Message.java

package com.evelus.galaxy;

import java.nio.ByteBuffer;

/**
 * A message. All implementations of messages must inherit this class and provide functions
 * to access various variables that are accessible from the buffer provided to the message.
 *
 * @author hadyn
 */
public class Message {
  private ByteBuffer buffer;
  private MessageStructure structure;
  private OffsetTable offsets;

  protected Message(ByteBuffer buffer, MessageStructure structure) {
    this.buffer = buffer;
    this.structure = structure;
    offsets = structure.createOffsetTable(buffer);
  }

  /**
   * Gets an integer variable from the message buffer.
   *
   * @param id the identifier of the variable to get.
   * @return the integer variable value.
   */
  protected int getInt(int id) {
    if(!structure.isFieldType(id, MessageField.TYPE_INT)) {
      throw new IllegalStateException("Expected field " + id + " to be an integer.");
    }
    buffer.position(offsets.getOffset(id));
    return buffer.getInt();
  }
}

MessageField.java

package com.evelus.galaxy;

import java.nio.ByteBuffer;

/**
 * @author hadyn
 */
public class MessageField {

  public static final int TYPE_INT = 0;

  private int id;
  private String name;
  private int type;

  public MessageField(int id, String name, int type) {
    this.id = id;
    this.name = name;
    this.type = type;
  }

  public int getId() {
    return id;
  }

  public int getType() {
    return type;
  }

  /**
   * Gets the length of a data type in bytes.
   *
   * @param type the data type.
   * @return the length in bytes.
   */
  public static int getLength(ByteBuffer buffer, int offset, int type) {
    switch (type) {
      case TYPE_INT:
        return 4;
      default:
        throw new UnsupportedOperationException("Unhandled data type.");
    }
  }
}

MessageStructure.java

package com.evelus.galaxy;

import java.nio.ByteBuffer;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import java.util.TreeSet;

/**
 * A structure which defines a message. Fields of a structure are parsed from buffers in ascending
 * order of identifiers. For backwards compatibility updated structures are expected to retain the
 * same identifiers for fields of previous versions. Fields being removed will break all backwards
 * compatibility.
 *
 * @author hadyn
 */
public class MessageStructure {

  /**
   * The fields that comprise the structure.
   */
  private Map<Integer, MessageField> fields = new HashMap<>();

  /**
   * Adds a field to the structure.
   *
   * @param field the field to add.
   * @return the field that was replaced if one already existed for the provided field's identifier.
   */
  public MessageField addField(MessageField field) {
    return fields.put(field.getId(), field);
  }

  /**
   * Gets a field.
   *
   * @param id the field identifier.
   * @return the field.
   */
  public MessageField getField(int id) {
    return fields.get(id);
  }

  /**
   * Gets if the structure contains a certain field.
   *
   * @param id the field identifier.
   * @return if the structure has a field with the specified id.
   */
  public boolean hasField(int id) {
    return fields.containsKey(id);
  }

  /**
   * Checks to see if a field is a certain type.
   *
   * @param id   the identifier of the field.
   * @param type the type to check.
   * @return if the field is of the specified type.
   */
  public boolean isFieldType(int id, int type) {
    if (!hasField(id)) {
      throw new NullPointerException("Field " + id + " does not exist.");
    }
    MessageField field = getField(id);
    return field.getType() == type;
  }

  /**
   * Creates an offset table for the structure. If the structure has variable length fields then the
   * byte buffer will have to be read to determine the offsets of all of the variables.
   *
   * @param buffer the buffer to read from.
   * @return the created offset table.
   */
  public OffsetTable createOffsetTable(ByteBuffer buffer) {
    OffsetTable table = new OffsetTable();
    int offset = 0;

    Set<Integer> ids = new TreeSet<>(fields.keySet());
    for(int id : ids) {
      MessageField field = fields.get(id);
      table.addOffset(id, offset);
      offset += MessageField.getLength(buffer, offset, field.getType());
    }

    return table;
  }
}

OffsetTable.java

package com.evelus.galaxy;

import java.util.HashMap;
import java.util.Map;

/**
 * @author hadyn
 */
public class OffsetTable {
  private Map<Integer, Integer> offsets = new HashMap<>();

  public void addOffset(int id, int offset) {
    offsets.put(id, offset);
  }

  public int getOffset(int id) {
    if(!offsets.containsKey(id)) {
      throw new IllegalArgumentException("Offset for variable " + id + " does not exist.");
    }
    return offsets.get(id);
  }
}

SilabMessage.java

package com.evelus.galaxy;

import java.nio.ByteBuffer;

/**
 * @author hadyn
 */
public class SilabMessage extends Message {
  public SilabMessage(ByteBuffer buffer, MessageStructure structure) {
    super(buffer, structure);
  }

  public int getPowerLevel() {
    return getInt(0);
  }
}