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diff --git a/src/core/SourceInterfaces.vala b/src/core/SourceInterfaces.vala
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+/* Copyright 2011-2014 Yorba Foundation
+ *
+ * This software is licensed under the GNU Lesser General Public License
+ * (version 2.1 or later).  See the COPYING file in this distribution.
+ */
+
+// See the note in MediaInterfaces.vala for some thoughts on the theory of expanding Shotwell's
+// features via interfaces rather than class heirarchies.
+
+// Indexable DataSources provide raw strings that may be searched against (and, in the future,
+// indexed) for free-text search queries.  DataSources implementing Indexable must prepare and
+// store (i.e. cache) these strings using prepare_indexable_string(s), as preparing the strings
+// for each call is expensive.
+//
+// When the indexable string has changed, the object should fire an alteration of
+// "indexable:keywords".  The prepare methods will not do this.
+
+public interface Indexable : DataSource {
+    public abstract unowned string? get_indexable_keywords();
+    
+    public static string? prepare_indexable_string(string? str) {
+        if(is_string_empty(str))
+            return null;
+        return String.remove_diacritics(str.down());
+    }
+    
+    public static string? prepare_indexable_strings(string[]? strs) {
+        if (strs == null || strs.length == 0)
+            return null;
+        
+        StringBuilder builder = new StringBuilder();
+        int ctr = 0;
+        do {
+            if (!is_string_empty(strs[ctr])) {
+                builder.append(strs[ctr].down());
+                if (ctr < strs.length - 1)
+                    builder.append_c(' ');
+            }
+        } while (++ctr < strs.length);
+        
+        return !is_string_empty(builder.str) ? builder.str : null;
+    }
+}
+