/*
 * 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.commons.compress.compressors.gzip;

import java.io.IOException;
import java.io.EOFException;
import java.io.InputStream;
import java.io.DataInputStream;
import java.io.BufferedInputStream;
import java.util.zip.DataFormatException;
import java.util.zip.Inflater;
import java.util.zip.CRC32;

import org.apache.commons.compress.compressors.CompressorInputStream;

/**
 * Input stream that decompresses .gz files.
 * This supports decompressing concatenated .gz files which is important
 * when decompressing standalone .gz files.
 * <p>
 * {@link java.util.zip.GZIPInputStream} doesn't decompress concatenated .gz
 * files: it stops after the first member and silently ignores the rest.
 * It doesn't leave the read position to point to the beginning of the next
 * member, which makes it difficult workaround the lack of concatenation
 * support.
 * <p>
 * Instead of using <code>GZIPInputStream</code>, this class has its own .gz
 * container format decoder. The actual decompression is done with
 * {@link java.util.zip.Inflater}.
 */
public class GzipCompressorInputStream extends CompressorInputStream {
    // Header flags
    // private static final int FTEXT = 0x01; // Uninteresting for us
    private static final int FHCRC = 0x02;
    private static final int FEXTRA = 0x04;
    private static final int FNAME = 0x08;
    private static final int FCOMMENT = 0x10;
    private static final int FRESERVED = 0xE0;

    // Compressed input stream, possibly wrapped in a BufferedInputStream
    private final InputStream in;

    // True if decompressing multimember streams.
    private final boolean decompressConcatenated;

    // Buffer to hold the input data
    private final byte[] buf = new byte[8192];

    // Amount of data in buf.
    private int bufUsed = 0;

    // Decompressor
    private Inflater inf = new Inflater(true);

    // CRC32 from uncompressed data
    private final CRC32 crc = new CRC32();

    private int memberSize;

    // True once everything has been decompressed
    private boolean endReached = false;

    // used in no-arg read method
    private final byte[] oneByte = new byte[1];

    /**
     * Constructs a new input stream that decompresses gzip-compressed data
     * from the specified input stream.
     * <p>
     * This is equivalent to
     * <code>GzipCompressorInputStream(inputStream, false)</code> and thus
     * will not decompress concatenated .gz files.
     *
     * @param inputStream  the InputStream from which this object should
     *                     be created of
     *
     * @throws IOException if the stream could not be created
     */
    public GzipCompressorInputStream(InputStream inputStream)
            throws IOException {
        this(inputStream, false);
    }

    /**
     * Constructs a new input stream that decompresses gzip-compressed data
     * from the specified input stream.
     * <p>
     * If <code>decompressConcatenated</code> is {@code false}:
     * This decompressor might read more input than it will actually use.
     * If <code>inputStream</code> supports <code>mark</code> and
     * <code>reset</code>, then the input position will be adjusted
     * so that it is right after the last byte of the compressed stream.
     * If <code>mark</code> isn't supported, the input position will be
     * undefined.
     *
     * @param inputStream  the InputStream from which this object should
     *                     be created of
     * @param decompressConcatenated
     *                     if true, decompress until the end of the input;
     *                     if false, stop after the first .gz member
     *
     * @throws IOException if the stream could not be created
     */
    public GzipCompressorInputStream(InputStream inputStream,
                                     boolean decompressConcatenated)
            throws IOException {
        // Mark support is strictly needed for concatenated files only,
        // but it's simpler if it is always available.
        if (inputStream.markSupported()) {
            in = inputStream;
        } else {
            in = new BufferedInputStream(inputStream);
        }

        this.decompressConcatenated = decompressConcatenated;
        init(true);
    }

    private boolean init(boolean isFirstMember) throws IOException {
        assert isFirstMember || decompressConcatenated;

        // Check the magic bytes without a possibility of EOFException.
        int magic0 = in.read();
        int magic1 = in.read();

        // If end of input was reached after decompressing at least
        // one .gz member, we have reached the end of the file successfully.
        if (magic0 == -1 && !isFirstMember) {
            return false;
        }

        if (magic0 != 31 || magic1 != 139) {
            throw new IOException(isFirstMember
                                  ? "Input is not in the .gz format"
                                  : "Garbage after a valid .gz stream");
        }

        // Parsing the rest of the header may throw EOFException.
        DataInputStream inData = new DataInputStream(in);
        int method = inData.readUnsignedByte();
        if (method != 8) {
            throw new IOException("Unsupported compression method "
                                  + method + " in the .gz header");
        }

        int flg = inData.readUnsignedByte();
        if ((flg & FRESERVED) != 0) {
            throw new IOException(
                    "Reserved flags are set in the .gz header");
        }

        inData.readInt(); // mtime, ignored
        inData.readUnsignedByte(); // extra flags, ignored
        inData.readUnsignedByte(); // operating system, ignored

        // Extra field, ignored
        if ((flg & FEXTRA) != 0) {
            int xlen = inData.readUnsignedByte();
            xlen |= inData.readUnsignedByte() << 8;

            // This isn't as efficient as calling in.skip would be,
            // but it's lazier to handle unexpected end of input this way.
            // Most files don't have an extra field anyway.
            while (xlen-- > 0) {
                inData.readUnsignedByte();
            }
        }

        // Original file name, ignored
        if ((flg & FNAME) != 0) {
            readToNull(inData);
        }

        // Comment, ignored
        if ((flg & FCOMMENT) != 0) {
            readToNull(inData);
        }

        // Header "CRC16" which is actually a truncated CRC32 (which isn't
        // as good as real CRC16). I don't know if any encoder implementation
        // sets this, so it's not worth trying to verify it. GNU gzip 1.4
        // doesn't support this field, but zlib seems to be able to at least
        // skip over it.
        if ((flg & FHCRC) != 0) {
            inData.readShort();
        }

        // Reset
        inf.reset();
        crc.reset();
        memberSize = 0;

        return true;
    }

    private void readToNull(DataInputStream inData) throws IOException {
        while (inData.readUnsignedByte() != 0x00) {}
    }

    @Override
    public int read() throws IOException {
        return read(oneByte, 0, 1) == -1 ? -1 : (oneByte[0] & 0xFF);
    }

    /**
     * {@inheritDoc}
     *
     * @since 1.1
     */
    @Override
    public int read(byte[] b, int off, int len) throws IOException {
        if (endReached) {
            return -1;
        }

        int size = 0;

        while (len > 0) {
            if (inf.needsInput()) {
                // Remember the current position because we may need to
                // rewind after reading too much input.
                in.mark(buf.length);

                bufUsed = in.read(buf);
                if (bufUsed == -1) {
                    throw new EOFException();
                }

                inf.setInput(buf, 0, bufUsed);
            }

            int ret;
            try {
                ret = inf.inflate(b, off, len);
            } catch (DataFormatException e) {
                throw new IOException("Gzip-compressed data is corrupt");
            }

            crc.update(b, off, ret);
            memberSize += ret;
            off += ret;
            len -= ret;
            size += ret;
            count(ret);

            if (inf.finished()) {
                // We may have read too many bytes. Rewind the read
                // position to match the actual amount used.
                //
                // NOTE: The "if" is there just in case. Since we used
                // in.mark earler, it should always skip enough.
                in.reset();

                int skipAmount = bufUsed - inf.getRemaining();
                if (in.skip(skipAmount) != skipAmount) {
                    throw new IOException();
                }

                bufUsed = 0;

                DataInputStream inData = new DataInputStream(in);

                // CRC32
                long crcStored = 0;
                for (int i = 0; i < 4; ++i) {
                    crcStored |= (long)inData.readUnsignedByte() << (i * 8);
                }

                if (crcStored != crc.getValue()) {
                    throw new IOException("Gzip-compressed data is corrupt "
                                          + "(CRC32 error)");
                }

                // Uncompressed size modulo 2^32 (ISIZE in the spec)
                int isize = 0;
                for (int i = 0; i < 4; ++i) {
                    isize |= inData.readUnsignedByte() << (i * 8);
                }

                if (isize != memberSize) {
                    throw new IOException("Gzip-compressed data is corrupt"
                                          + "(uncompressed size mismatch)");
                }

                // See if this is the end of the file.
                if (!decompressConcatenated || !init(false)) {
                    inf.end();
                    inf = null;
                    endReached = true;
                    return size == 0 ? -1 : size;
                }
            }
        }

        return size;
    }

    /**
     * Checks if the signature matches what is expected for a .gz file.
     *
     * @param signature the bytes to check
     * @param length    the number of bytes to check
     * @return          true if this is a .gz stream, false otherwise
     *
     * @since 1.1
     */
    public static boolean matches(byte[] signature, int length) {

        if (length < 2) {
            return false;
        }

        if (signature[0] != 31) {
            return false;
        }

        if (signature[1] != -117) {
            return false;
        }

        return true;
    }

    /**
     * Closes the input stream (unless it is System.in).
     *
     * @since 1.2
     */
    @Override
    public void close() throws IOException {
        if (inf != null) {
            inf.end();
            inf = null;
        }

        if (this.in != System.in) {
            this.in.close();
        }
    }
}