Swap MD5 hash to CRC32 for checksum calculation (#39)

Swap cryptographic MD5 hash for NCHF CRC32 for checksum calculation

lib/src/main/java/com/dynatrace/file/util/PollBasedFilePoller.java

@@ -16,38 +16,30 @@
 import java.io.IOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
-import java.security.MessageDigest;
-import java.security.NoSuchAlgorithmException;
 import java.time.Duration;
-import java.util.Arrays;
 import java.util.concurrent.*;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.logging.Logger;
+import java.util.zip.CRC32;
 
 class PollBasedFilePoller extends FilePoller {
   private static final Logger LOGGER = Logger.getLogger(PollBasedFilePoller.class.getName());
   private static final String LOG_MESSAGE_FAILED_FILE_READ = "Failed to read file %s. Error: %s";
 
   private final AtomicBoolean changedSinceLastInquiry = new AtomicBoolean(false);
-  private byte[] prevChecksumBytes;
 
   private final ScheduledFuture<?> worker;
   private final ScheduledExecutorService executorService;
 
-  private MessageDigest md5;
+  private final CRC32 crc32 = new CRC32();
+  private Long prevChecksumValue = null;
 
   protected PollBasedFilePoller(Path filePath, Duration pollInterval) {
     super(filePath);
     if (pollInterval == null) {
       throw new IllegalArgumentException("Poll interval cannot be null");
     }
 
-    try {
-      md5 = MessageDigest.getInstance("MD5");
-    } catch (NoSuchAlgorithmException ignored) {
-      // ignore, since this is not something dependent on the code. MD5 should always be there.
-    }
-
     executorService =
         Executors.newSingleThreadScheduledExecutor(
             new ThreadFactory() {
@@ -63,9 +55,6 @@ public Thread newThread(Runnable r) {
     worker =
         executorService.scheduleAtFixedRate(
             this::poll, pollInterval.toNanos(), pollInterval.toNanos(), TimeUnit.NANOSECONDS);
-
-    // read the initial checksum
-    prevChecksumBytes = getChecksumBytes();
   }
 
   @Override
@@ -75,36 +64,36 @@ public boolean fileContentsUpdated() {
   }
 
   private synchronized void poll() {
-    byte[] latestChecksumBytes = getChecksumBytes();
-
-    if (!Arrays.equals(latestChecksumBytes, prevChecksumBytes)) {
-      prevChecksumBytes = latestChecksumBytes;
-
-      // The file did exist before (prevChecksumBytes != null) but doesn't anymore
-      // (latestChecksumBytes == null).
-      // This code here is only reached when prevChecksumBytes != latestChecksumBytes.
-      // This means the file has been deleted since the last poll, which should not trigger a
-      // state change.
-      if (latestChecksumBytes != null) {
+    Long latestChecksumValue = getChecksumValue();
+
+    // | latest | previous | outcome                 |
+    // | ------ | -------- | ----------------------- |
+    // | null   | null     | do nothing              |
+    // | null   | !null    | do nothing              |
+    // | !null  | null     | update (no file before) |
+    // | !null  | !null    | update if changed       |
+    if (latestChecksumValue != null) {
+      if (!latestChecksumValue.equals(prevChecksumValue)) {
         changedSinceLastInquiry.set(true);
       }
+      prevChecksumValue = latestChecksumValue;
     }
   }
 
-  private synchronized byte[] getChecksumBytes() {
-    byte[] bytes = null;
+  private synchronized Long getChecksumValue() {
+    crc32.reset();
     try {
-      bytes = md5.digest(Files.readAllBytes(absoluteFilePath));
+      byte[] fileBytes = Files.readAllBytes(absoluteFilePath);
+      crc32.update(fileBytes, 0, fileBytes.length);
     } catch (IOException e) {
       LOGGER.warning(() -> String.format(LOG_MESSAGE_FAILED_FILE_READ, absoluteFilePath, e));
-    } finally {
-      md5.reset();
+      return null;
     }
-    return bytes;
+    return crc32.getValue();
   }
 
   @Override
-  public void close() throws IOException {
+  public void close() {
     worker.cancel(true);
     executorService.shutdown();
     try {

lib/src/test/java/com/dynatrace/file/util/FilePollerTestHelpers.java

@@ -75,8 +75,10 @@ static void filePollerUpdatesOnlyOnCreateAndChangeNotOnDelete(FilePoller poller,
 
     // create the file again
     Files.createFile(path);
-    await().atMost(1, TimeUnit.SECONDS).until(poller::fileContentsUpdated);
+    // creating an empty file does not trigger an update (no new information to be read)
+    await().atMost(1, TimeUnit.SECONDS).until(() -> !poller.fileContentsUpdated());
 
+    // adding new content to the new file will trigger an update.
     Files.write(path, "Some content".getBytes());
     await().atMost(1, TimeUnit.SECONDS).until(poller::fileContentsUpdated);
   }