Add a more generic system for handling early messages.

app/src/main/java/org/thoughtcrime/securesms/util/EarlyMessageCache.java

@@ -0,0 +1,60 @@
+package org.thoughtcrime.securesms.util;
+
+import androidx.annotation.NonNull;
+
+import org.thoughtcrime.securesms.recipients.RecipientId;
+import org.whispersystems.libsignal.util.guava.Optional;
+import org.whispersystems.signalservice.api.messages.SignalServiceContent;
+
+import java.util.Objects;
+
+/**
+ * Sometimes a message that is referencing another message can arrive out of order. In these cases,
+ * we want to temporarily hold on (i.e. keep a memory cache) to these messages and apply them after
+ * we receive the referenced message.
+ */
+public final class EarlyMessageCache {
+
+  private final LRUCache<MessageId, SignalServiceContent> cache = new LRUCache<>(100);
+
+  /**
+   * @param targetSender        The sender of the message this message depends on.
+   * @param targetSentTimestamp The sent timestamp of the message this message depends on.
+   */
+  public void store(@NonNull RecipientId targetSender, long targetSentTimestamp, @NonNull SignalServiceContent content) {
+    cache.put(new MessageId(targetSender, targetSentTimestamp), content);
+  }
+
+  /**
+   * Returns and removes any content that is dependent on the provided message id.
+   * @param sender        The sender of the message in question.
+   * @param sentTimestamp The sent timestamp of the message in question.
+   */
+  public Optional<SignalServiceContent> retrieve(@NonNull RecipientId sender, long sentTimestamp) {
+    return Optional.fromNullable(cache.remove(new MessageId(sender, sentTimestamp)));
+  }
+
+  private static final class MessageId {
+    private final RecipientId sender;
+    private final long        sentTimestamp;
+
+    private MessageId(@NonNull RecipientId sender, long sentTimestamp) {
+      this.sender        = sender;
+      this.sentTimestamp = sentTimestamp;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+      if (this == o) return true;
+      if (o == null || getClass() != o.getClass()) return false;
+      MessageId messageId = (MessageId) o;
+      return sentTimestamp == messageId.sentTimestamp &&
+          Objects.equals(sender, messageId.sender);
+    }
+
+    @Override
+    public int hashCode() {
+      return Objects.hash(sentTimestamp, sender);
+    }
+  }
+}