Provide folder threading support. When threading is on, messages in

folders are sorted first by subject, then by date.  Subject sorting is
case insensitive and disregards leading re:, fw: and fwd: prefixes.

Threading can be toggled via the T hotkey or the option menu.

The icons could use improvement.

Threading state is maintained during a run of K-9, among all
accounts.

res/menu/folder_message_list_option.xml

@@ -28,10 +28,10 @@
 		android:title="@string/account_settings_action"
 		android:icon="@android:drawable/ic_menu_preferences"
 	/>
-    <!-- 
-		<item android:id="@+id/search"
-		android:title="@string/search_action" />
-		<item android:id="@+id/preferences"
-		android:title="@string/preferences_action" />
-	-->
+	<item
+		android:id="@+id/thread"
+		android:title="@string/thread_action"
+		android:icon="@drawable/ic_menu_thread"
+	/>
+
 </menu>

res/values/strings.xml

@@ -60,6 +60,8 @@
     <string name="add_attachment_action">Add attachment</string>
     <string name="dump_settings_action">Dump settings</string>
     <string name="empty_trash_action">Empty Trash</string>
+    <string name="thread_action">Thread</string>
+    <string name="unthread_action">Unthread</string>
     <string name="about_action">About</string>
 
     <string name="accounts_context_menu_title">Account options</string>
@@ -111,6 +113,8 @@ Welcome to K-9 Mail setup.  K-9 is an open source email client for Android based
  * Keyboard shortcuts
  * Better IMAP support 
  * Saving attachments to SD
+ * Empty Trash
+ * Threading
  * ...and more
 \nPlease note that K-9 does not support most free hotmail accounts and, like many email clients, has some quirks when talking to Microsoft Exchange.
 \nSubmit bug reports, contribute new features and ask questions at http://code.google.com/p/k9mail
@@ -371,6 +375,6 @@ Welcome to K-9 Mail setup.  K-9 is an open source email client for Android based
     Message\u000AK, N - Next Message\u000AZ - Zoom Out\u000AShift-Z -
     Zoom In\u000aG - Flag</string>
     <string name="message_list_help_key">Del (or D) - Delete\u000AR -
-    Reply\u000AA - Reply All\u000AC - Compose\u000AF - Forward\u000aG - Flag\u000AQ
+    Reply\u000AA - Reply All\u000AC - Compose\u000AF - Forward\u000aG - Flag\u000AT - Thread\u000AQ
     - Return to Accounts</string>
 </resources>

src/com/android/email/MessagingController.java

@@ -108,6 +108,8 @@ public class MessagingController implements Runnable {
     //private Set<MessagingListener> mListeners = Collections.synchronizedSet(new HashSet<MessagingListener>());
     private Set<MessagingListener> mListeners = new CopyOnWriteArraySet<MessagingListener>();
     
+    private boolean threading = false;
+    
     private MessagingListener checkMailListener = null;
     
     private boolean mBusy;
@@ -2477,4 +2479,14 @@ s             * critical data as fast as possible, and then we'll fill in the de
         addListener(this.checkMailListener);
       }
     }
+
+    public boolean isThreading()
+    {
+      return threading;
+    }
+
+    public void setThreading(boolean threading)
+    {
+      this.threading = threading;
+    }
 }

src/com/android/email/activity/FolderMessageList.java

@@ -7,6 +7,8 @@ import java.util.Collections;
 import java.util.Date;
 import java.util.List;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
 
 import android.app.ExpandableListActivity;
 import android.app.NotificationManager;
@@ -125,6 +127,9 @@ public class FolderMessageList extends ExpandableListActivity
     private LayoutInflater mInflater;
 
     private Account mAccount;
+    
+    private Menu optionsMenu = null;
+
 
     /**
 	 * Stores the name of the folder that we want to open as soon as possible
@@ -143,6 +148,8 @@ public class FolderMessageList extends ExpandableListActivity
 	private DateFormat dateFormat = null;
 
 	private DateFormat timeFormat = null;
+	
+	private boolean thread = false;
 
 	private DateFormat getDateFormat()
 	{
@@ -527,6 +534,7 @@ public class FolderMessageList extends ExpandableListActivity
 	{
 		final int expandedGroup = savedInstanceState.getInt(
 				STATE_KEY_EXPANDED_GROUP, -1);
+		
 		if (expandedGroup >= 0 && mAdapter.getGroupCount() > expandedGroup)
 		{
             mListView.expandGroup(expandedGroup);
@@ -573,6 +581,8 @@ public class FolderMessageList extends ExpandableListActivity
 		
 		NotificationManager notifMgr = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
 		notifMgr.cancel(mAccount.getAccountNumber());
+    thread = MessagingController.getInstance(getApplication()).isThreading();
+
 
   }
 
@@ -630,6 +640,7 @@ public class FolderMessageList extends ExpandableListActivity
 	        case KeyEvent.KEYCODE_C: { onCompose(); return true;}
 	        case KeyEvent.KEYCODE_Q: { onAccounts(); return true; }
 	        case KeyEvent.KEYCODE_S: { onEditAccount(); return true; }
+	        case KeyEvent.KEYCODE_T: { onToggleThread(); return true; }
 	        case KeyEvent.KEYCODE_H: {
 	            Toast toast = Toast.makeText(this, R.string.message_list_help_key, Toast.LENGTH_LONG);
 	            toast.show();
@@ -769,6 +780,20 @@ public class FolderMessageList extends ExpandableListActivity
 	{
         MessageCompose.actionCompose(this, mAccount);
     }
+	
+	private void onToggleThread()
+	{
+	  thread = !thread;
+	  
+	  for (FolderInfoHolder folder : mAdapter.mFolders)
+	  { 
+  	  Collections.sort(folder.messages);
+	  }
+	  mAdapter.notifyDataSetChanged();
+	  setMenuThread();
+	  MessagingController.getInstance(getApplication()).setThreading(thread);
+	  
+	}
 
 	private void onDelete(MessageInfoHolder holder)
 	{
@@ -879,6 +904,9 @@ public class FolderMessageList extends ExpandableListActivity
             case R.id.account_settings:
                 onEditAccount();
                 return true;
+            case R.id.thread:
+              onToggleThread();
+              return true;
 			case R.id.empty_trash:
 				onEmptyTrash(mAccount);
 				return true;
@@ -893,8 +921,24 @@ public class FolderMessageList extends ExpandableListActivity
 	{
         super.onCreateOptionsMenu(menu);
         getMenuInflater().inflate(R.menu.folder_message_list_option, menu);
+        optionsMenu = menu;
+        setMenuThread();
         return true;
     }
+    
+    private void setMenuThread()
+    {
+      Menu menu = optionsMenu;
+      if (menu != null)
+      {
+        MenuItem threadItem = menu.findItem(R.id.thread);
+        if (threadItem != null)
+        {
+          threadItem.setTitle(thread ? R.string.unthread_action : R.string.thread_action);
+          threadItem.setIcon(thread ? R.drawable.ic_menu_unthread : R.drawable.ic_menu_thread);
+        }
+      }
+    }
 
     @Override
 	public boolean onContextItemSelected(MenuItem item)
@@ -1864,6 +1908,8 @@ public class FolderMessageList extends ExpandableListActivity
             public String date;
 
             public Date compareDate;
+            
+            public String compareSubject;
 
             public String sender;
             
@@ -1929,8 +1975,52 @@ public class FolderMessageList extends ExpandableListActivity
 
 			public int compareTo(MessageInfoHolder o)
 			{
-                return this.compareDate.compareTo(o.compareDate) * -1;
+			  if (thread)
+			  {
+  			  if (compareSubject == null)
+  			  {
+  			    compareSubject = stripPrefixes(subject).toLowerCase();
+  			  }
+  			  if (o.compareSubject == null)
+  			  {
+  			    o.compareSubject = stripPrefixes(o.subject).toLowerCase();
+  			  }
+  			  int subjCompare = this.compareSubject.compareTo(o.compareSubject);
+  			  if (subjCompare != 0)
+  			  {
+  			    return subjCompare;
+  			  }
+			  }
+			  
+        return this.compareDate.compareTo(o.compareDate) * -1;
             }
+			Pattern pattern = null;
+			String patternString = "^ *(re|fw|fwd): *";
+			private String stripPrefixes(String in)
+			{
+			  synchronized(patternString)
+			  {
+  			  if (pattern == null)
+          {
+            pattern = Pattern.compile(patternString, Pattern.CASE_INSENSITIVE );
+          }
+			  }
+			  Matcher matcher = pattern.matcher(in);
+			  int lastPrefix = -1;
+			  while (matcher.find())
+			  {
+			    lastPrefix = matcher.end();
+			  }
+			  if (lastPrefix > -1 && lastPrefix < in.length() - 1)
+			  {
+			    return in.substring(lastPrefix);
+			  }
+			  else
+			  {
+			    return in;
+			  }
+			}
+			
         }
 
 		class FolderViewHolder