source: josm/trunk/src/org/tukaani/xz/common/DecoderUtil.java@ 14039

/*
 * DecoderUtil
 *
 * Author: Lasse Collin <lasse.collin@tukaani.org>
 *
 * This file has been put into the public domain.
 * You can do whatever you want with this file.
 */

package org.tukaani.xz.common;

import java.io.InputStream;
import java.io.IOException;
import java.io.EOFException;
import java.util.zip.CRC32;
import org.tukaani.xz.XZ;
import org.tukaani.xz.XZFormatException;
import org.tukaani.xz.CorruptedInputException;
import org.tukaani.xz.UnsupportedOptionsException;

public class DecoderUtil extends Util {
    public static boolean isCRC32Valid(byte[] buf, int off, int len,
                                       int ref_off) {
        CRC32 crc32 = new CRC32();
        crc32.update(buf, off, len);
        long value = crc32.getValue();

        for (int i = 0; i < 4; ++i)
            if ((byte)(value >>> (i * 8)) != buf[ref_off + i])
                return false;

        return true;
    }

    public static StreamFlags decodeStreamHeader(byte[] buf)
            throws IOException {
        for (int i = 0; i < XZ.HEADER_MAGIC.length; ++i)
            if (buf[i] != XZ.HEADER_MAGIC[i])
                throw new XZFormatException();

        if (!isCRC32Valid(buf, XZ.HEADER_MAGIC.length, 2,
                          XZ.HEADER_MAGIC.length + 2))
            throw new CorruptedInputException("XZ Stream Header is corrupt");

        try {
            return decodeStreamFlags(buf, XZ.HEADER_MAGIC.length);
        } catch (UnsupportedOptionsException e) {
            throw new UnsupportedOptionsException(
                    "Unsupported options in XZ Stream Header");
        }
    }

    public static StreamFlags decodeStreamFooter(byte[] buf)
            throws IOException {
        if (buf[10] != XZ.FOOTER_MAGIC[0] || buf[11] != XZ.FOOTER_MAGIC[1]) {
            // NOTE: The exception could be XZFormatException too.
            // It depends on the situation which one is better.
            throw new CorruptedInputException("XZ Stream Footer is corrupt");
        }

        if (!isCRC32Valid(buf, 4, 6, 0))
            throw new CorruptedInputException("XZ Stream Footer is corrupt");

        StreamFlags streamFlags;
        try {
            streamFlags = decodeStreamFlags(buf, 8);
        } catch (UnsupportedOptionsException e) {
            throw new UnsupportedOptionsException(
                    "Unsupported options in XZ Stream Footer");
        }

        streamFlags.backwardSize = 0;
        for (int i = 0; i < 4; ++i)
            streamFlags.backwardSize |= (buf[i + 4] & 0xFF) << (i * 8);

        streamFlags.backwardSize = (streamFlags.backwardSize + 1) * 4;

        return streamFlags;
    }

    private static StreamFlags decodeStreamFlags(byte[] buf, int off)
            throws UnsupportedOptionsException {
        if (buf[off] != 0x00 || (buf[off + 1] & 0xFF) >= 0x10)
            throw new UnsupportedOptionsException();

        StreamFlags streamFlags = new StreamFlags();
        streamFlags.checkType = buf[off + 1];

        return streamFlags;
    }

    public static boolean areStreamFlagsEqual(StreamFlags a, StreamFlags b) {
        // backwardSize is intentionally not compared.
        return a.checkType == b.checkType;
    }

    public static long decodeVLI(InputStream in) throws IOException {
        int b = in.read();
        if (b == -1)
            throw new EOFException();

        long num = b & 0x7F;
        int i = 0;

        while ((b & 0x80) != 0x00) {
            if (++i >= VLI_SIZE_MAX)
                throw new CorruptedInputException();

            b = in.read();
            if (b == -1)
                throw new EOFException();

            if (b == 0x00)
                throw new CorruptedInputException();

            num |= (long)(b & 0x7F) << (i * 7);
        }

        return num;
    }
}