WelcomeService: Show a specific welcome message only once

We should avoid pestering users with the same message over and over again.
Better to show it once, remember the SHA-1 checksum of the displayed
message, and then only show the welcome message again when the application
version number *and* the welcome message changes.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

src/main/java/org/scijava/welcome/DefaultWelcomeService.java

@@ -33,6 +33,9 @@
 
 import java.io.File;
 import java.io.IOException;
+import java.io.UnsupportedEncodingException;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
 
 import org.scijava.app.AppService;
 import org.scijava.command.CommandService;
@@ -61,6 +64,7 @@ public class DefaultWelcomeService extends AbstractService implements
 {
 
 	private final static String WELCOME_FILE = "WELCOME.md";
+	private final static String CHECKSUM_PREFS_KEY = "checksum";
 
 	@Parameter
 	private LogService log;
@@ -89,6 +93,10 @@ public void displayWelcome() {
 		try {
 			if (welcomeFile.exists()) {
 				final String welcomeText = textService.asHTML(welcomeFile);
+				final String checksum = getChecksum(welcomeText);
+				final String previousChecksum = Prefs.get(getClass(), CHECKSUM_PREFS_KEY);
+				if (checksum.equals(previousChecksum)) return;
+				Prefs.put(getClass(), CHECKSUM_PREFS_KEY, checksum);
 				displayService.createDisplay(welcomeText);
 			}
 		}
@@ -126,4 +134,32 @@ private String firstRunPrefKey() {
 		return "firstRun-" + appService.getApp().getVersion();
 	}
 
+	// get digest of the file as according to fullPath
+	private String getChecksum(final String text)
+	{
+		try {
+			final MessageDigest digest = MessageDigest.getInstance("SHA-1");
+			digest.update(text.getBytes("UTF-8"));
+			return toHex(digest.digest());
+		}
+		catch (NoSuchAlgorithmException e) {
+			return "" + text.hashCode();
+		}
+		catch (UnsupportedEncodingException e) {
+			return "" + text.hashCode();
+		}
+	}
+
+	private final static char[] hex = { '0', '1', '2', '3', '4', '5', '6', '7',
+		'8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };
+
+	private String toHex(final byte[] bytes) {
+		final char[] buffer = new char[bytes.length * 2];
+		for (int i = 0; i < bytes.length; i++) {
+			buffer[i * 2] = hex[(bytes[i] & 0xf0) >> 4];
+			buffer[i * 2 + 1] = hex[bytes[i] & 0xf];
+		}
+		return new String(buffer);
+	}
+
 }