source: trunk/eraser6/Eraser.Util.FileSystem/Fat12Api.cpp @ 1239

/* 
 * $Id$
 * Copyright 2009 The Eraser Project
 * Original Author: Joel Low <lowjoel@users.sourceforge.net>
 * Modified By: 
 * 
 * This file is part of Eraser.
 * 
 * Eraser is free software: you can redistribute it and/or modify it under the
 * terms of the GNU General Public License as published by the Free Software
 * Foundation, either version 3 of the License, or (at your option) any later
 * version.
 * 
 * Eraser 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 General Public License for more details.
 * 
 * A copy of the GNU General Public License can be found at
 * <http://www.gnu.org/licenses/>.
 */

#include <stdafx.h>
#include "FatApi.h"

using namespace System::IO;
using namespace System::Runtime::InteropServices;

namespace Eraser {
namespace Util {
    Fat12Api::Fat12Api(VolumeInfo^ info) : Fat12Or16Api(info)
    {
        //Sanity checks: check that this volume is FAT16!
        if (!IsFat12() || info->VolumeFormat == L"FAT16")
            throw gcnew ArgumentException(L"The volume provided is not a FAT12 volume.");
    }

    Fat12Api::Fat12Api(VolumeInfo^ info, IO::Stream^ stream) : Fat12Or16Api(info, stream)
    {
        //Sanity checks: check that this volume is FAT16!
        if (!IsFat12() || info->VolumeFormat == L"FAT16")
            throw gcnew ArgumentException(L"The volume provided is not a FAT12 volume.");
    }

    bool Fat12Api::IsClusterAllocated(unsigned cluster)
    {
        unsigned nextCluster = GetFatValue(cluster);

        if (
            nextCluster <= 0x001 ||
            (nextCluster >= 0xFF0 && nextCluster <= 0xFF6) ||
            nextCluster == 0xFF7
        )
            return false;

        return true;
    }

    unsigned Fat12Api::GetNextCluster(unsigned cluster)
    {
        unsigned nextCluster = GetFatValue(cluster);
        if (nextCluster <= 0x001 || (nextCluster >= 0xFF0 && nextCluster <= 0xFF6))
            throw gcnew ArgumentException(L"Invalid FAT cluster: cluster is marked free.");
        else if (nextCluster == 0xFF7)
            throw gcnew ArgumentException(L"Invalid FAT cluster: cluster is marked bad.");
        else if (nextCluster >= 0xFF8)
            return 0xFFFFFFFF;
        else
            return nextCluster;
    }

    unsigned Fat12Api::FileSize(unsigned cluster)
    {
        for (unsigned result = 1; ; ++result)
        {
            unsigned nextCluster = GetFatValue(cluster);
            if (nextCluster <= 0x001 || (nextCluster >= 0xFFF0 && nextCluster <= 0xFF6))
                throw gcnew ArgumentException(L"Invalid FAT cluster: cluster is marked free.");
            else if (nextCluster == 0xFF7)
                throw gcnew ArgumentException(L"Invalid FAT cluster: cluster is marked bad.");
            else if (nextCluster >= 0xFF8)
                return ClusterSizeToSize(result);
            else
                cluster = nextCluster;
        }
    }

    unsigned Fat12Api::GetFatValue(unsigned cluster)
    {
        //Get the pointer to the FAT entry. Round the cluster value down to the nearest
        //even number (since 2 clusters share 3 bytes)
        char* fatEntry = Fat + ((cluster & ~1) / 2) * 3;
        unsigned fatValue = 0;
        for (size_t i = 0; i < 3; ++i)
            fatValue |= static_cast<unsigned>(static_cast<unsigned char>(*(fatEntry + i))) << (i * 8);

        //Get the correct half of the 24 bits. If the cluster is odd we take the 12 least significant bits
        if (cluster & 1)
            fatValue >>= 12;
        else
            fatValue &= 0xFFF;

        //Return the result.
        return fatValue;
    }
}
}