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Apache Jena Fuseki server Kafka connector
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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF 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 org.apache.kafka.common.utils;

import java.nio.ByteBuffer;
import java.util.ArrayDeque;
import java.util.Deque;
import java.util.HashMap;
import java.util.Map;

/**
 * Simple non-threadsafe interface for caching byte buffers. This is suitable for simple cases like ensuring that
 * a given KafkaConsumer reuses the same decompression buffer when iterating over fetched records. For small record
 * batches, allocating a potentially large buffer (64 KB for LZ4) will dominate the cost of decompressing and
 * iterating over the records in the batch.
 */
public abstract class BufferSupplier implements AutoCloseable {

    public static final BufferSupplier NO_CACHING = new BufferSupplier() {
        @Override
        public ByteBuffer get(int capacity) {
            return ByteBuffer.allocate(capacity);
        }

        @Override
        public void release(ByteBuffer buffer) {}

        @Override
        public void close() {}
    };

    public static BufferSupplier create() {
        return new DefaultSupplier();
    }

    /**
     * Supply a buffer with the required capacity. This may return a cached buffer or allocate a new instance.
     */
    public abstract ByteBuffer get(int capacity);

    /**
     * Return the provided buffer to be reused by a subsequent call to `get`.
     */
    public abstract void release(ByteBuffer buffer);

    /**
     * Release all resources associated with this supplier.
     */
    public abstract void close();

    private static class DefaultSupplier extends BufferSupplier {
        // We currently use a single block size, so optimise for that case
        private final Map> bufferMap = new HashMap<>(1);

        @Override
        public ByteBuffer get(int size) {
            Deque bufferQueue = bufferMap.get(size);
            if (bufferQueue == null || bufferQueue.isEmpty())
                return ByteBuffer.allocate(size);
            else
                return bufferQueue.pollFirst();
        }

        @Override
        public void release(ByteBuffer buffer) {
            buffer.clear();
            // We currently keep a single buffer in flight, so optimise for that case
            Deque bufferQueue = bufferMap.computeIfAbsent(buffer.capacity(), k -> new ArrayDeque<>(1));
            bufferQueue.addLast(buffer);
        }

        @Override
        public void close() {
            bufferMap.clear();
        }
    }

    /**
     * Simple buffer supplier for single-threaded usage. It caches a single buffer, which grows
     * monotonically as needed to fulfill the allocation request.
     */
    public static class GrowableBufferSupplier extends BufferSupplier {
        private ByteBuffer cachedBuffer;

        @Override
        public ByteBuffer get(int minCapacity) {
            if (cachedBuffer != null && cachedBuffer.capacity() >= minCapacity) {
                ByteBuffer res = cachedBuffer;
                cachedBuffer = null;
                return res;
            } else {
                cachedBuffer = null;
                return ByteBuffer.allocate(minCapacity);
            }
        }

        @Override
        public void release(ByteBuffer buffer) {
            buffer.clear();
            cachedBuffer = buffer;
        }

        @Override
        public void close() {
            cachedBuffer = null;
        }
    }

}