Use FTS vocabulary for search suggestions

app/src/main/java/eu/faircode/email/FragmentDialogSearch.java

@@ -56,6 +56,8 @@ import java.util.ArrayList;
 import java.util.Calendar;
 import java.util.List;
 
+import io.requery.android.database.sqlite.SQLiteDatabase;
+
 public class FragmentDialogSearch extends FragmentDialogBase {
     private static final int MAX_SUGGESTIONS = 3;
 
@@ -137,15 +139,23 @@ public class FragmentDialogSearch extends FragmentDialogBase {
                 MatrixCursor cursor = new MatrixCursor(new String[]{"_id", "suggestion"});
                 if (TextUtils.isEmpty(typed))
                     return cursor;
-                if (fts && pro)
+
+                if (cbSearchIndex.isEnabled() && cbSearchIndex.isChecked()) {
+                    SQLiteDatabase db = FtsDbHelper.getInstance(context);
+                    List<String> suggestions = FtsDbHelper.getSuggestions(
+                            db,
+                            typed + "%",
+                            MAX_SUGGESTIONS);
+                    for (int i = 0; i < suggestions.size(); i++)
+                        cursor.addRow(new Object[]{i + 1, suggestions.get(i)});
                     return cursor;
+                }
 
-                String query = "%" + typed + "%";
                 DB db = DB.getInstance(context);
                 return db.message().getSuggestions(
                         account < 0 ? null : account,
                         folder < 0 ? null : folder,
-                        "%" + query + "%",
+                        "%" + typed + "%",
                         MAX_SUGGESTIONS);
             }
         });

app/src/main/java/eu/faircode/email/FtsDbHelper.java

@@ -39,7 +39,7 @@ public class FtsDbHelper extends SQLiteOpenHelper {
 
     private static FtsDbHelper instance = null;
 
-    private static final int DATABASE_VERSION = 4;
+    private static final int DATABASE_VERSION = 5;
     private static final String DATABASE_NAME = "fts.db";
 
     private FtsDbHelper(Context context) {
@@ -68,12 +68,18 @@ public class FtsDbHelper extends SQLiteOpenHelper {
                 ", `notes`" +
                 ", tokenize = \"unicode61 remove_diacritics 2\")");
         // https://www.sqlite.org/fts5.html#unicode61_tokenizer
+
+        // https://www.sqlite.org/fts5.html#the_fts5vocab_virtual_table_module
+        db.execSQL("CREATE VIRTUAL TABLE message_terms USING fts5vocab('message', 'row');");
     }
 
     @Override
     public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
         Log.i("FTS upgrade from " + oldVersion + " to " + newVersion);
-        db.execSQL("DROP TABLE `message`");
+
+        db.execSQL("DROP TABLE IF EXISTS `message`");
+        db.execSQL("DROP TABLE IF EXISTS `message_terms`");
+
         onCreate(db);
 
         DB.getInstance(context).message().resetFts();
@@ -114,6 +120,21 @@ public class FtsDbHelper extends SQLiteOpenHelper {
         db.delete("message", "rowid = ?", new Object[]{id});
     }
 
+    static List<String> getSuggestions(SQLiteDatabase db, String query, int max) {
+        List<String> result = new ArrayList<>();
+
+        Cursor cursor = db.query(
+                "SELECT term FROM message_terms" +
+                        " WHERE term LIKE ?" +
+                        " ORDER BY cnt" +
+                        " LIMIT " + max,
+                new Object[]{query});
+        while (cursor != null && cursor.moveToNext())
+            result.add(cursor.getString(0));
+
+        return result;
+    }
+
     static List<Long> match(
             SQLiteDatabase db,
             Long account, Long folder,