C++ VSIVirtualHandle::Seek方法代码示例

int VSIFSeekL( VSILFILE * fp, vsi_l_offset nOffset, int nWhence )

{
    VSIVirtualHandle *poFileHandle = (VSIVirtualHandle *) fp;
    
    return poFileHandle->Seek( nOffset, nWhence );
}

int VSIBufferedReaderHandle::Seek( vsi_l_offset nOffset, int nWhence )
{
    //CPLDebug( "BUFFERED", "Seek(%d,%d)", (int)nOffset, (int)nWhence);
    bEOF = FALSE;
    if (nWhence == SEEK_CUR)
        nCurOffset += nOffset;
    else if (nWhence == SEEK_END)
    {
        poBaseHandle->Seek(nOffset, nWhence);
        nCurOffset = poBaseHandle->Tell();
        bNeedBaseHandleSeek = TRUE;
    }
    else
        nCurOffset = nOffset;

    return 0;
}

int VSISparseFileFilesystemHandler::Stat( const char * pszFilename, 
                                          VSIStatBufL * psStatBuf,
                                          int nFlags )

{
    VSIVirtualHandle *poFile = Open( pszFilename, "r" );

    memset( psStatBuf, 0, sizeof(VSIStatBufL) );

    if( poFile == NULL )
        return -1;

    poFile->Seek( 0, SEEK_END );
    size_t nLength = (size_t) poFile->Tell();
    delete poFile;

    int nResult = VSIStatExL( pszFilename + strlen("/vsisparse/"), 
                            psStatBuf, nFlags );

    psStatBuf->st_size = nLength;

    return nResult;
}

int VSICachedFile::LoadBlocks( size_t nStartBlock, size_t nBlockCount,
                               void *pBuffer, size_t nBufferSize )

{
    if( nBlockCount == 0 )
        return 1;

    if( apoCache.size() < nStartBlock + nBlockCount )
        apoCache.resize( nStartBlock + nBlockCount );

/* -------------------------------------------------------------------- */
/*      When we want to load only one block, we can directly load it    */
/*      into the target buffer with no concern about intermediaries.    */
/* -------------------------------------------------------------------- */
    if( nBlockCount == 1 )
    {
        poBase->Seek( nStartBlock * CHUNK_SIZE, SEEK_SET );

        apoCache[nStartBlock] = new VSICacheChunk();

        VSICacheChunk *poBlock = apoCache[nStartBlock];

        poBlock->iBlock = nStartBlock;
        poBlock->nDataFilled = poBase->Read( poBlock->abyData, 1, CHUNK_SIZE );
        nCacheUsed += poBlock->nDataFilled;

        // Merges into the LRU list. 
        Demote( poBlock );

        return 1;
    }

/* -------------------------------------------------------------------- */
/*      If the buffer is quite large but not quite large enough to      */
/*      hold all the blocks we will take the pain of splitting the      */
/*      io request in two in order to avoid allocating a large          */
/*      temporary buffer.                                               */
/* -------------------------------------------------------------------- */
    if( nBufferSize > CHUNK_SIZE * 20 
        && nBufferSize < nBlockCount * CHUNK_SIZE )
    {
        if( !LoadBlocks( nStartBlock, 2, pBuffer, nBufferSize ) )
            return 0;

        return LoadBlocks( nStartBlock+2, nBlockCount-2, pBuffer, nBufferSize );
    }

/* -------------------------------------------------------------------- */
/*      Do we need to allocate our own buffer?                          */
/* -------------------------------------------------------------------- */
    GByte *pabyWorkBuffer = (GByte *) pBuffer;

    if( nBufferSize < CHUNK_SIZE * nBlockCount )
        pabyWorkBuffer = (GByte *) CPLMalloc(CHUNK_SIZE * nBlockCount);

/* -------------------------------------------------------------------- */
/*      Read the whole request into the working buffer.                 */
/* -------------------------------------------------------------------- */
    if( poBase->Seek( nStartBlock * CHUNK_SIZE, SEEK_SET ) != 0 )
        return 0;

    size_t nDataRead = poBase->Read( pabyWorkBuffer, 1, nBlockCount*CHUNK_SIZE);

    if( nBlockCount * CHUNK_SIZE > nDataRead + CHUNK_SIZE - 1 )
        nBlockCount = (nDataRead + CHUNK_SIZE - 1) / CHUNK_SIZE;

    for( size_t i = 0; i < nBlockCount; i++ )
    {
        VSICacheChunk *poBlock = new VSICacheChunk();

        poBlock->iBlock = nStartBlock + i;

        CPLAssert( apoCache[i+nStartBlock] == NULL );

        apoCache[i + nStartBlock] = poBlock;

        if( nDataRead >= (i+1) * CHUNK_SIZE )
            poBlock->nDataFilled = CHUNK_SIZE;
        else
            poBlock->nDataFilled = nDataRead - i*CHUNK_SIZE;

        memcpy( poBlock->abyData, pabyWorkBuffer + i*CHUNK_SIZE,
                (size_t) poBlock->nDataFilled );

        nCacheUsed += poBlock->nDataFilled;

        // Merges into the LRU list. 
        Demote( poBlock );
    }

    if( pabyWorkBuffer != pBuffer )
        CPLFree( pabyWorkBuffer );

    return 1;
}

size_t VSIBufferedReaderHandle::Read( void *pBuffer, size_t nSize, size_t nMemb )
{
    const size_t nTotalToRead = nSize * nMemb;
    //CPLDebug( "BUFFERED", "Read(%d)", (int)nTotalToRead);

    if (nSize == 0)
        return 0;

    if (nBufferSize != 0 &&
            nCurOffset >= nBufferOffset && nCurOffset <= nBufferOffset + nBufferSize)
    {
        /* We try to read from an offset located within the buffer */
        const int nReadInBuffer = (int) MIN(nTotalToRead, nBufferOffset + nBufferSize - nCurOffset);
        memcpy(pBuffer, pabyBuffer + nCurOffset - nBufferOffset, nReadInBuffer);
        const int nToReadInFile = nTotalToRead - nReadInBuffer;
        if (nToReadInFile > 0)
        {
            /* The beginning of the the data to read is located in the buffer */
            /* but the end must be read from the file */
            if (bNeedBaseHandleSeek)
                poBaseHandle->Seek(nBufferOffset + nBufferSize, SEEK_SET);
            bNeedBaseHandleSeek = FALSE;
            //CPLAssert(poBaseHandle->Tell() == nBufferOffset + nBufferSize);

            const int nReadInFile = poBaseHandle->Read((GByte*)pBuffer + nReadInBuffer, 1, nToReadInFile);
            const int nRead = nReadInBuffer + nReadInFile;

            nBufferSize = MIN(nRead, MAX_BUFFER_SIZE);
            nBufferOffset = nCurOffset + nRead - nBufferSize;
            memcpy(pabyBuffer, (GByte*)pBuffer + nRead - nBufferSize, nBufferSize);

            nCurOffset += nRead;
            //CPLAssert(poBaseHandle->Tell() == nBufferOffset + nBufferSize);
            //CPLAssert(poBaseHandle->Tell() == nCurOffset);

            bEOF = poBaseHandle->Eof();

            return nRead / nSize;
        }
        else
        {
            /* The data to read is completely located within the buffer */
            nCurOffset += nTotalToRead;
            return nTotalToRead / nSize;
        }
    }
    else
    {
        /* We try either to read before or after the buffer, so a seek is necessary */
        poBaseHandle->Seek(nCurOffset, SEEK_SET);
        bNeedBaseHandleSeek = FALSE;
        const int nReadInFile = poBaseHandle->Read(pBuffer, 1, nTotalToRead);
        nBufferSize = MIN(nReadInFile, MAX_BUFFER_SIZE);
        nBufferOffset = nCurOffset + nReadInFile - nBufferSize;
        memcpy(pabyBuffer, (GByte*)pBuffer + nReadInFile - nBufferSize, nBufferSize);

        nCurOffset += nReadInFile;
        //CPLAssert(poBaseHandle->Tell() == nBufferOffset + nBufferSize);
        //CPLAssert(poBaseHandle->Tell() == nCurOffset);

        bEOF = poBaseHandle->Eof();

        return nReadInFile / nSize;
    }

}