/*
    * Copyright (C) 2021, Thomas Wolf <thomas.wolf@paranor.ch> and others
    *
    * This program and the accompanying materials are made available under the
    * terms of the Eclipse Distribution License v. 1.0 which is available at
    * https://www.eclipse.org/org/documents/edl-v10.php.
    *
    * SPDX-License-Identifier: BSD-3-Clause
    */
  package org.eclipse.jgit.transport.sshd.agent;
  
  import java.io.IOException;
  import java.text.MessageFormat;
  import java.util.Objects;
  
  import org.apache.sshd.agent.SshAgentConstants;
  import org.apache.sshd.common.SshException;
  import org.apache.sshd.common.util.buffer.BufferUtils;
  import org.eclipse.jgit.internal.transport.sshd.SshdText;
  
  /**
   * Provides some utility methods for implementing {@link Connector}s.
   *
   * @since 6.0
   */
  public abstract class AbstractConnector implements Connector {
  
  	// A somewhat sane lower bound for the maximum reply length
  	private static final int MIN_REPLY_LENGTH = 8 * 1024;
  
  	/**
  	 * Default maximum reply length. 256kB is the OpenSSH limit.
  	 */
  	protected static final int DEFAULT_MAX_REPLY_LENGTH = 256 * 1024;
  
  	private final int maxReplyLength;
  
  	/**
  	 * Creates a new instance using the {@link #DEFAULT_MAX_REPLY_LENGTH}.
  	 */
  	protected AbstractConnector() {
  		this(DEFAULT_MAX_REPLY_LENGTH);
  	}
  
  	/**
  	 * Creates a new instance.
  	 *
  	 * @param maxReplyLength
  	 *            maximum number of payload bytes we're ready to accept
  	 */
  	protected AbstractConnector(int maxReplyLength) {
  		if (maxReplyLength < MIN_REPLY_LENGTH) {
  			throw new IllegalArgumentException(
  					"Maximum payload length too small"); //$NON-NLS-1$
  		}
  		this.maxReplyLength = maxReplyLength;
  	}
  
  	/**
  	 * Retrieves the maximum message length this {@link AbstractConnector} is
  	 * configured for.
  	 *
  	 * @return the maximum message length
  	 */
  	protected int getMaximumMessageLength() {
  		return this.maxReplyLength;
  	}
  
  	/**
  	 * Prepares a message for sending by inserting the command and message
  	 * length.
  	 *
  	 * @param command
  	 *            SSH agent command the request is for
  	 * @param message
  	 *            about to be sent, including the 5 spare bytes at the front
  	 * @throws IllegalArgumentException
  	 *             if {@code message} has less than 5 bytes
  	 */
  	protected void prepareMessage(byte command, byte[] message)
  			throws IllegalArgumentException {
  		Objects.requireNonNull(message);
  		if (message.length < 5) {
  			// No translation; internal error
  			throw new IllegalArgumentException("Message buffer for " //$NON-NLS-1$
  					+ SshAgentConstants.getCommandMessageName(command)
  					+ " must have at least 5 bytes; have only " //$NON-NLS-1$
  					+ message.length);
  		}
  		BufferUtils.putUInt(message.length - 4, message);
  		message[4] = command;
  	}
  
  	/**
  	 * Checks the received length of a reply.
  	 *
  	 * @param command
  	 *            SSH agent command the reply is for
  	 * @param length
 	 *            length as received: number of payload bytes
 	 * @return the length as an {@code int}
 	 * @throws IOException
 	 *             if the length is invalid
 	 */
 	protected int toLength(byte command, byte[] length)
 			throws IOException {
 		long l = BufferUtils.getUInt(length);
 		if (l <= 0 || l > maxReplyLength - 4) {
 			throw new SshException(MessageFormat.format(
 					SshdText.get().sshAgentReplyLengthError,
 					Long.toString(l),
 					SshAgentConstants.getCommandMessageName(command)));
 		}
 		return (int) l;
 	}
 }