofValue: UInt32())))
             payloadToSend?.append(payload!)
+            // pre-calculate the expected CRC sum for that packet (must include header and offset)
+            currentChunkToSendCrc32 = CRC32.checksum(bytes: payloadToSend!.bytes)
             lastPackageSentAt = NSDate.init().timeIntervalSinceNow
 
         case .CHUNK_ACK:
@@ -181,17 +184,8 @@ class DataTransferManager: NSObject {
     }
 
     func checkChunkWithResponse(_ response: Data) -> Bool {
-
-        return true
-
-        //FIXME: This CRC32 implementation doesn't work. Disabled for now
-        let responseInUInt32 = response.withUnsafeBytes { $0.load(as: UInt8.self) }
-        let chunkInUInt8 = [UInt8](self.currentChunkToSend!)
-        let checksum = CRC32.checksum(bytes: chunkInUInt8)
-        if responseInUInt32 == checksum {
-            return true
-        }
-        return false
+        // compares received crc32 with the previously calculated checksum
+        return response.withUnsafeBytes { $0.load(as: UInt32.self) } == currentChunkToSendCrc32
     }
 
     func updateProgress() {

card10badge/Bluetooth/UInt+Extension.swift

@@ -21,3 +21,9 @@ extension isDataInt {
 
 extension UInt16: isDataInt { }
 extension UInt64: isDataInt { }
+
+extension Data {
+    var bytes: [UInt8] {
+        return [UInt8](self)
+    }
+}