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Transactional event fabric for the JVM
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package com.obsidiandynamics.blackstrom.ledger;

import static com.obsidiandynamics.func.Functions.*;

import java.util.*;
import java.util.concurrent.*;

import com.obsidiandynamics.blackstrom.handler.*;
import com.obsidiandynamics.blackstrom.model.*;
import com.obsidiandynamics.blackstrom.retention.*;
import com.obsidiandynamics.func.*;

public final class StackLedger implements Ledger {
  private final Set subscribedHandlerIds = new CopyOnWriteArraySet<>();
  
  private final LinkedList queue = new LinkedList<>();
  
  private final Object lock = new Object();
  
  /** Tracks presence of group members. */
  private final Set groups = new CopyOnWriteArraySet<>();
  
  private boolean delivering;
  
  private static final class ContextualHandler {
    final MessageHandler handler;
    
    final MessageContext context;

    ContextualHandler(MessageHandler handler, MessageContext context) {
      this.handler = handler;
      this.context = context;
    }
  }
  
  private volatile ContextualHandler[] contextualHandlers = new ContextualHandler[0];

  @Override
  public Object attach(MessageHandler handler) {
    final UUID handlerId = handler.getGroupId() != null ? UUID.randomUUID() : null;
    
    if (handler.getGroupId() == null || groups.add(handler.getGroupId())) {
      final MessageContext context = new DefaultMessageContext(this, handlerId, NopRetention.getInstance());
      contextualHandlers = ArrayCopy.append(contextualHandlers, new ContextualHandler(handler, context));
      ifPresent(handlerId, subscribedHandlerIds::add);
    }
    
    return handlerId;
  }

  @Override
  public void append(Message message, AppendCallback callback) {
    synchronized (lock) {
      queue.addLast(message);
      callback.onAppend(message.getMessageId(), null);
      if (! delivering) {
        delivering = true;
        do {
          final Message head = queue.removeFirst();
          for (ContextualHandler contextualHandler : contextualHandlers) {
            contextualHandler.handler.onMessage(contextualHandler.context, head);
          }
        } while (! queue.isEmpty());
        delivering = false;
      }
    }
  }

  @Override
  public boolean isAssigned(Object handlerId, int shard) {
    return handlerId == null || subscribedHandlerIds.contains(handlerId);
  }
}