package com.c2kernel.utils;

import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.util.*;

/*******************************************************************************
 * SoftReferences are reaped if not strong references are left and the vm is
 * running out of memory. Most caches in the kernel use this.
 * 
 * $Revision: 1.5 $ $Date: 2004/10/29 13:29:09 $
 ******************************************************************************/
public class SoftCache extends AbstractMap {

    private final Map hash = new HashMap();
    private final int minSize;
    private final LinkedList hardCache = new LinkedList();
    private final ReferenceQueue queue = new ReferenceQueue();

    public SoftCache() {
        this(0);
    }

    public SoftCache(int minSize) {
        this.minSize = minSize;
    }

    public Object get(Object key) {
        Object result = null;
        SoftReference soft_ref = (SoftReference) hash.get(key);
        if (soft_ref != null) {
            result = soft_ref.get();
            if (result == null)
                hash.remove(key);
            else
                if (minSize > 0) { // add to hard cache so it's not reaped for a while
                    hardCache.addFirst(result);
                    if (hardCache.size() > minSize) // trim last one off
                        hardCache.removeLast();
                }
        }
        return result;
    }

    public Object put(Object key, Object value) {
        processQueue();
        if (minSize > 0) {
            hardCache.addFirst(value);
            if (hardCache.size() > minSize)
                hardCache.removeLast();
        }
        return hash.put(key, new SoftValue(value, key, queue));
    }

    public Object remove(Object key) {
        processQueue();
        return hash.remove(key);
    }

    public void clear() {
        hardCache.clear();
        while(queue.poll()!=null);
        hash.clear();
    }

    public int size() {
        processQueue();
        return hash.size();
    }
    
    public Set keySet() {
        processQueue();
        return hash.keySet();
    }

    public Set entrySet() {
        // Would have to create another Map to do this - too expensive
        // Throwing runtime expensive is dangerous, but better than nulls
        throw new UnsupportedOperationException();
    }    
    
    private static class SoftValue extends SoftReference {
        private final Object key;
        private SoftValue(Object k, Object key, ReferenceQueue q) {
            super(k, q);
            this.key = key;
        }
    }
    
    /**
     * Look for values that have been reaped, and remove their keys from the cache
     */
    private void processQueue() {
        SoftValue sv;
        while ((sv = (SoftValue) queue.poll()) != null) {
            hash.remove(sv.key);
        }
    }
    
}