Cache delivery receipts when they arrive before sync message

// FREEBIE

src/org/thoughtcrime/securesms/database/EarlyReceiptCache.java

@@ -0,0 +1,56 @@
+package org.thoughtcrime.securesms.database;
+
+import android.support.annotation.NonNull;
+import android.util.Log;
+
+import org.thoughtcrime.securesms.util.LRUCache;
+
+public class EarlyReceiptCache {
+
+  private static final String TAG = EarlyReceiptCache.class.getSimpleName();
+
+  private final LRUCache<Placeholder, Long> cache = new LRUCache<>(100);
+
+  public synchronized void increment(long timestamp, String address) {
+    Log.w(TAG, this+"");
+    Log.w(TAG, String.format("Early receipt: %d,%s", timestamp, address));
+    Placeholder tuple = new Placeholder(timestamp, address);
+    Long        count = cache.get(tuple);
+
+    if (count != null) {
+      cache.put(tuple, ++count);
+    } else {
+      cache.put(tuple, 1L);
+    }
+  }
+
+  public synchronized long remove(long timestamp, String address) {
+    Long count = cache.remove(new Placeholder(timestamp, address));
+    Log.w(TAG, this+"");
+    Log.w(TAG, String.format("Checking early receipts (%d, %s): %d", timestamp, address, count));
+    return count != null ? count : 0;
+  }
+
+  private class Placeholder {
+
+    private final          long   timestamp;
+    private final @NonNull String address;
+
+    private Placeholder(long timestamp, @NonNull  String address) {
+      this.timestamp = timestamp;
+      this.address   = address;
+    }
+
+    @Override
+    public boolean equals(Object other) {
+      return other != null && other instanceof Placeholder &&
+          ((Placeholder)other).timestamp == this.timestamp &&
+          ((Placeholder)other).address.equals(this.address);
+    }
+
+    @Override
+    public int hashCode() {
+      return (int)timestamp ^ address.hashCode();
+    }
+  }
+}