Fix bug related to gallery selection state.

TreeSets are annoying. contains() is calculated with the comparator,
which can lead to some weird bugs. Made sure the comparator didn't think
two items with the same date were identical.

Also fixed stableId generation to avoid any potential weirdness there.

src/org/thoughtcrime/securesms/mediapreview/MediaRailAdapter.java

@@ -10,6 +10,7 @@ import org.thoughtcrime.securesms.R;
 import org.thoughtcrime.securesms.components.ThumbnailView;
 import org.thoughtcrime.securesms.mediasend.Media;
 import org.thoughtcrime.securesms.mms.GlideRequests;
+import org.thoughtcrime.securesms.util.StableIdGenerator;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -20,6 +21,7 @@ public class MediaRailAdapter extends RecyclerView.Adapter<MediaRailAdapter.Medi
   private final List<Media>              media;
   private final RailItemListener         listener;
   private final boolean                  deleteEnabled;
+  private final StableIdGenerator<Media> stableIdGenerator;
 
   private int activePosition;
 
@@ -28,6 +30,9 @@ public class MediaRailAdapter extends RecyclerView.Adapter<MediaRailAdapter.Medi
     this.media             = new ArrayList<>();
     this.listener          = listener;
     this.deleteEnabled     = deleteEnabled;
+    this.stableIdGenerator = new StableIdGenerator<>();
+
+    setHasStableIds(true);
   }
 
   @NonNull
@@ -51,6 +56,11 @@ public class MediaRailAdapter extends RecyclerView.Adapter<MediaRailAdapter.Medi
     return media.size();
   }
 
+  @Override
+  public long getItemId(int position) {
+    return stableIdGenerator.getId(media.get(position));
+  }
+
   public void setMedia(@NonNull List<Media> media) {
     setMedia(media, activePosition);
   }

src/org/thoughtcrime/securesms/mediasend/MediaPickerItemAdapter.java

@@ -13,6 +13,7 @@ import com.bumptech.glide.load.resource.drawable.DrawableTransitionOptions;
 import org.thoughtcrime.securesms.R;
 import org.thoughtcrime.securesms.mms.GlideRequests;
 import org.thoughtcrime.securesms.util.MediaUtil;
+import org.thoughtcrime.securesms.util.StableIdGenerator;
 
 import java.util.ArrayList;
 import java.util.Collection;
@@ -27,6 +28,7 @@ public class MediaPickerItemAdapter extends RecyclerView.Adapter<MediaPickerItem
   private final List<Media>              media;
   private final Set<Media>               selected;
   private final int                      maxSelection;
+  private final StableIdGenerator<Media> stableIdGenerator;
 
   private boolean forcedMultiSelect;
 
@@ -35,8 +37,10 @@ public class MediaPickerItemAdapter extends RecyclerView.Adapter<MediaPickerItem
     this.eventListener     = eventListener;
     this.media             = new ArrayList<>();
     this.maxSelection      = maxSelection;
+    this.stableIdGenerator = new StableIdGenerator<>();
     this.selected          = new TreeSet<>((m1, m2) -> {
       if      (m1.equals(m2))                                 return 0;
+      else if (Long.compare(m2.getDate(), m1.getDate()) == 0) return m2.getUri().compareTo(m1.getUri());
       else                                                    return Long.compare(m2.getDate(), m1.getDate());
     });
 
@@ -65,7 +69,7 @@ public class MediaPickerItemAdapter extends RecyclerView.Adapter<MediaPickerItem
 
   @Override
   public long getItemId(int position) {
-    return media.get(position).getDate();
+    return stableIdGenerator.getId(media.get(position));
   }
 
   void setMedia(@NonNull List<Media> media) {

src/org/thoughtcrime/securesms/util/StableIdGenerator.java

@@ -0,0 +1,31 @@
+package org.thoughtcrime.securesms.util;
+
+import android.support.annotation.MainThread;
+import android.support.annotation.NonNull;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Useful for generate ID's to be used with
+ * {@link android.support.v7.widget.RecyclerView.Adapter#getItemId(int)} when you otherwise don't
+ * have a good way to generate an ID.
+ */
+public class StableIdGenerator<E> {
+
+  private final Map<E, Long> keys = new HashMap<>();
+
+  private long index = 1;
+
+  @MainThread
+  public long getId(@NonNull E item) {
+    if (keys.containsKey(item)) {
+      return keys.get(item);
+    }
+
+    long key = index++;
+    keys.put(item, key);
+
+    return key;
+  }
+}