/*
 * The MIT License
 *
 * Copyright (c) 2009 The Broad Institute
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */
package htsjdk.samtools.util;


import htsjdk.samtools.FileTruncatedException;
import htsjdk.samtools.SAMException;
import htsjdk.samtools.seekablestream.SeekableBufferedStream;
import htsjdk.samtools.seekablestream.SeekableFileStream;
import htsjdk.samtools.seekablestream.SeekableHTTPStream;
import htsjdk.samtools.seekablestream.SeekableStream;
import htsjdk.samtools.util.zip.InflaterFactory;

import java.io.*;
import java.net.URL;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.channels.SeekableByteChannel;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.StandardOpenOption;
import java.util.Arrays;

/**
 * Utility class for reading BGZF block compressed files.  The caller can treat this file like any other InputStream.
 * It probably is not necessary to wrap this stream in a buffering stream, because there is internal buffering.
 * The advantage of BGZF over conventional GZip format is that BGZF allows for seeking without having to read the
 * entire file up to the location being sought.  Note that seeking is only possible if the input stream is seekable.
 * 
 * Note that this implementation is not synchronized. If multiple threads access an instance concurrently, it must be synchronized externally. 
 *
 * c.f. http://samtools.sourceforge.net/SAM1.pdf for details of BGZF format
 */
public class BlockCompressedInputStream extends InputStream implements LocationAware {

    public final static String INCORRECT_HEADER_SIZE_MSG = "Incorrect header size for file: ";
    public final static String UNEXPECTED_BLOCK_LENGTH_MSG = "Unexpected compressed block length: ";
    public final static String PREMATURE_END_MSG = "Premature end of file: ";
    public final static String CANNOT_SEEK_STREAM_MSG = "Cannot seek a position for a non-file stream";
    public final static String CANNOT_SEEK_CLOSED_STREAM_MSG = "Cannot seek a position for a closed stream";
    public final static String INVALID_FILE_PTR_MSG = "Invalid file pointer: ";

    private InputStream mStream = null;
    private boolean mIsClosed = false;
    private SeekableStream mFile = null;
    private byte[] mFileBuffer = null;
    private DecompressedBlock mCurrentBlock = null;
    private int mCurrentOffset = 0;
    private long mStreamOffset = 0;
    private final BlockGunzipper blockGunzipper;

    /**
     * Note that seek() is not supported if this ctor is used.
     * @param stream source of bytes
     */
    public BlockCompressedInputStream(final InputStream stream) {
        this(stream, true, BlockGunzipper.getDefaultInflaterFactory());
    }

    /**
     * Note that seek() is not supported if this ctor is used.
     * @param stream source of bytes
     * @param inflaterFactory {@link InflaterFactory} used by {@link BlockGunzipper}
     */
    public BlockCompressedInputStream(final InputStream stream, final InflaterFactory inflaterFactory) {
        this(stream, true, inflaterFactory);
    }

    /**
     * Note that seek() is not supported if this ctor is used.
     * @param stream source of bytes
     * @param allowBuffering if true, allow buffering
     */
    public BlockCompressedInputStream(final InputStream stream, final boolean allowBuffering) {
        this(stream, allowBuffering, BlockGunzipper.getDefaultInflaterFactory());
    }

    /**
     * Note that seek() is not supported if this ctor is used.
     * @param stream source of bytes
     * @param allowBuffering if true, allow buffering
     * @param inflaterFactory {@link InflaterFactory} used by {@link BlockGunzipper}
     */
    public BlockCompressedInputStream(final InputStream stream, final boolean allowBuffering, final InflaterFactory inflaterFactory) {
        if (allowBuffering) {
            mStream = IOUtil.toBufferedStream(stream);
        }
        else {
            mStream = stream;
        }

        mFile = null;
        blockGunzipper = new BlockGunzipper(inflaterFactory);
    }

    /**
     * Use this ctor if you wish to call seek()
     * @param file source of bytes
     * @throws IOException
     */
    public BlockCompressedInputStream(final File file) throws IOException {
        this(file, BlockGunzipper.getDefaultInflaterFactory());
    }

    /**
     * Use this ctor if you wish to call seek()
     * @param file source of bytes
     * @param inflaterFactory {@link InflaterFactory} used by {@link BlockGunzipper}
     * @throws IOException
     */
    public BlockCompressedInputStream(final File file, final InflaterFactory inflaterFactory) throws IOException {
        mFile = new SeekableFileStream(file);
        mStream = null;
        blockGunzipper = new BlockGunzipper(inflaterFactory);
    }

    /**
     * @param url source of bytes
     */
    public BlockCompressedInputStream(final URL url) {
        this(url, BlockGunzipper.getDefaultInflaterFactory());
    }

    /**
     * @param url source of bytes
     * @param inflaterFactory {@link InflaterFactory} used by {@link BlockGunzipper}
     */
    public BlockCompressedInputStream(final URL url, final InflaterFactory inflaterFactory) {
        mFile = new SeekableBufferedStream(new SeekableHTTPStream(url));
        mStream = null;
        blockGunzipper = new BlockGunzipper(inflaterFactory);
    }

    /**
     * For providing some arbitrary data source.  No additional buffering is
     * provided, so if the underlying source is not buffered, wrap it in a
     * SeekableBufferedStream before passing to this ctor.
     * @param strm source of bytes
     */
    public BlockCompressedInputStream(final SeekableStream strm) {
        this(strm, BlockGunzipper.getDefaultInflaterFactory());
    }

    /**
     * For providing some arbitrary data source.  No additional buffering is
     * provided, so if the underlying source is not buffered, wrap it in a
     * SeekableBufferedStream before passing to this ctor.
     * @param strm source of bytes
     * @param inflaterFactory {@link InflaterFactory} used by {@link BlockGunzipper}
     */
    public BlockCompressedInputStream(final SeekableStream strm, final InflaterFactory inflaterFactory) {
        mFile = strm;
        mStream = null;
        blockGunzipper = new BlockGunzipper(inflaterFactory);
    }

    /**
     * Determines whether or not the inflater will re-calculated the CRC on the decompressed data
     * and check it against the value stored in the GZIP header.  CRC checking is an expensive
     * operation and should be used accordingly.
     */
    public void setCheckCrcs(final boolean check) {
        this.blockGunzipper.setCheckCrcs(check);
    }

    /**
     * @return the number of bytes that can be read (or skipped over) from this input stream without blocking by the
     * next caller of a method for this input stream. The next caller might be the same thread or another thread.
     * Note that although the next caller can read this many bytes without blocking, the available() method call itself
     * may block in order to fill an internal buffer if it has been exhausted.
     */
    @Override
    public int available() throws IOException {
        if (mCurrentBlock == null || mCurrentOffset == mCurrentBlock.mBlock.length) {
            readBlock();
        }
        if (mCurrentBlock == null) {
            return 0;
        }
        return mCurrentBlock.mBlock.length - mCurrentOffset;
    }

    /**
     * @return <code>true</code> if the stream is at the end of a BGZF block,
     * <code>false</code> otherwise.
     */
    public boolean endOfBlock() {
        return (mCurrentBlock != null && mCurrentOffset == mCurrentBlock.mBlock.length);
    }

    /**
     * Closes the underlying InputStream or RandomAccessFile
     */
    @Override
    public void close() throws IOException {
        if (mFile != null) {
            mFile.close();
            mFile = null;
        } else if (mStream != null) {
            mStream.close();
            mStream = null;
        }
        // Encourage garbage collection
        mFileBuffer = null;
        mCurrentBlock = null;

        // Mark as closed
        mIsClosed = true;
    }

    /**
     * Reads the next byte of data from the input stream. The value byte is returned as an int in the range 0 to 255.
     * If no byte is available because the end of the stream has been reached, the value -1 is returned.
     * This method blocks until input data is available, the end of the stream is detected, or an exception is thrown.

     * @return the next byte of data, or -1 if the end of the stream is reached.
     */
    @Override
    public int read() throws IOException {
        return (available() > 0) ? (mCurrentBlock.mBlock[mCurrentOffset++] & 0xFF) : -1;
    }

    /**
     * Reads some number of bytes from the input stream and stores them into the buffer array b. The number of bytes
     * actually read is returned as an integer. This method blocks until input data is available, end of file is detected,
     * or an exception is thrown.
     *
     * read(buf) has the same effect as read(buf, 0, buf.length).
     *
     * @param buffer the buffer into which the data is read.
     * @return the total number of bytes read into the buffer, or -1 is there is no more data because the end of
     * the stream has been reached.
     */
    @Override
    public int read(final byte[] buffer) throws IOException {
        return read(buffer, 0, buffer.length);
    }

    private volatile ByteArrayOutputStream buf = null;
    private static final byte eol = '\n';
    private static final byte eolCr = '\r';
    
    /**
     * Reads a whole line. A line is considered to be terminated by either a line feed ('\n'), 
     * carriage return ('\r') or carriage return followed by a line feed ("\r\n").
     *
     * @return  A String containing the contents of the line, excluding the line terminating
     *          character, or null if the end of the stream has been reached
     *
     * @exception  IOException  If an I/O error occurs
     */
    public String readLine() throws IOException {
        int available = available();
        if (available == 0) {
            return null;
        }
        if(null == buf){ // lazy initialisation 
            buf = new ByteArrayOutputStream(8192);
        }
        buf.reset();
        boolean done = false;
        boolean foundCr = false; // \r found flag
        while (!done) {
            int linetmpPos = mCurrentOffset;
            int bCnt = 0;
            while((available-- > 0)){
                final byte c = mCurrentBlock.mBlock[linetmpPos++];
                if(c == eol){ // found \n
                    done = true;
                    break;
                } else if(foundCr){  // previous char was \r
                    --linetmpPos; // current char is not \n so put it back
                    done = true;
                    break;
                } else if(c == eolCr){ // found \r
                    foundCr = true;
                    continue; // no ++bCnt
                }
                ++bCnt;
            }
            if(mCurrentOffset < linetmpPos) {
                buf.write(mCurrentBlock.mBlock, mCurrentOffset, bCnt);
                mCurrentOffset = linetmpPos;
            }
            available = available();
            if(available == 0) {
                // EOF
                done = true;
            }
        }
        return buf.toString();
    }

    /**
     * Reads up to len bytes of data from the input stream into an array of bytes. An attempt is made to read
     * as many as len bytes, but a smaller number may be read. The number of bytes actually read is returned as an integer.
     *
     * This method blocks until input data is available, end of file is detected, or an exception is thrown.
     *
     * @param buffer buffer into which data is read.
     * @param offset the start offset in array b  at which the data is written.
     * @param length the maximum number of bytes to read.
     * @return the total number of bytes read into the buffer, or -1 if there is no more data because the end of
     * the stream has been reached.
     */
    @Override
    public int read(final byte[] buffer, int offset, int length) throws IOException {
        final int originalLength = length;
        while (length > 0) {
            final int available = available();
            if (available == 0) {
                // Signal EOF to caller
                if (originalLength == length) {
                    return -1;
                }
                break;
            }
            final int copyLength = Math.min(length, available);
            System.arraycopy(mCurrentBlock.mBlock, mCurrentOffset, buffer, offset, copyLength);
            mCurrentOffset += copyLength;
            offset += copyLength;
            length -= copyLength;
        }
        return originalLength - length;
    }

    /**
     * Seek to the given position in the file.  Note that pos is a special virtual file pointer,
     * not an actual byte offset.
     *
     * @param pos virtual file pointer position
     * @throws IOException if stream is closed or not a file based stream
     */
    public void seek(final long pos) throws IOException {
        // Must be before the mFile == null check because mFile == null for closed files and streams
        if (mIsClosed) {
            throw new IOException(CANNOT_SEEK_CLOSED_STREAM_MSG);
        }

        // Cannot seek on streams that are not file based
        if (mFile == null) {
            throw new IOException(CANNOT_SEEK_STREAM_MSG);
        }

        // Decode virtual file pointer
        // Upper 48 bits is the byte offset into the compressed stream of a
        // block.
        // Lower 16 bits is the byte offset into the uncompressed stream inside
        // the block.
        final long compressedOffset = BlockCompressedFilePointerUtil.getBlockAddress(pos);
        final int uncompressedOffset = BlockCompressedFilePointerUtil.getBlockOffset(pos);
        final int available;
        if (mCurrentBlock != null && mCurrentBlock.mBlockAddress == compressedOffset) {
            available = mCurrentBlock.mBlock.length;
        } else {
            prepareForSeek();
            mFile.seek(compressedOffset);
            mStreamOffset = compressedOffset;
            mCurrentBlock = nextBlock(getBufferForReuse(mCurrentBlock));
            mCurrentOffset = 0;
            available = available();
        }
        if (uncompressedOffset > available || (uncompressedOffset == available && !eof())) {
            throw new IOException(INVALID_FILE_PTR_MSG + pos + " for " + getSource());
        }
        mCurrentOffset = uncompressedOffset;
    }
    
    /**
     * Performs cleanup required before seek is called on the underlying stream 
     */
    protected void prepareForSeek() {
    }

    private boolean eof() throws IOException {
        if (mFile.eof()) {
            return true;
        }
        // If the last remaining block is the size of the EMPTY_GZIP_BLOCK, this is the same as being at EOF.
        return (mFile.length() - (mCurrentBlock.mBlockAddress
                + mCurrentBlock.mBlockCompressedSize) == BlockCompressedStreamConstants.EMPTY_GZIP_BLOCK.length);
    }

    /**
     * @return virtual file pointer that can be passed to seek() to return to the current position.  This is
     * not an actual byte offset, so arithmetic on file pointers cannot be done to determine the distance between
     * the two.
     */
    public long getFilePointer() {
        if (mCurrentBlock == null) {
            // Haven't read anything yet = at start of stream
            return BlockCompressedFilePointerUtil.makeFilePointer(0, 0);
        }
        if (mCurrentOffset > 0 && mCurrentOffset == mCurrentBlock.mBlock.length) {
            // If current offset is at the end of the current block, file
            // pointer should point
            // to the beginning of the next block.
            return BlockCompressedFilePointerUtil.makeFilePointer(mCurrentBlock.mBlockAddress + mCurrentBlock.mBlockCompressedSize, 0);
        }
        return BlockCompressedFilePointerUtil.makeFilePointer(mCurrentBlock.mBlockAddress, mCurrentOffset);
    }

    @Override
    public long getPosition() {
        return getFilePointer();
    }

    public static long getFileBlock(final long bgzfOffset) {
        return BlockCompressedFilePointerUtil.getBlockAddress(bgzfOffset);
    }
    
    /**
     * @param stream Must be at start of file.  Throws RuntimeException if !stream.markSupported().
     * @return true if the given file looks like a valid BGZF file.
     */
    public static boolean isValidFile(final InputStream stream) throws IOException {
        if (!stream.markSupported()) {
            throw new RuntimeException("Cannot test non-buffered stream");
        }
        stream.mark(BlockCompressedStreamConstants.BLOCK_HEADER_LENGTH);
        final byte[] buffer = new byte[BlockCompressedStreamConstants.BLOCK_HEADER_LENGTH];
        final int count = readBytes(stream, buffer, 0, BlockCompressedStreamConstants.BLOCK_HEADER_LENGTH);
        stream.reset();
        return count == BlockCompressedStreamConstants.BLOCK_HEADER_LENGTH && isValidBlockHeader(buffer);
    }

    private static boolean isValidBlockHeader(final byte[] buffer) {
        return (buffer[0] == BlockCompressedStreamConstants.GZIP_ID1 &&
                (buffer[1] & 0xFF) == BlockCompressedStreamConstants.GZIP_ID2 &&
                (buffer[3] & BlockCompressedStreamConstants.GZIP_FLG) != 0 &&
                buffer[10] == BlockCompressedStreamConstants.GZIP_XLEN &&
                buffer[12] == BlockCompressedStreamConstants.BGZF_ID1 &&
                buffer[13] == BlockCompressedStreamConstants.BGZF_ID2);
    }

    private void readBlock() throws IOException {
        mCurrentBlock = nextBlock(getBufferForReuse(mCurrentBlock));
        mCurrentOffset = 0;
        checkAndRethrowDecompressionException();
    }
    /**
     * Reads and decompresses the next block
     * @param bufferAvailableForReuse decompression buffer available for reuse
     * @return next block in the decompressed stream
     */
    protected DecompressedBlock nextBlock(byte[] bufferAvailableForReuse) {
        return processNextBlock(bufferAvailableForReuse);
    }
    /**
     * Rethrows an exception encountered during decompression
     * @throws IOException
     */
    private void checkAndRethrowDecompressionException() throws IOException {
        if (mCurrentBlock.mException != null) {
            if (mCurrentBlock.mException instanceof IOException) {
                throw (IOException) mCurrentBlock.mException;
            } else if (mCurrentBlock.mException instanceof RuntimeException) {
                throw (RuntimeException) mCurrentBlock.mException;
            } else {
                throw new RuntimeException(mCurrentBlock.mException);
            }
        }
    }
    
    /**
     * Attempt to reuse the buffer of the given block 
     * @param block owning block
     * @return null decompressing buffer to reuse, null if no buffer is available
     */
    private byte[] getBufferForReuse(DecompressedBlock block) {
        if (block == null) return null;
        return block.mBlock;
    }
    
    /**
     * Decompress the next block from the input stream. When using asynchronous 
     * IO, this will be called by the background thread.
     * @param bufferAvailableForReuse buffer in which to place decompressed block. A null or
     *  incorrectly sized buffer will result in the buffer being ignored and
     *  a new buffer allocated for decompression.
     * @return next block in input stream
     */
    protected DecompressedBlock processNextBlock(byte[] bufferAvailableForReuse) {
        if (mFileBuffer == null) {
            mFileBuffer = new byte[BlockCompressedStreamConstants.MAX_COMPRESSED_BLOCK_SIZE];
        }
        long blockAddress = mStreamOffset;
        try {
            final int headerByteCount = readBytes(mFileBuffer, 0, BlockCompressedStreamConstants.BLOCK_HEADER_LENGTH);
            mStreamOffset += headerByteCount;
            if (headerByteCount == 0) {
                // Handle case where there is no empty gzip block at end.
                return new DecompressedBlock(blockAddress, new byte[0], 0);
            }
            if (headerByteCount != BlockCompressedStreamConstants.BLOCK_HEADER_LENGTH) {
                return new DecompressedBlock(blockAddress, headerByteCount, new IOException(INCORRECT_HEADER_SIZE_MSG + getSource()));
            }
            final int blockLength = unpackInt16(mFileBuffer, BlockCompressedStreamConstants.BLOCK_LENGTH_OFFSET) + 1;
            if (blockLength < BlockCompressedStreamConstants.BLOCK_HEADER_LENGTH || blockLength > mFileBuffer.length) {
                return new DecompressedBlock(blockAddress, blockLength,
                        new IOException(UNEXPECTED_BLOCK_LENGTH_MSG + blockLength + " for " + getSource()));
            }
            final int remaining = blockLength - BlockCompressedStreamConstants.BLOCK_HEADER_LENGTH;
            final int dataByteCount = readBytes(mFileBuffer, BlockCompressedStreamConstants.BLOCK_HEADER_LENGTH,
                    remaining);
            mStreamOffset += dataByteCount;
            if (dataByteCount != remaining) {
                return new DecompressedBlock(blockAddress, blockLength,
                        new FileTruncatedException(PREMATURE_END_MSG + getSource()));
            }
            final byte[] decompressed = inflateBlock(mFileBuffer, blockLength, bufferAvailableForReuse);
            return new DecompressedBlock(blockAddress, decompressed, blockLength);
        } catch (IOException e) {
            return new DecompressedBlock(blockAddress, 0, e);
        }
    }

    private byte[] inflateBlock(final byte[] compressedBlock, final int compressedLength,
            final byte[] bufferAvailableForReuse) throws IOException {
        final int uncompressedLength = unpackInt32(compressedBlock, compressedLength - 4);
        if (uncompressedLength < 0) {
            throw new RuntimeIOException(getSource() + " has invalid uncompressedLength: " + uncompressedLength);
        }
        byte[] buffer = bufferAvailableForReuse;
        if (buffer == null || uncompressedLength != buffer.length) {
        	// can't reuse the buffer since the size is incorrect
            buffer = new byte[uncompressedLength];
        }
        blockGunzipper.unzipBlock(buffer, compressedBlock, compressedLength);
        return buffer;
    }

    private String getSource() {
        return mFile == null ? "data stream" : mFile.getSource();
    }

    private int readBytes(final byte[] buffer, final int offset, final int length) throws IOException {
        if (mFile != null) {
            return readBytes(mFile, buffer, offset, length);
        } else if (mStream != null) {
            return readBytes(mStream, buffer, offset, length);
        } else {
            return 0;
        }
    }

    private static int readBytes(final SeekableStream file, final byte[] buffer, final int offset, final int length) throws IOException {
        int bytesRead = 0;
        while (bytesRead < length) {
            final int count = file.read(buffer, offset + bytesRead, length - bytesRead);
            if (count <= 0) {
                break;
            }
            bytesRead += count;
        }
        return bytesRead;
    }

    private static int readBytes(final InputStream stream, final byte[] buffer, final int offset, final int length) throws IOException {
        int bytesRead = 0;
        while (bytesRead < length) {
            final int count = stream.read(buffer, offset + bytesRead, length - bytesRead);
            if (count <= 0) {
                break;
            }
            bytesRead += count;
        }
        return bytesRead;
    }

    private int unpackInt16(final byte[] buffer, final int offset) {
        return ((buffer[offset] & 0xFF) |
                ((buffer[offset+1] & 0xFF) << 8));
    }

    private int unpackInt32(final byte[] buffer, final int offset) {
        return ((buffer[offset] & 0xFF) |
                ((buffer[offset+1] & 0xFF) << 8) |
                ((buffer[offset+2] & 0xFF) << 16) |
                ((buffer[offset+3] & 0xFF) << 24));
    }

    public enum FileTermination {HAS_TERMINATOR_BLOCK, HAS_HEALTHY_LAST_BLOCK, DEFECTIVE}

    /**
     *
     * @param file the file to check
     * @return status of the last compressed block
     * @throws IOException
     */
    public static FileTermination checkTermination(final File file) throws IOException {
        return checkTermination(IOUtil.toPath(file));
    }

    /**
     *
     * @param path to the file to check
     * @return status of the last compressed block
     * @throws IOException
     */
    public static FileTermination checkTermination(final Path path) throws IOException {
        try( final SeekableByteChannel channel = Files.newByteChannel(path, StandardOpenOption.READ) ){
            return checkTermination(channel);
        }
    }

    /**
     * check the status of the final bzgipped block for the given bgzipped resource
     *
     * @param channel an open channel to read from,
     * the channel will remain open and the initial position will be restored when the operation completes
     * this makes no guarantee about the state of the channel if an exception is thrown during reading
     *
     * @return the status of the last compressed black
     * @throws IOException
     */
    public static FileTermination checkTermination(SeekableByteChannel channel) throws IOException {
        final long fileSize = channel.size();
        if (fileSize < BlockCompressedStreamConstants.EMPTY_GZIP_BLOCK.length) {
            return FileTermination.DEFECTIVE;
        }
        final long initialPosition = channel.position();
        boolean exceptionThrown = false;
        try {
            channel.position(fileSize - BlockCompressedStreamConstants.EMPTY_GZIP_BLOCK.length);

            //Check if the end of the file is an empty gzip block which is used as the terminator for a bgzipped file
            final ByteBuffer lastBlockBuffer = ByteBuffer.allocate(BlockCompressedStreamConstants.EMPTY_GZIP_BLOCK.length);
            readFully(channel, lastBlockBuffer);
            if (Arrays.equals(lastBlockBuffer.array(), BlockCompressedStreamConstants.EMPTY_GZIP_BLOCK)) {
                return FileTermination.HAS_TERMINATOR_BLOCK;
            }

            //if the last block isn't an empty gzip block, check to see if it is a healthy compressed block or if it's corrupted
            final int bufsize = (int) Math.min(fileSize, BlockCompressedStreamConstants.MAX_COMPRESSED_BLOCK_SIZE);
            final byte[] bufferArray = new byte[bufsize];
            channel.position(fileSize - bufsize);
            readFully(channel, ByteBuffer.wrap(bufferArray));
            for (int i = bufferArray.length - BlockCompressedStreamConstants.EMPTY_GZIP_BLOCK.length;
                 i >= 0; --i) {
                if (!preambleEqual(BlockCompressedStreamConstants.GZIP_BLOCK_PREAMBLE,
                                   bufferArray, i, BlockCompressedStreamConstants.GZIP_BLOCK_PREAMBLE.length)) {
                    continue;
                }
                final ByteBuffer byteBuffer = ByteBuffer.wrap(bufferArray,
                                                              i + BlockCompressedStreamConstants.GZIP_BLOCK_PREAMBLE.length,
                                                              4);
                byteBuffer.order(ByteOrder.LITTLE_ENDIAN);
                final int totalBlockSizeMinusOne = byteBuffer.getShort() & 0xFFFF;
                if (bufferArray.length - i == totalBlockSizeMinusOne + 1) {
                    return FileTermination.HAS_HEALTHY_LAST_BLOCK;
                } else {
                    return FileTermination.DEFECTIVE;
                }
            }
            return FileTermination.DEFECTIVE;
        } catch (final Throwable e) {
            exceptionThrown = true;
            throw e;
        } finally {
            //if an exception was thrown we don't want to reset the position because that would be likely to throw again
            //and suppress the initial exception
            if(!exceptionThrown) {
                channel.position(initialPosition);
            }
        }
    }

    /**
     * read as many bytes as dst's capacity into dst or throw if that's not possible
     *
     * @throws EOFException if channel has fewer bytes available than dst's capacity
     */
    static void readFully(SeekableByteChannel channel, ByteBuffer dst) throws IOException {
        int totalBytesRead = 0;
        final int capacity = dst.capacity();
        while (totalBytesRead < capacity) {
            final int bytesRead = channel.read(dst);
            if (bytesRead == -1) {
                throw new EOFException();
            }
            totalBytesRead += bytesRead;
        }
    }

    public static void assertNonDefectiveFile(final File file) throws IOException {
        if (checkTermination(file) == FileTermination.DEFECTIVE) {
            throw new SAMException(file.getAbsolutePath() + " does not have a valid GZIP block at the end of the file.");
        }
    }

    private static boolean preambleEqual(final byte[] preamble, final byte[] buf, final int startOffset, final int length) {
        for (int i = 0; i < length; ++i) {
            if (preamble[i] != buf[i + startOffset]) {
                return false;
            }
        }
        return true;
    }

    protected static class DecompressedBlock {
        /**
         * Decompressed block
         */
        private final byte[] mBlock;
        /**
         * Compressed size of block (the uncompressed size can be found using
         * mBlock.length)
         */
        private final int mBlockCompressedSize;
        /**
         * Stream offset of start of block
         */
        private final long mBlockAddress;
        /**
         * Exception thrown (if any) when attempting to decompress block
         */
        private final Exception mException;

        public DecompressedBlock(long blockAddress, byte[] block, int compressedSize) {
            mBlock = block;
            mBlockAddress = blockAddress;
            mBlockCompressedSize = compressedSize;
            mException = null;
        }

        public DecompressedBlock(long blockAddress, int compressedSize, Exception exception) {
            mBlock = new byte[0];
            mBlockAddress = blockAddress;
            mBlockCompressedSize = compressedSize;
            mException = exception;
        }
    }
}
