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package io.questdb.std;

import java.util.ArrayDeque;

/**
 * Weak object pool can be used in multithreaded environment. While
 * weak object pool is a factory of new instances, when object is popped
 * out of pool there is no reference to tie this object back to pool.
 * 
 * In multithreaded environment weak pools must be local to thread and
 * objects they create can travel between pools, e.g. one thread gets
 * object from pool and last thread to finish with this object then
 * returns it to its own local pool.
 * 
 * In an extreme situation this object migration could keep feeding some
 * pools until we run out of memory. Hence, we limit the maximum number of
 * objects a pool can hold.
 */
abstract class WeakObjectPoolBase {
    // package private for testing
    final ArrayDeque cache = new ArrayDeque<>();

    private final int initSize;
    private final int maxSize;

    public WeakObjectPoolBase(int initSize) {
        this.initSize = initSize;
        this.maxSize = 2 * initSize;
    }

    public T pop() {
        final T obj = cache.poll();
        return obj == null ? newInstance() : obj;
    }

    public boolean push(T obj) {
        assert obj != null;
        if (cache.size() < maxSize) {
            clear(obj);
            cache.push(obj);
            return true;
        } else {
            close(obj);
            return false;
        }
    }

    public int size() {
        return cache.size();
    }

    void clear(T obj) {
    }

    void close(T obj) {
    }

    final void fill() {
        for (int i = 0; i < initSize; i++) {
            cache.add(newInstance());
        }
    }

    abstract T newInstance();
}