/*
 * This file contains the coefficient buffer controller for decompression.
 * This controller is the top level of the JPEG decompressor proper.
 * The coefficient buffer lies between entropy decoding and inverse-DCT steps.
 *
 * In buffered-image mode, this controller is the interface between
 * input-oriented processing and output-oriented processing.
 * Also, the input side (only) is used when reading a file for transcoding.
 */

namespace BitMiracle.LibJpeg.Classic.Internal;

/// <summary>
/// Coefficient buffer control
/// 
/// This code applies interblock smoothing as described by section K.8
/// of the JPEG standard: the first 5 AC coefficients are estimated from
/// the DC values of a DCT block and its 8 neighboring blocks.
/// We apply smoothing only for progressive JPEG decoding, and only if
/// the coefficients it can estimate are not yet known to full precision.
/// </summary>
struct jpeg_d_coef_controller
{
    private const int SAVED_COEFS = 6; /* we save coef_bits[0..5] */

    /* Natural-order array positions of the first 5 zigzag-order coefficients */
    private const int Q01_POS = 1;
    private const int Q10_POS = 8;
    private const int Q20_POS = 16;
    private const int Q11_POS = 9;
    private const int Q02_POS = 2;

    private enum DecompressorType
    {
        Ordinary,
        Smooth,
        OnePass
    }

    private jpeg_decompress_struct m_cinfo;
    private bool m_useDummyConsumeData;
    private DecompressorType m_decompressor;

    /* These variables keep track of the current location of the input side. */
    /* cinfo.input_iMCU_row is also used for this. */
    private int m_MCU_ctr;     /* counts MCUs processed in current row */
    private int m_MCU_vert_offset;        /* counts MCU rows within iMCU row */
    private int m_MCU_rows_per_iMCU_row;  /* number of such rows needed */

    /* The output side's location is represented by cinfo.output_iMCU_row. */

    /* In single-pass modes, it's sufficient to buffer just one MCU.
    * We allocate a workspace of D_MAX_BLOCKS_IN_MCU coefficient blocks,
    * and let the entropy decoder write into that workspace each time.
    * (On 80x86, the workspace is FAR even though it's not really very big;
    * this is to keep the module interfaces unchanged when a large coefficient
    * buffer is necessary.)
    * In multi-pass modes, this array points to the current MCU's blocks
    * within the virtual arrays; it is used only by the input side.
    */
    private JBLOCK[] m_MCU_buffer = new JBLOCK[JpegConstants.D_MAX_BLOCKS_IN_MCU];

    /* In multi-pass modes, we need a virtual block array for each component. */
    private jvirt_array<JBLOCK>[] m_whole_image = new jvirt_array<JBLOCK>[JpegConstants.MAX_COMPONENTS];
    private jvirt_array<JBLOCK>[]? m_coef_arrays;

    /* When doing block smoothing, we latch coefficient Al values here */
    private int[]? m_coef_bits_latch;
    private int m_coef_bits_savedOffset;
    private readonly JvirtArrayWindow<JBLOCK>[] m_accessBuffer = new JvirtArrayWindow<JBLOCK>[JpegConstants.MAX_COMPS_IN_SCAN];

    public jpeg_d_coef_controller(jpeg_decompress_struct cinfo, bool need_full_buffer) : this()
    {
        m_cinfo = cinfo;
        /* Create the coefficient buffer. */
        if (need_full_buffer)
        {
            /* Allocate a full-image virtual array for each component, */
            /* padded to a multiple of samp_factor DCT blocks in each direction. */
            /* Note we ask for a pre-zeroed array. */
            for (int ci = 0; ci < cinfo.m_num_components; ci++)
            {
                m_whole_image[ci] = jpeg_common_struct.CreateBlocksArray(
                    JpegUtils.jround_up(cinfo.Comp_info![ci].Width_in_blocks, cinfo.Comp_info![ci].H_samp_factor),
                    JpegUtils.jround_up(cinfo.Comp_info![ci].height_in_blocks, cinfo.Comp_info![ci].V_samp_factor));
                m_whole_image[ci].ErrorProcessor = cinfo;
            }
            m_useDummyConsumeData = false;
            m_decompressor = DecompressorType.Ordinary;
            m_coef_arrays = m_whole_image; /* link to virtual arrays */
        }
        else
        {
            /* We only need a single-MCU buffer. */
            m_useDummyConsumeData = true;
            m_decompressor = DecompressorType.OnePass;
            m_coef_arrays = null; /* flag for no virtual arrays */
        }
    }

    /// <summary>
    /// Initialize for an input processing pass.
    /// </summary>
    public void start_input_pass()
    {
        m_cinfo.m_input_iMCU_row = 0;
        start_iMCU_row();
    }

    /// <summary>
    /// Consume input data and store it in the full-image coefficient buffer.
    /// We read as much as one fully interleaved MCU row ("iMCU" row) per call,
    /// ie, v_samp_factor block rows for each component in the scan.
    /// </summary>
    public ReadResult consume_data()
    {
        if (m_useDummyConsumeData)
        {
            return ReadResult.JPEG_SUSPENDED;  /* Always indicate nothing was done */
        }

        var buffer = m_accessBuffer;

        /* Align the virtual buffers for the components used in this scan. */
        for (int ci = 0; ci < m_cinfo.m_comps_in_scan; ci++)
        {
            jpeg_component_info componentInfo = m_cinfo.Comp_info![m_cinfo.m_cur_comp_info[ci]];

            buffer[ci] = m_whole_image[componentInfo.Component_index].AccessWindow(
                m_cinfo.m_input_iMCU_row * componentInfo.V_samp_factor, componentInfo.V_samp_factor);

            /* Note: entropy decoder expects buffer to be zeroed,
             * but this is handled automatically by the memory manager
             * because we requested a pre-zeroed array.
             */
        }

        /* Loop to process one whole iMCU row */
        for (int yoffset = m_MCU_vert_offset; yoffset < m_MCU_rows_per_iMCU_row; yoffset++)
        {
            for (int MCU_col_num = m_MCU_ctr; MCU_col_num < m_cinfo.m_MCUs_per_row; MCU_col_num++)
            {
                /* Construct list of pointers to DCT blocks belonging to this MCU */
                int blkn = 0;           /* index of current DCT block within MCU */
                for (int ci = 0; ci < m_cinfo.m_comps_in_scan; ci++)
                {
                    jpeg_component_info componentInfo = m_cinfo.Comp_info![m_cinfo.m_cur_comp_info[ci]];
                    int start_col = MCU_col_num * componentInfo.MCU_width;
                    for (int yindex = 0; yindex < componentInfo.MCU_height; yindex++)
                    {
                        for (int xindex = 0; xindex < componentInfo.MCU_width; xindex++)
                        {
                            m_MCU_buffer[blkn] = buffer[ci][yindex + yoffset][start_col + xindex];
                            blkn++;
                        }
                    }
                }
                /* Try to fetch the MCU. */
                if (!m_cinfo.m_entropy.decode_mcu(m_MCU_buffer))
                {
                    /* Suspension forced; update state counters and exit */
                    m_MCU_vert_offset = yoffset;
                    m_MCU_ctr = MCU_col_num;
                    return ReadResult.JPEG_SUSPENDED;
                }
            }
            /* Completed an MCU row, but perhaps not an iMCU row */
            m_MCU_ctr = 0;
        }

        /* Completed the iMCU row, advance counters for next one */
        m_cinfo.m_input_iMCU_row++;
        if (m_cinfo.m_input_iMCU_row < m_cinfo.m_total_iMCU_rows)
        {
            start_iMCU_row();
            return ReadResult.JPEG_ROW_COMPLETED;
        }

        /* Completed the scan */
        m_cinfo.m_inputctl.finish_input_pass();
        return ReadResult.JPEG_SCAN_COMPLETED;
    }

    /// <summary>
    /// Initialize for an output processing pass.
    /// </summary>
    public void start_output_pass()
    {
        /* If multipass, check to see whether to use block smoothing on this pass */
        if (m_coef_arrays != null)
        {
            if (m_cinfo.m_do_block_smoothing && smoothing_ok())
            {
                m_decompressor = DecompressorType.Smooth;
            }
            else
            {
                m_decompressor = DecompressorType.Ordinary;
            }
        }

        m_cinfo.m_output_iMCU_row = 0;
    }

    public ReadResult decompress_data(ComponentBuffer[] output_buf)
    {
        switch (m_decompressor)
        {
            case DecompressorType.Ordinary:
                return decompress_data_ordinary(output_buf);

            case DecompressorType.Smooth:
                return decompress_smooth_data(output_buf);

            case DecompressorType.OnePass:
                return decompress_onepass(output_buf);
        }

        m_cinfo.ERREXIT(J_MESSAGE_CODE.JERR_NOTIMPL);
        return 0;
    }

    /* Pointer to array of coefficient virtual arrays, or null if none */
    public jvirt_array<JBLOCK>[] GetCoefArrays()
    {
        return m_coef_arrays!;
    }

    /// <summary>
    /// Decompress and return some data in the single-pass case.
    /// Always attempts to emit one fully interleaved MCU row ("iMCU" row).
    /// Input and output must run in lockstep since we have only a one-MCU buffer.
    /// Return value is JPEG_ROW_COMPLETED, JPEG_SCAN_COMPLETED, or JPEG_SUSPENDED.
    /// 
    /// NB: output_buf contains a plane for each component in image,
    /// which we index according to the component's SOF position.
    /// </summary>
    private ReadResult decompress_onepass(ComponentBuffer[] output_buf)
    {
        int last_MCU_col = m_cinfo.m_MCUs_per_row - 1;
        int last_iMCU_row = m_cinfo.m_total_iMCU_rows - 1;

        /* Loop to process as much as one whole iMCU row */
        for (int yoffset = m_MCU_vert_offset; yoffset < m_MCU_rows_per_iMCU_row; yoffset++)
        {
            for (int MCU_col_num = m_MCU_ctr; MCU_col_num <= last_MCU_col; MCU_col_num++)
            {
                /* Try to fetch an MCU.  Entropy decoder expects buffer to be zeroed. */
                if (m_cinfo.lim_Se != 0)
                {
                    /* can bypass in DC only case */
                    for (int i = 0; i < m_cinfo.m_blocks_in_MCU; i++)
                    {
                        m_MCU_buffer[i].Clear();
                    }
                }

                if (!m_cinfo.m_entropy.decode_mcu(m_MCU_buffer))
                {
                    /* Suspension forced; update state counters and exit */
                    m_MCU_vert_offset = yoffset;
                    m_MCU_ctr = MCU_col_num;
                    return ReadResult.JPEG_SUSPENDED;
                }

                /* Determine where data should go in output_buf and do the IDCT thing.
                 * We skip dummy blocks at the right and bottom edges (but blkn gets
                 * incremented past them!).  Note the inner loop relies on having
                 * allocated the MCU_buffer[] blocks sequentially.
                 */
                int blkn = 0;           /* index of current DCT block within MCU */
                for (int ci = 0; ci < m_cinfo.m_comps_in_scan; ci++)
                {
                    jpeg_component_info componentInfo = m_cinfo.Comp_info![m_cinfo.m_cur_comp_info[ci]];

                    /* Don't bother to IDCT an uninteresting component. */
                    if (!componentInfo.component_needed)
                    {
                        blkn += componentInfo.MCU_blocks;
                        continue;
                    }

                    int useful_width = (MCU_col_num < last_MCU_col) ? componentInfo.MCU_width : componentInfo.last_col_width;
                    int outputIndex = yoffset * componentInfo.DCT_v_scaled_size;
                    int start_col = MCU_col_num * componentInfo.MCU_sample_width;
                    for (int yindex = 0; yindex < componentInfo.MCU_height; yindex++)
                    {
                        if (m_cinfo.m_input_iMCU_row < last_iMCU_row || yoffset + yindex < componentInfo.last_row_height)
                        {
                            int output_col = start_col;
                            for (int xindex = 0; xindex < useful_width; xindex++)
                            {
                                m_cinfo.m_idct.inverse(
                                    componentInfo.Component_index,
                                    m_MCU_buffer[blkn + xindex].ReadOnlyCoefficients,
                                    output_buf[componentInfo.Component_index],
                                    outputIndex,
                                    output_col);

                                output_col += componentInfo.DCT_h_scaled_size;
                            }
                        }

                        blkn += componentInfo.MCU_width;
                        outputIndex += componentInfo.DCT_v_scaled_size;
                    }
                }
            }

            /* Completed an MCU row, but perhaps not an iMCU row */
            m_MCU_ctr = 0;
        }

        /* Completed the iMCU row, advance counters for next one */
        m_cinfo.m_output_iMCU_row++;
        m_cinfo.m_input_iMCU_row++;
        if (m_cinfo.m_input_iMCU_row < m_cinfo.m_total_iMCU_rows)
        {
            start_iMCU_row();
            return ReadResult.JPEG_ROW_COMPLETED;
        }

        /* Completed the scan */
        m_cinfo.m_inputctl.finish_input_pass();
        return ReadResult.JPEG_SCAN_COMPLETED;
    }

    /// <summary>
    /// Decompress and return some data in the multi-pass case.
    /// Always attempts to emit one fully interleaved MCU row ("iMCU" row).
    /// Return value is JPEG_ROW_COMPLETED, JPEG_SCAN_COMPLETED, or JPEG_SUSPENDED.
    /// 
    /// NB: output_buf contains a plane for each component in image.
    /// </summary>
    private ReadResult decompress_data_ordinary(ComponentBuffer[] output_buf)
    {
        /* Force some input to be done if we are getting ahead of the input. */
        while (m_cinfo.m_input_scan_number < m_cinfo.m_output_scan_number ||
               (m_cinfo.m_input_scan_number == m_cinfo.m_output_scan_number &&
                m_cinfo.m_input_iMCU_row <= m_cinfo.m_output_iMCU_row))
        {
            if (m_cinfo.m_inputctl.consume_input() == ReadResult.JPEG_SUSPENDED)
            {
                return ReadResult.JPEG_SUSPENDED;
            }
        }

        int last_iMCU_row = m_cinfo.m_total_iMCU_rows - 1;

        /* OK, output from the virtual arrays. */
        for (int ci = 0; ci < m_cinfo.m_num_components; ci++)
        {
            jpeg_component_info componentInfo = m_cinfo.Comp_info![ci];

            /* Don't bother to IDCT an uninteresting component. */
            if (!componentInfo.component_needed)
            {
                continue;
            }

            /* Align the virtual buffer for this component. */
            var buffer = m_whole_image[ci].AccessWindow(m_cinfo.m_output_iMCU_row * componentInfo.V_samp_factor,
                componentInfo.V_samp_factor);

            /* Count non-dummy DCT block rows in this iMCU row. */
            int block_rows;
            if (m_cinfo.m_output_iMCU_row < last_iMCU_row)
            {
                block_rows = componentInfo.V_samp_factor;
            }
            else
            {
                /* NB: can't use last_row_height here; it is input-side-dependent! */
                block_rows = componentInfo.height_in_blocks % componentInfo.V_samp_factor;
                if (block_rows == 0)
                {
                    block_rows = componentInfo.V_samp_factor;
                }
            }

            /* Loop over all DCT blocks to be processed. */
            int rowIndex = 0;
            for (int block_row = 0; block_row < block_rows; block_row++)
            {
                int output_col = 0;
                for (int block_num = 0; block_num < componentInfo.Width_in_blocks; block_num++)
                {
                    m_cinfo.m_idct.inverse(
                        componentInfo.Component_index,
                        buffer[block_row][block_num].ReadOnlyCoefficients,
                        output_buf[ci],
                        rowIndex,
                        output_col);

                    output_col += componentInfo.DCT_h_scaled_size;
                }

                rowIndex += componentInfo.DCT_v_scaled_size;
            }
        }

        m_cinfo.m_output_iMCU_row++;
        if (m_cinfo.m_output_iMCU_row < m_cinfo.m_total_iMCU_rows)
        {
            return ReadResult.JPEG_ROW_COMPLETED;
        }

        return ReadResult.JPEG_SCAN_COMPLETED;
    }

    /// <summary>
    /// Variant of decompress_data for use when doing block smoothing.
    /// </summary>
    private ReadResult decompress_smooth_data(ComponentBuffer[] output_buf)
    {
        /* Force some input to be done if we are getting ahead of the input. */
        while (m_cinfo.m_input_scan_number <= m_cinfo.m_output_scan_number && !m_cinfo.m_inputctl.EOIReached())
        {
            if (m_cinfo.m_input_scan_number == m_cinfo.m_output_scan_number)
            {
                /* If input is working on current scan, we ordinarily want it to
                 * have completed the current row.  But if input scan is DC,
                 * we want it to keep one row ahead so that next block row's DC
                 * values are up to date.
                 */
                int delta = (m_cinfo.m_Ss == 0) ? 1 : 0;
                if (m_cinfo.m_input_iMCU_row > m_cinfo.m_output_iMCU_row + delta)
                {
                    break;
                }
            }

            if (m_cinfo.m_inputctl.consume_input() == ReadResult.JPEG_SUSPENDED)
            {
                return ReadResult.JPEG_SUSPENDED;
            }
        }

        int last_iMCU_row = m_cinfo.m_total_iMCU_rows - 1;

        var coefBitsLatch = m_coef_bits_latch!;

        /* OK, output from the virtual arrays. */
        for (int ci = 0; ci < m_cinfo.m_num_components; ci++)
        {
            jpeg_component_info componentInfo = m_cinfo.Comp_info![ci];

            /* Don't bother to IDCT an uninteresting component. */
            if (!componentInfo.component_needed)
            {
                continue;
            }

            int block_rows;
            int access_rows;
            bool last_row;
            /* Count non-dummy DCT block rows in this iMCU row. */
            if (m_cinfo.m_output_iMCU_row < last_iMCU_row)
            {
                block_rows = componentInfo.V_samp_factor;
                access_rows = block_rows * 2; /* this and next iMCU row */
                last_row = false;
            }
            else
            {
                /* NB: can't use last_row_height here; it is input-side-dependent! */
                block_rows = componentInfo.height_in_blocks % componentInfo.V_samp_factor;
                if (block_rows == 0)
                {
                    block_rows = componentInfo.V_samp_factor;
                }

                access_rows = block_rows; /* this iMCU row only */
                last_row = true;
            }

            /* Align the virtual buffer for this component. */
            var buffer = default(JvirtArrayWindow<JBLOCK>);
            bool first_row;
            int bufferRowOffset = 0;
            if (m_cinfo.m_output_iMCU_row > 0)
            {
                access_rows += componentInfo.V_samp_factor; /* prior iMCU row too */
                buffer = m_whole_image[ci].AccessWindow((m_cinfo.m_output_iMCU_row - 1) * componentInfo.V_samp_factor, access_rows);
                bufferRowOffset = componentInfo.V_samp_factor; /* point to current iMCU row */
                first_row = false;
            }
            else
            {
                buffer = m_whole_image[ci].AccessWindow(0, access_rows);
                first_row = true;
            }

            /* Fetch component-dependent info */
            int coefBitsOffset = ci * SAVED_COEFS;
            JQUANT_TBL qtbl = componentInfo.quant_table!;
            int Q00 = qtbl.quantval[0];
            int Q01 = qtbl.quantval[Q01_POS];
            int Q10 = qtbl.quantval[Q10_POS];
            int Q20 = qtbl.quantval[Q20_POS];
            int Q11 = qtbl.quantval[Q11_POS];
            int Q02 = qtbl.quantval[Q02_POS];
            int outputIndex = ci;

            /* Loop over all DCT blocks to be processed. */
            for (int block_row = 0; block_row < block_rows; block_row++)
            {
                int bufferIndex = bufferRowOffset + block_row;

                int prev_block_row;
                if (first_row && block_row == 0)
                {
                    prev_block_row = bufferIndex;
                }
                else
                {
                    prev_block_row = bufferIndex - 1;
                }

                int next_block_row;
                if (last_row && block_row == block_rows - 1)
                {
                    next_block_row = bufferIndex;
                }
                else
                {
                    next_block_row = bufferIndex + 1;
                }

                /* We fetch the surrounding DC values using a sliding-register approach.
                 * Initialize all nine here so as to do the right thing on narrow pics.
                 */
                int DC1 = buffer[prev_block_row][0][0];
                int DC2 = DC1;
                int DC3 = DC1;

                int DC4 = buffer[bufferIndex][0][0];
                int DC5 = DC4;
                int DC6 = DC4;

                int DC7 = buffer[next_block_row][0][0];
                int DC8 = DC7;
                int DC9 = DC7;

                int output_col = 0;
                int last_block_column = componentInfo.Width_in_blocks - 1;
                for (int block_num = 0; block_num <= last_block_column; block_num++)
                {
                    /* Fetch current DCT block into workspace so we can modify it. */
                    JBLOCK workspace = new JBLOCK();
                    workspace.CopyFrom(in buffer[bufferIndex][0]);

                    /* Update DC values */
                    if (block_num < last_block_column)
                    {
                        DC3 = buffer[prev_block_row][1][0];
                        DC6 = buffer[bufferIndex][1][0];
                        DC9 = buffer[next_block_row][1][0];
                    }

                    /* Compute coefficient estimates per K.8.
                     * An estimate is applied only if coefficient is still zero,
                     * and is not known to be fully accurate.
                     */
                    /* AC01 */
                    int Al = coefBitsLatch[m_coef_bits_savedOffset + coefBitsOffset + 1];
                    if (Al != 0 && workspace[1] == 0)
                    {
                        int pred;
                        int num = 36 * Q00 * (DC4 - DC6);
                        if (num >= 0)
                        {
                            pred = ((Q01 << 7) + num) / (Q01 << 8);
                            if (Al > 0 && pred >= (1 << Al))
                            {
                                pred = (1 << Al) - 1;
                            }
                        }
                        else
                        {
                            pred = ((Q01 << 7) - num) / (Q01 << 8);
                            if (Al > 0 && pred >= (1 << Al))
                            {
                                pred = (1 << Al) - 1;
                            }

                            pred = -pred;
                        }
                        workspace[1] = (short)pred;
                    }

                    /* AC10 */
                    Al = coefBitsLatch[m_coef_bits_savedOffset + coefBitsOffset + 2];
                    if (Al != 0 && workspace[8] == 0)
                    {
                        int pred;
                        int num = 36 * Q00 * (DC2 - DC8);
                        if (num >= 0)
                        {
                            pred = ((Q10 << 7) + num) / (Q10 << 8);
                            if (Al > 0 && pred >= (1 << Al))
                            {
                                pred = (1 << Al) - 1;
                            }
                        }
                        else
                        {
                            pred = ((Q10 << 7) - num) / (Q10 << 8);
                            if (Al > 0 && pred >= (1 << Al))
                            {
                                pred = (1 << Al) - 1;
                            }

                            pred = -pred;
                        }
                        workspace[8] = (short)pred;
                    }

                    /* AC20 */
                    Al = coefBitsLatch[m_coef_bits_savedOffset + coefBitsOffset + 3];
                    if (Al != 0 && workspace[16] == 0)
                    {
                        int pred;
                        int num = 9 * Q00 * (DC2 + DC8 - 2 * DC5);
                        if (num >= 0)
                        {
                            pred = ((Q20 << 7) + num) / (Q20 << 8);
                            if (Al > 0 && pred >= (1 << Al))
                            {
                                pred = (1 << Al) - 1;
                            }
                        }
                        else
                        {
                            pred = ((Q20 << 7) - num) / (Q20 << 8);
                            if (Al > 0 && pred >= (1 << Al))
                            {
                                pred = (1 << Al) - 1;
                            }

                            pred = -pred;
                        }
                        workspace[16] = (short)pred;
                    }

                    /* AC11 */
                    Al = coefBitsLatch[m_coef_bits_savedOffset + coefBitsOffset + 4];
                    if (Al != 0 && workspace[9] == 0)
                    {
                        int pred;
                        int num = 5 * Q00 * (DC1 - DC3 - DC7 + DC9);
                        if (num >= 0)
                        {
                            pred = ((Q11 << 7) + num) / (Q11 << 8);
                            if (Al > 0 && pred >= (1 << Al))
                            {
                                pred = (1 << Al) - 1;
                            }
                        }
                        else
                        {
                            pred = ((Q11 << 7) - num) / (Q11 << 8);
                            if (Al > 0 && pred >= (1 << Al))
                            {
                                pred = (1 << Al) - 1;
                            }

                            pred = -pred;
                        }
                        workspace[9] = (short)pred;
                    }

                    /* AC02 */
                    Al = coefBitsLatch[m_coef_bits_savedOffset + coefBitsOffset + 5];
                    if (Al != 0 && workspace[2] == 0)
                    {
                        int pred;
                        int num = 9 * Q00 * (DC4 + DC6 - 2 * DC5);
                        if (num >= 0)
                        {
                            pred = ((Q02 << 7) + num) / (Q02 << 8);
                            if (Al > 0 && pred >= (1 << Al))
                            {
                                pred = (1 << Al) - 1;
                            }
                        }
                        else
                        {
                            pred = ((Q02 << 7) - num) / (Q02 << 8);
                            if (Al > 0 && pred >= (1 << Al))
                            {
                                pred = (1 << Al) - 1;
                            }

                            pred = -pred;
                        }
                        workspace[2] = (short)pred;
                    }

                    /* OK, do the IDCT */
                    m_cinfo.m_idct.inverse(
                        componentInfo.Component_index,
                        workspace.ReadOnlyCoefficients,
                        output_buf[outputIndex],
                        0,
                        output_col);

                    /* Advance for next column */
                    DC1 = DC2;
                    DC2 = DC3;
                    DC4 = DC5;
                    DC5 = DC6;
                    DC7 = DC8;
                    DC8 = DC9;

                    bufferIndex++;
                    prev_block_row++;
                    next_block_row++;

                    output_col += componentInfo.DCT_h_scaled_size;
                }

                outputIndex += componentInfo.DCT_v_scaled_size;
            }
        }

        m_cinfo.m_output_iMCU_row++;
        if (m_cinfo.m_output_iMCU_row < m_cinfo.m_total_iMCU_rows)
        {
            return ReadResult.JPEG_ROW_COMPLETED;
        }

        return ReadResult.JPEG_SCAN_COMPLETED;
    }

    /// <summary>
    /// Determine whether block smoothing is applicable and safe.
    /// We also latch the current states of the coef_bits[] entries for the
    /// AC coefficients; otherwise, if the input side of the decompressor
    /// advances into a new scan, we might think the coefficients are known
    /// more accurately than they really are.
    /// </summary>
    private bool smoothing_ok()
    {
        if (!m_cinfo.m_progressive_mode || m_cinfo.m_coef_bits == null)
        {
            return false;
        }

        /* Allocate latch area if not already done */
        if (m_coef_bits_latch == null)
        {
            m_coef_bits_latch = new int[m_cinfo.m_num_components * SAVED_COEFS];
            m_coef_bits_savedOffset = 0;
        }

        bool smoothing_useful = false;
        int[][] coefBits = m_cinfo.m_coef_bits!;
        for (int ci = 0; ci < m_cinfo.m_num_components; ci++)
        {
            /* All components' quantization values must already be latched. */
            JQUANT_TBL? qtable = m_cinfo.Comp_info![ci].quant_table;
            if (qtable == null)
            {
                return false;
            }

            /* Verify DC & first 5 AC quantizers are nonzero to avoid zero-divide. */
            if (qtable.quantval[0] == 0 || qtable.quantval[Q01_POS] == 0 ||
                qtable.quantval[Q10_POS] == 0 || qtable.quantval[Q20_POS] == 0 ||
                qtable.quantval[Q11_POS] == 0 || qtable.quantval[Q02_POS] == 0)
            {
                return false;
            }

            /* DC values must be at least partly known for all components. */
            if (coefBits[ci][0] < 0)
            {
                return false;
            }

            /* Block smoothing is helpful if some AC coefficients remain inaccurate. */
            for (int coefi = 1; coefi <= 5; coefi++)
            {
                m_coef_bits_latch![m_coef_bits_savedOffset + coefi] = coefBits[ci][coefi];
                if (coefBits[ci][coefi] != 0)
                {
                    smoothing_useful = true;
                }
            }

            m_coef_bits_savedOffset += SAVED_COEFS;
        }

        return smoothing_useful;
    }

    /// <summary>
    /// Reset within-iMCU-row counters for a new row (input side)
    /// </summary>
    private void start_iMCU_row()
    {
        /* In an interleaved scan, an MCU row is the same as an iMCU row.
         * In a noninterleaved scan, an iMCU row has v_samp_factor MCU rows.
         * But at the bottom of the image, process only what's left.
         */
        if (m_cinfo.m_comps_in_scan > 1)
        {
            m_MCU_rows_per_iMCU_row = 1;
        }
        else
        {
            jpeg_component_info componentInfo = m_cinfo.Comp_info![m_cinfo.m_cur_comp_info[0]];

            if (m_cinfo.m_input_iMCU_row < (m_cinfo.m_total_iMCU_rows - 1))
            {
                m_MCU_rows_per_iMCU_row = componentInfo.V_samp_factor;
            }
            else
            {
                m_MCU_rows_per_iMCU_row = componentInfo.last_row_height;
            }
        }

        m_MCU_ctr = 0;
        m_MCU_vert_offset = 0;
    }
}
