Renderer/src/main/java/electrosphere/server/terrain/manager/ServerChunkCache.java

package electrosphere.server.terrain.manager;

import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.concurrent.Semaphore;

/**
 * Caches chunk data on the server
 */
public class ServerChunkCache {

    /**
     * Number of chunks to cache
     */
    static final int CACHE_SIZE = 5000;

    /**
     * The size of the cache
     */
    int cacheSize = CACHE_SIZE;

    /**
     * The cached data
     */
    Map<String, ServerTerrainChunk> chunkCache = new HashMap<String,ServerTerrainChunk>();

    /**
     * Tracks how recently a chunk has been queries for (used for evicting old chunks from cache)
     */
    List<String> queryRecencyQueue = new LinkedList<String>();

    /**
     * Tracks what chunks are already queued to be asynchronously loaded. Used so we don't have two threads generating/fetching the same chunk
     */
    Map<String, Boolean> queuedChunkMap = new HashMap<String,Boolean>();

    /**
     * The lock for thread safety
     */
    Semaphore lock = new Semaphore(1);

    /**
     * Gets the collection of server terrain chunks that are cached
     * @return The collection of chunks
     */
    public Collection<ServerTerrainChunk> getContents(){
        lock.acquireUninterruptibly();
        Collection<ServerTerrainChunk> rVal = Collections.unmodifiableCollection(chunkCache.values());
        lock.release();
        return rVal;
    }

    /**
     * Evicts all chunks in the cache
     */
    public void clear(){
        lock.acquireUninterruptibly();
        chunkCache.clear();
        lock.release();
    }

    /**
     * Gets the chunk at a given world position
     * @param worldX The world x coordinate
     * @param worldY The world y coordinate
     * @param worldZ The world z coordinate
     * @return The chunk
     */
    public ServerTerrainChunk get(int worldX, int worldY, int worldZ){
        ServerTerrainChunk rVal = null;
        String key = this.getKey(worldX, worldY, worldZ);
        lock.acquireUninterruptibly();
        queryRecencyQueue.remove(key);
        queryRecencyQueue.add(0, key);
        rVal = this.chunkCache.get(key);
        lock.release();
        return rVal;
    }

    /**
     * Adds a chunk to the cache
     * @param worldX The world x coordinate of the chunk
     * @param worldY The world y coordinate of the chunk
     * @param worldZ The world z coordinate of the chunk
     * @param chunk The chunk itself
     */
    public void add(int worldX, int worldY, int worldZ, ServerTerrainChunk chunk){
        String key = this.getKey(worldX, worldY, worldZ);
        lock.acquireUninterruptibly();
        queryRecencyQueue.add(0, key);
        this.chunkCache.put(key, chunk);
        lock.release();
    }

    /**
     * Checks if the cache contains the chunk at a given world position
     * @param worldX The world x coordinate
     * @param worldY The world y coordinate
     * @param worldZ The world z coordinate
     * @return true if the cache contains this chunk, false otherwise
     */
    public boolean containsChunk(int worldX, int worldY, int worldZ){
        String key = this.getKey(worldX,worldY,worldZ);
        lock.acquireUninterruptibly();
        boolean rVal = this.chunkCache.containsKey(key);
        lock.release();
        return rVal;
    }

    /**
     * Gets the key for a given world position
     * @param worldX The x component
     * @param worldY The y component
     * @param worldZ The z component
     * @return The key
     */
    public String getKey(int worldX, int worldY, int worldZ){
        return worldX + "_" + worldY + "_" + worldZ;
    }

    /**
     * Checks if the chunk is already queued or not 
     * @param worldX The world x position of the chunk
     * @param worldY The world y position of the chunk
     * @param worldZ The world z position of the chunk
     * @return true if the chunk is already queued, false otherwise
     */
    public boolean chunkIsQueued(int worldX, int worldY, int worldZ){
        String key = this.getKey(worldX,worldY,worldZ);
        lock.acquireUninterruptibly();
        boolean rVal = this.queuedChunkMap.containsKey(key);
        lock.release();
        return rVal;
    }

    /**
     * Flags a chunk as queued
     * @param worldX The world x position of the chunk
     * @param worldY The world y position of the chunk
     * @param worldZ The world z position of the chunk
     */
    public void queueChunk(int worldX, int worldY, int worldZ){
        String key = this.getKey(worldX,worldY,worldZ);
        lock.acquireUninterruptibly();
        this.queuedChunkMap.put(key,true);
        lock.release();
    }

    /**
     * Unflags a chunk as queued
     * @param worldX The world x position of the chunk
     * @param worldY The world y position of the chunk
     * @param worldZ The world z position of the chunk
     */
    public void unqueueChunk(int worldX, int worldY, int worldZ){
        String key = this.getKey(worldX,worldY,worldZ);
        lock.acquireUninterruptibly();
        this.queuedChunkMap.remove(key);
        lock.release();
    }
    
}