/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.juneau.rest.client.mcp;


import java.util.concurrent.atomic.*;

import org.apache.juneau.bean.jsonrpc.*;
import org.apache.juneau.commons.inject.*;
import static org.apache.juneau.commons.utils.Shorts.*;

/**
 * Dispatches server-initiated JSON-RPC requests delivered over an MCP client's duplex event-stream channel to a
 * single registered {@link McpServerRequestHandler}.
 *
 * <p>
 * Requests are handed to the registered handler untouched, with their {@code params} left as the generic,
 * revision-neutral wire representation - never projected into a typed request bean - since the set of
 * server-initiated methods (for example {@code sampling/createMessage}; illustrative only, as sampling is
 * deprecated per SEP-2577) is revision- and deployment-specific and this dispatcher has no knowledge of it.
 *
 * @since 10.0.0
 */
public class McpDuplexDispatcher {

   private final AtomicReference<McpServerRequestHandler> handler = new AtomicReference<>();

   /**
    * Registers (or de-registers) the handler invoked by {@link #dispatch(JsonRpcRequest, BeanStore)}.
    *
    * @param value The handler to register. Passing <jk>null</jk> de-registers any previously-registered handler -
    *    a subsequent {@link #dispatch(JsonRpcRequest, BeanStore)} call then behaves as if no handler was ever
    *    registered (see the "no handler" behavior documented there).
    * @return This object.
    */
   public McpDuplexDispatcher register(McpServerRequestHandler value) {
      handler.set(value);
      return this;
   }

   /**
    * Dispatches a single server-initiated request to the registered handler.
    *
    * <p>
    * When no handler is registered: a notification (a <jk>null</jk> {@link JsonRpcRequest#getId() id}) is
    * silently dropped (returns <jk>null</jk>); a request is rejected with a {@code -32601} {@link McpException}.
    *
    * @param request The inbound request. Must not be <jk>null</jk>.
    * @param ctx The request-scoped bean store to hand to the handler. Can be <jk>null</jk>, in which case
    *    {@link BasicBeanStore#INSTANCE} is used instead.
    * @return The handler's result, or <jk>null</jk> for a notification with no registered handler.
    * @throws McpException If no handler is registered and {@code request} is not a notification, or if the
    *    handler itself throws (an {@link McpException} thrown by the handler propagates as-is; any other
    *    exception is wrapped as a {@code -32603} internal-error {@link McpException} with the original
    *    exception preserved as its cause).
    */
   public Object dispatch(JsonRpcRequest request, BeanStore ctx) {
      reqnn("request", request);
      var h = handler.get();
      if (h == null) {
         if (JsonRpcResponse.notification(request.getId()))
            return null;
         throw new McpException(-32601, f("No server-request handler registered for method '%s'", request.getMethod()));
      }
      try {
         return h.handle(request, ctx == null ? BasicBeanStore.INSTANCE : ctx);
      } catch (McpException e) {
         throw e;
      } catch (Exception e) {
         var e2 = new McpException(-32603, "Internal error handling server request '" + request.getMethod() + "'");
         e2.initCause(e);
         throw e2;
      }
   }
}