/*
 * 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.cassandra.hints;

import java.io.EOFException;
import java.io.File;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.util.Iterator;

import javax.annotation.Nullable;

import com.google.common.primitives.Ints;
import com.google.common.util.concurrent.RateLimiter;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import org.apache.cassandra.db.UnknownColumnFamilyException;
import org.apache.cassandra.io.FSReadError;
import org.apache.cassandra.service.StorageService;
import org.apache.cassandra.utils.AbstractIterator;
import org.apache.cassandra.utils.ByteBufferUtil;

/**
 * A paged non-compressed hints reader that provides two iterators:
 * - a 'raw' ByteBuffer iterator that doesn't deserialize the hints, but returns the pre-encoded hints verbatim
 * - a decoded iterator, that deserializes the underlying bytes into {@link Hint} instances.
 *
 * The former is an optimisation for when the messaging version of the file matches the messaging version of the destination
 * node. Extra decoding and reencoding is a waste of effort in this scenario, so we avoid it.
 *
 * The latter is required for dispatch of hints to nodes that have a different messaging version, and in general is just an
 * easy way to enable backward and future compatibilty.
 */
class HintsReader implements AutoCloseable, Iterable<HintsReader.Page>
{
    private static final Logger logger = LoggerFactory.getLogger(HintsReader.class);

    // don't read more than 512 KB of hints at a time.
    private static final int PAGE_SIZE = 512 << 10;

    private final HintsDescriptor descriptor;
    private final File file;
    private final ChecksummedDataInput input;

    // we pass the RateLimiter into HintsReader itself because it's cheaper to calculate the size before the hint is deserialized
    @Nullable
    private final RateLimiter rateLimiter;

    protected HintsReader(HintsDescriptor descriptor, File file, ChecksummedDataInput reader, RateLimiter rateLimiter)
    {
        this.descriptor = descriptor;
        this.file = file;
        this.input = reader;
        this.rateLimiter = rateLimiter;
    }

    @SuppressWarnings("resource") // HintsReader owns input
    static HintsReader open(File file, RateLimiter rateLimiter)
    {
        ChecksummedDataInput reader = ChecksummedDataInput.open(file);
        try
        {
            HintsDescriptor descriptor = HintsDescriptor.deserialize(reader);
            if (descriptor.isCompressed())
            {
                // since the hints descriptor is always uncompressed, it needs to be read with the normal ChecksummedDataInput.
                // The compressed input is instantiated with the uncompressed input's position
                reader = CompressedChecksummedDataInput.upgradeInput(reader, descriptor.createCompressor());
            }
            return new HintsReader(descriptor, file, reader, rateLimiter);
        }
        catch (IOException e)
        {
            reader.close();
            throw new FSReadError(e, file);
        }
    }

    static HintsReader open(File file)
    {
        return open(file, null);
    }

    public void close()
    {
        input.close();
    }

    public HintsDescriptor descriptor()
    {
        return descriptor;
    }

    void seek(InputPosition newPosition)
    {
        input.seek(newPosition);
    }

    public Iterator<Page> iterator()
    {
        return new PagesIterator();
    }

    public ChecksummedDataInput getInput()
    {
        return input;
    }

    final class Page
    {
        public final InputPosition position;

        private Page(InputPosition inputPosition)
        {
            this.position = inputPosition;
        }

        Iterator<Hint> hintsIterator()
        {
            return new HintsIterator(position);
        }

        Iterator<ByteBuffer> buffersIterator()
        {
            return new BuffersIterator(position);
        }
    }

    final class PagesIterator extends AbstractIterator<Page>
    {
        @SuppressWarnings("resource")
        protected Page computeNext()
        {
            input.tryUncacheRead();

            if (input.isEOF())
                return endOfData();

            return new Page(input.getSeekPosition());
        }
    }

    /**
     * A decoding iterator that deserializes the hints as it goes.
     */
    final class HintsIterator extends AbstractIterator<Hint>
    {
        private final InputPosition offset;

        HintsIterator(InputPosition offset)
        {
            super();
            this.offset = offset;
        }

        protected Hint computeNext()
        {
            Hint hint;

            do
            {
                InputPosition position = input.getSeekPosition();

                if (input.isEOF())
                    return endOfData(); // reached EOF

                if (position.subtract(offset) >= PAGE_SIZE)
                    return endOfData(); // read page size or more bytes

                try
                {
                    hint = computeNextInternal();
                }
                catch (EOFException e)
                {
                    logger.warn("Unexpected EOF replaying hints ({}), likely due to unflushed hint file on shutdown; continuing", descriptor.fileName(), e);
                    return endOfData();
                }
                catch (IOException e)
                {
                    throw new FSReadError(e, file);
                }
            }
            while (hint == null);

            return hint;
        }

        private Hint computeNextInternal() throws IOException
        {
            input.resetCrc();
            input.resetLimit();

            int size = input.readInt();
            if (size == 0)
            {
                // Avoid throwing IOException when a hint file ends with a run of zeros - this
                // can happen when hard-rebooting unresponsive machines.
                if (!verifyAllZeros(input))
                    throw new IOException("Corrupt hint file found");
                throw new EOFException("Unexpected end of file (size == 0)");
            }

            // if we cannot corroborate the size via crc, then we cannot safely skip this hint
            if (!input.checkCrc())
                throw new IOException("Digest mismatch exception");

            return readHint(size);
        }

        private Hint readHint(int size) throws IOException
        {
            if (rateLimiter != null)
                rateLimiter.acquire(size);
            input.limit(size);

            Hint hint;
            try
            {
                hint = Hint.serializer.deserialize(input, descriptor.messagingVersion());
                input.checkLimit(0);
            }
            catch (UnknownColumnFamilyException e)
            {
                logger.warn("Failed to read a hint for {}: {} - table with id {} is unknown in file {}",
                            StorageService.instance.getEndpointForHostId(descriptor.hostId),
                            descriptor.hostId,
                            e.cfId,
                            descriptor.fileName());
                input.skipBytes(Ints.checkedCast(size - input.bytesPastLimit()));

                hint = null; // set the return value to null and let following code to update/check the CRC
            }

            if (input.checkCrc())
                return hint;

            // log a warning and skip the corrupted entry
            logger.warn("Failed to read a hint for {}: {} - digest mismatch for hint at position {} in file {}",
                        StorageService.instance.getEndpointForHostId(descriptor.hostId),
                        descriptor.hostId,
                        input.getPosition() - size - 4,
                        descriptor.fileName());
            return null;
        }
    }

    /**
     * A verbatim iterator that simply returns the underlying ByteBuffers.
     */
    final class BuffersIterator extends AbstractIterator<ByteBuffer>
    {
        private final InputPosition offset;

        BuffersIterator(InputPosition offset)
        {
            super();
            this.offset = offset;
        }

        protected ByteBuffer computeNext()
        {
            ByteBuffer buffer;

            do
            {
                InputPosition position = input.getSeekPosition();

                if (input.isEOF())
                    return endOfData(); // reached EOF

                if (position.subtract(offset) >= PAGE_SIZE)
                    return endOfData(); // read page size or more bytes

                try
                {
                    buffer = computeNextInternal();
                }
                catch (EOFException e)
                {
                    logger.warn("Unexpected EOF replaying hints ({}), likely due to unflushed hint file on shutdown; continuing", descriptor.fileName(), e);
                    return endOfData();
                }
                catch (IOException e)
                {
                    throw new FSReadError(e, file);
                }
            }
            while (buffer == null);

            return buffer;
        }

        private ByteBuffer computeNextInternal() throws IOException
        {
            input.resetCrc();
            input.resetLimit();

            int size = input.readInt();
            if (size == 0)
            {
                // Avoid throwing IOException when a hint file ends with a run of zeros - this
                // can happen when hard-rebooting unresponsive machines.
                if (!verifyAllZeros(input))
                    throw new IOException("Corrupt hint file found");
                throw new EOFException("Unexpected end of file (size == 0)");
            }

            // if we cannot corroborate the size via crc, then we cannot safely skip this hint
            if (!input.checkCrc())
                throw new IOException("Digest mismatch exception");

            return readBuffer(size);
        }

        private ByteBuffer readBuffer(int size) throws IOException
        {
            if (rateLimiter != null)
                rateLimiter.acquire(size);
            input.limit(size);

            ByteBuffer buffer = ByteBufferUtil.read(input, size);
            if (input.checkCrc())
                return buffer;

            // log a warning and skip the corrupted entry
            logger.warn("Failed to read a hint for {} - digest mismatch for hint at position {} in file {}",
                        descriptor.hostId,
                        input.getPosition() - size - 4,
                        descriptor.fileName());
            return null;
        }
    }

    private static boolean verifyAllZeros(ChecksummedDataInput input) throws IOException
    {
        while (!input.isEOF())
        {
            if (input.readByte() != 0)
                return false;
        }
        return true;
    }
}