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/*
 * Copyright 2013 The Netty Project
 *
 * The Netty Project licenses this file to you under the Apache License,
 * version 2.0 (the "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at:
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations
 * under the License.
 */

package io.netty.buffer;

import io.netty.util.internal.PlatformDependent;

import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;

/**
 * Abstract base class for {@link ByteBuf} implementations that count references.
 */
public abstract class AbstractReferenceCountedByteBuf extends AbstractByteBuf {

    private static final AtomicIntegerFieldUpdater refCntUpdater =
            AtomicIntegerFieldUpdater.newUpdater(AbstractReferenceCountedByteBuf.class, "refCnt");

    private static final long REFCNT_FIELD_OFFSET;

    static {
        long refCntFieldOffset = -1;
        try {
            if (PlatformDependent.hasUnsafe()) {
                refCntFieldOffset = PlatformDependent.objectFieldOffset(
                        AbstractReferenceCountedByteBuf.class.getDeclaredField("refCnt"));
            }
        } catch (Throwable t) {
            // Ignored
        }

        REFCNT_FIELD_OFFSET = refCntFieldOffset;
    }

    @SuppressWarnings("FieldMayBeFinal")
    private volatile int refCnt = 1;

    protected AbstractReferenceCountedByteBuf(int maxCapacity) {
        super(maxCapacity);
    }

    @Override
    public final int refCnt() {
        if (REFCNT_FIELD_OFFSET >= 0) {
            // Try to do non-volatile read for performance.
            return PlatformDependent.getInt(this, REFCNT_FIELD_OFFSET);
        } else {
            return refCnt;
        }
    }

    @Override
    public ByteBuf retain() {
        for (;;) {
            int refCnt = this.refCnt;
            if (refCnt == 0) {
                throw new IllegalBufferAccessException();
            }
            if (refCnt == Integer.MAX_VALUE) {
                throw new IllegalBufferAccessException("refCnt overflow");
            }
            if (refCntUpdater.compareAndSet(this, refCnt, refCnt + 1)) {
                break;
            }
        }
        return this;
    }

    @Override
    public ByteBuf retain(int increment) {
        if (increment <= 0) {
            throw new IllegalArgumentException("increment: " + increment + " (expected: > 0)");
        }

        for (;;) {
            int refCnt = this.refCnt;
            if (refCnt == 0) {
                throw new IllegalBufferAccessException();
            }
            if (refCnt > Integer.MAX_VALUE - increment) {
                throw new IllegalBufferAccessException("refCnt overflow");
            }
            if (refCntUpdater.compareAndSet(this, refCnt, refCnt + increment)) {
                break;
            }
        }
        return this;
    }

    @Override
    public final boolean release() {
        for (;;) {
            int refCnt = this.refCnt;
            if (refCnt == 0) {
                throw new IllegalBufferAccessException();
            }

            if (refCntUpdater.compareAndSet(this, refCnt, refCnt - 1)) {
                if (refCnt == 1) {
                    deallocate();
                    return true;
                }
                return false;
            }
        }
    }

    @Override
    public final boolean release(int decrement) {
        if (decrement <= 0) {
            throw new IllegalArgumentException("decrement: " + decrement + " (expected: > 0)");
        }

        for (;;) {
            int refCnt = this.refCnt;
            if (refCnt < decrement) {
                throw new IllegalBufferAccessException();
            }

            if (refCntUpdater.compareAndSet(this, refCnt, refCnt - decrement)) {
                if (refCnt == decrement) {
                    deallocate();
                    return true;
                }
                return false;
            }
        }
    }

    /**
     * Called once {@link #refCnt()} is equals 0.
     */
    protected abstract void deallocate();
}