Aaru/Aaru.Images/HDCopy/Helpers.cs

// /***************************************************************************
// Aaru Data Preservation Suite
// ----------------------------------------------------------------------------
//
// Filename       : Helpers.cs
// Author(s)      : Natalia Portillo <claunia@claunia.com>
//
// Component      : Disk image plugins.
//
// --[ Description ] ----------------------------------------------------------
//
//     Contains helpers for HD-Copy disk images.
//
// --[ License ] --------------------------------------------------------------
//
//     This library is free software; you can redistribute it and/or modify
//     it under the terms of the GNU Lesser General Public License as
//     published by the Free Software Foundation; either version 2.1 of the
//     License, or (at your option) any later version.
//
//     This library is distributed in the hope that it will be useful, but
//     WITHOUT ANY WARRANTY; without even the implied warranty of
//     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
//     Lesser General Public License for more details.
//
//     You should have received a copy of the GNU Lesser General Public
//     License along with this library; if not, see <http://www.gnu.org/licenses/>.
//
// ----------------------------------------------------------------------------
// Copyright © 2017 Michael Drüing
// Copyright © 2011-2021 Natalia Portillo
// ****************************************************************************/

using System;
using System.IO;

namespace Aaru.DiscImages
{
    public sealed partial class HdCopy
    {
        bool TryReadHeader(Stream stream, ref FileHeader fhdr, ref long dataStartOffset)
        {
            int numTracks = 82;

            stream.Seek(0, SeekOrigin.Begin);

            if(stream.Length < 16 + (2 * 82))
                return false;

            FileHeader fheader;

            /* assume it's a regular HD-Copy file without the disk name */
            dataStartOffset = 2 + 2 * numTracks;
            fheader.lastCylinder = (byte)stream.ReadByte();
            fheader.sectorsPerTrack = (byte)stream.ReadByte();
            if (fheader.lastCylinder == 0xff && fheader.sectorsPerTrack == 0x18)
            {
                /* This is an "extended" HD-Copy file with filename information and 84 tracks */
                stream.Seek(0x0e, SeekOrigin.Begin);
                fheader.lastCylinder = (byte)stream.ReadByte();
                fheader.sectorsPerTrack = (byte)stream.ReadByte();
                numTracks = 84;
                dataStartOffset = 16 + 2 * numTracks;
            }
            fheader.trackMap = new byte[2 * numTracks];
            stream.Read(fheader.trackMap, 0, 2 * numTracks);

            /* Some sanity checks on the values we just read.
             * We know the image is from a DOS floppy disk, so assume
             * some sane cylinder and sectors-per-track count.
             */
            if(fheader.sectorsPerTrack < 8 ||
               fheader.sectorsPerTrack > 40)
                return false;

            if(fheader.lastCylinder < 37 ||
               fheader.lastCylinder >= 84)
                return false;

            // Validate the trackmap. First two tracks need to be present
            if(fheader.trackMap[0] != 1 ||
               fheader.trackMap[1] != 1)
                return false;

            // all other tracks must be either present (=1) or absent (=0)
            for(int i = 0; i < 2 * numTracks; i++)
                if(fheader.trackMap[i] > 1)
                    return false;

            /* return success */
            fhdr = fheader;
            return true;
        }

        void ReadTrackIntoCache(Stream stream, int trackNum)
        {
            byte[] trackData = new byte[_imageInfo.SectorSize * _imageInfo.SectorsPerTrack];
            byte[] blkHeader = new byte[3];

            // check that track is present
            if(_trackOffset[trackNum] == -1)
                throw new InvalidDataException("Tried reading a track that is not present in image");

            stream.Seek(_trackOffset[trackNum], SeekOrigin.Begin);

            // read the compressed track data
            stream.Read(blkHeader, 0, 3);
            short compressedLength = (short)(BitConverter.ToInt16(blkHeader, 0) - 1);
            byte  escapeByte       = blkHeader[2];

            byte[] cBuffer = new byte[compressedLength];
            stream.Read(cBuffer, 0, compressedLength);

            // decompress the data
            int sIndex = 0; // source buffer position
            int dIndex = 0; // destination buffer position

            while(sIndex < compressedLength)
                if(cBuffer[sIndex] == escapeByte)
                {
                    sIndex++; // skip over escape byte
                    byte fillByte  = cBuffer[sIndex++];
                    byte fillCount = cBuffer[sIndex++];

                    // fill destination buffer
                    for(int i = 0; i < fillCount; i++)
                        trackData[dIndex++] = fillByte;
                }
                else
                    trackData[dIndex++] = cBuffer[sIndex++];

            // check that the number of bytes decompressed matches a whole track
            if(dIndex != _imageInfo.SectorSize * _imageInfo.SectorsPerTrack)
                throw new InvalidDataException("Track decompression yielded incomplete data");

            // store track in cache
            _trackCache[trackNum] = trackData;
        }
    }
}