/*

 * jdapistd.c

 *

 * This file was part of the Independent JPEG Group's software:

 * Copyright (C) 1994-1996, Thomas G. Lane.

 * libjpeg-turbo Modifications:

 * Copyright (C) 2010, 2015-2020, 2022-2024, D. R. Commander.

 * Copyright (C) 2015, Google, Inc.

 * For conditions of distribution and use, see the accompanying README.ijg

 * file.

 *

 * This file contains application interface code for the decompression half

 * of the JPEG library.  These are the "standard" API routines that are

 * used in the normal full-decompression case.  They are not used by a

 * transcoding-only application.  Note that if an application links in

 * jpeg_start_decompress, it will end up linking in the entire decompressor.

 * We thus must separate this file from jdapimin.c to avoid linking the

 * whole decompression library into a transcoder.

 */

#include "jinclude.h"

#if BITS_IN_JSAMPLE != 16 || defined(D_LOSSLESS_SUPPORTED)

#include "jdmainct.h"

#include "jdcoefct.h"

#else

#define JPEG_INTERNALS

#include "jpeglib.h"

#endif

#include "jdmaster.h"

#include "jdmerge.h"

#include "jdsample.h"

#include "jmemsys.h"

#if BITS_IN_JSAMPLE == 8

/* Forward declarations */

LOCAL(boolean) output_pass_setup(j_decompress_ptr cinfo);

/*

 * Decompression initialization.

 * jpeg_read_header must be completed before calling this.

 *

 * If a multipass operating mode was selected, this will do all but the

 * last pass, and thus may take a great deal of time.

 *

 * Returns FALSE if suspended.  The return value need be inspected only if

 * a suspending data source is used.

 */

GLOBAL(boolean)

jpeg_start_decompress(j_decompress_ptr cinfo)

{

  if (cinfo->global_state == DSTATE_READY) {

    /* First call: initialize master control, select active modules */

    jinit_master_decompress(cinfo);

    if (cinfo->buffered_image) {

      /* No more work here; expecting jpeg_start_output next */

      cinfo->global_state = DSTATE_BUFIMAGE;

      return TRUE;

    }

    cinfo->global_state = DSTATE_PRELOAD;

  }

  if (cinfo->global_state == DSTATE_PRELOAD) {

    /* If file has multiple scans, absorb them all into the coef buffer */

    if (cinfo->inputctl->has_multiple_scans) {

#ifdef D_MULTISCAN_FILES_SUPPORTED

      for (;;) {

        int retcode;

        /* Call progress monitor hook if present */

        if (cinfo->progress != NULL)

          (*cinfo->progress->progress_monitor) ((j_common_ptr)cinfo);

        /* Absorb some more input */

        retcode = (*cinfo->inputctl->consume_input) (cinfo);

        if (retcode == JPEG_SUSPENDED)

          return FALSE;

        if (retcode == JPEG_REACHED_EOI)

          break;

        /* Advance progress counter if appropriate */

        if (cinfo->progress != NULL &&

            (retcode == JPEG_ROW_COMPLETED || retcode == JPEG_REACHED_SOS)) {

          if (++cinfo->progress->pass_counter >= cinfo->progress->pass_limit) {

            /* jdmaster underestimated number of scans; ratchet up one scan */

            cinfo->progress->pass_limit += (long)cinfo->total_iMCU_rows;

          }

        }

      }

#else

      ERREXIT(cinfo, JERR_NOT_COMPILED);

#endif /* D_MULTISCAN_FILES_SUPPORTED */

    }

    cinfo->output_scan_number = cinfo->input_scan_number;

  } else if (cinfo->global_state != DSTATE_PRESCAN)

    ERREXIT1(cinfo, JERR_BAD_STATE, cinfo->global_state);

  /* Perform any dummy output passes, and set up for the final pass */

  return output_pass_setup(cinfo);

}

/*

 * Set up for an output pass, and perform any dummy pass(es) needed.

 * Common subroutine for jpeg_start_decompress and jpeg_start_output.

 * Entry: global_state = DSTATE_PRESCAN only if previously suspended.

 * Exit: If done, returns TRUE and sets global_state for proper output mode.

 *       If suspended, returns FALSE and sets global_state = DSTATE_PRESCAN.

 */

LOCAL(boolean)

output_pass_setup(j_decompress_ptr cinfo)

{

  if (cinfo->global_state != DSTATE_PRESCAN) {

    /* First call: do pass setup */

    (*cinfo->master->prepare_for_output_pass) (cinfo);

    cinfo->output_scanline = 0;

    cinfo->global_state = DSTATE_PRESCAN;

  }

  /* Loop over any required dummy passes */

  while (cinfo->master->is_dummy_pass) {

#ifdef QUANT_2PASS_SUPPORTED

    /* Crank through the dummy pass */

    while (cinfo->output_scanline < cinfo->output_height) {

      JDIMENSION last_scanline;

      /* Call progress monitor hook if present */

      if (cinfo->progress != NULL) {

        cinfo->progress->pass_counter = (long)cinfo->output_scanline;

        cinfo->progress->pass_limit = (long)cinfo->output_height;

        (*cinfo->progress->progress_monitor) ((j_common_ptr)cinfo);

      }

      /* Process some data */

      last_scanline = cinfo->output_scanline;

#ifdef D_LOSSLESS_SUPPORTED

      if (cinfo->data_precision == 16)

        (*cinfo->main->process_data_16) (cinfo, (J16SAMPARRAY)NULL,

                                         &cinfo->output_scanline,

                                         (JDIMENSION)0);

      else

#endif

      if (cinfo->data_precision == 12)

        (*cinfo->main->process_data_12) (cinfo, (J12SAMPARRAY)NULL,

                                         &cinfo->output_scanline,

                                         (JDIMENSION)0);

      else

        (*cinfo->main->process_data) (cinfo, (JSAMPARRAY)NULL,

                                      &cinfo->output_scanline, (JDIMENSION)0);

      if (cinfo->output_scanline == last_scanline)

        return FALSE;           /* No progress made, must suspend */

    }

    /* Finish up dummy pass, and set up for another one */

    (*cinfo->master->finish_output_pass) (cinfo);

    (*cinfo->master->prepare_for_output_pass) (cinfo);

    cinfo->output_scanline = 0;

#else

    ERREXIT(cinfo, JERR_NOT_COMPILED);

#endif /* QUANT_2PASS_SUPPORTED */

  }

  /* Ready for application to drive output pass through

   * _jpeg_read_scanlines or _jpeg_read_raw_data.

   */

  cinfo->global_state = cinfo->raw_data_out ? DSTATE_RAW_OK : DSTATE_SCANNING;

  return TRUE;

}

#endif /* BITS_IN_JSAMPLE == 8 */

#if BITS_IN_JSAMPLE != 16

/*

 * Enable partial scanline decompression

 *

 * Must be called after jpeg_start_decompress() and before any calls to

 * _jpeg_read_scanlines() or _jpeg_skip_scanlines().

 *

 * Refer to libjpeg.txt for more information.

 */

GLOBAL(void)

_jpeg_crop_scanline(j_decompress_ptr cinfo, JDIMENSION *xoffset,

                    JDIMENSION *width)

{

  int ci, align, orig_downsampled_width;

  JDIMENSION input_xoffset;

  boolean reinit_upsampler = FALSE;

  jpeg_component_info *compptr;

#ifdef UPSAMPLE_MERGING_SUPPORTED

  my_master_ptr master = (my_master_ptr)cinfo->master;

#endif

  if (cinfo->data_precision != BITS_IN_JSAMPLE)

    ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);

  if (cinfo->master->lossless)

    ERREXIT(cinfo, JERR_NOTIMPL);

  if ((cinfo->global_state != DSTATE_SCANNING &&

       cinfo->global_state != DSTATE_BUFIMAGE) || cinfo->output_scanline != 0)

    ERREXIT1(cinfo, JERR_BAD_STATE, cinfo->global_state);

  if (!xoffset || !width)

    ERREXIT(cinfo, JERR_BAD_CROP_SPEC);

  /* xoffset and width must fall within the output image dimensions. */

  if (*width == 0 ||

      (unsigned long long)(*xoffset) + *width > cinfo->output_width)

    ERREXIT(cinfo, JERR_WIDTH_OVERFLOW);

  /* No need to do anything if the caller wants the entire width. */

  if (*width == cinfo->output_width)

    return;

  /* Ensuring the proper alignment of xoffset is tricky.  At minimum, it

   * must align with an MCU boundary, because:

   *

   *   (1) The IDCT is performed in blocks, and it is not feasible to modify

   *       the algorithm so that it can transform partial blocks.

   *   (2) Because of the SIMD extensions, any input buffer passed to the

   *       upsampling and color conversion routines must be aligned to the

   *       SIMD word size (for instance, 128-bit in the case of SSE2.)  The

   *       easiest way to accomplish this without copying data is to ensure

   *       that upsampling and color conversion begin at the start of the

   *       first MCU column that will be inverse transformed.

   *

   * In practice, we actually impose a stricter alignment requirement.  We

   * require that xoffset be a multiple of the maximum MCU column width of all

   * of the components (the "iMCU column width.")  This is to simplify the

   * single-pass decompression case, allowing us to use the same MCU column

   * width for all of the components.

   */

  if (cinfo->comps_in_scan == 1 && cinfo->num_components == 1)

    align = cinfo->_min_DCT_scaled_size;

  else

    align = cinfo->_min_DCT_scaled_size * cinfo->max_h_samp_factor;

  /* Adjust xoffset to the nearest iMCU boundary <= the requested value */

  input_xoffset = *xoffset;

  *xoffset = (input_xoffset / align) * align;

  /* Adjust the width so that the right edge of the output image is as

   * requested (only the left edge is altered.)  It is important that calling

   * programs check this value after this function returns, so that they can

   * allocate an output buffer with the appropriate size.

   */

  *width = *width + input_xoffset - *xoffset;

  cinfo->output_width = *width;

#ifdef UPSAMPLE_MERGING_SUPPORTED

  if (master->using_merged_upsample && cinfo->max_v_samp_factor == 2) {

    my_merged_upsample_ptr upsample = (my_merged_upsample_ptr)cinfo->upsample;

    upsample->out_row_width =

      cinfo->output_width * cinfo->out_color_components;

  }

#endif

  /* Set the first and last iMCU columns that we must decompress.  These values

   * will be used in single-scan decompressions.

   */

  cinfo->master->first_iMCU_col = (JDIMENSION)(long)(*xoffset) / (long)align;

  cinfo->master->last_iMCU_col =

    (JDIMENSION)jdiv_round_up((long)(*xoffset + cinfo->output_width),

                              (long)align) - 1;

  for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;

       ci++, compptr++) {

    int hsf = (cinfo->comps_in_scan == 1 && cinfo->num_components == 1) ?

              1 : compptr->h_samp_factor;

    /* Set downsampled_width to the new output width. */

    orig_downsampled_width = compptr->downsampled_width;

    compptr->downsampled_width =

      (JDIMENSION)jdiv_round_up((long)cinfo->output_width *

                                (long)(compptr->h_samp_factor *

                                       compptr->_DCT_scaled_size),

                                (long)(cinfo->max_h_samp_factor *

                                       cinfo->_min_DCT_scaled_size));

    if (compptr->downsampled_width < 2 && orig_downsampled_width >= 2)

      reinit_upsampler = TRUE;

    /* Set the first and last iMCU columns that we must decompress.  These

     * values will be used in multi-scan decompressions.

     */

    cinfo->master->first_MCU_col[ci] =

      (JDIMENSION)(long)(*xoffset * hsf) / (long)align;

    cinfo->master->last_MCU_col[ci] =

      (JDIMENSION)jdiv_round_up((long)((*xoffset + cinfo->output_width) * hsf),

                                (long)align) - 1;

  }

  if (reinit_upsampler) {

    cinfo->master->jinit_upsampler_no_alloc = TRUE;

    _jinit_upsampler(cinfo);

    cinfo->master->jinit_upsampler_no_alloc = FALSE;

  }

}

#endif /* BITS_IN_JSAMPLE != 16 */

/*

 * Read some scanlines of data from the JPEG decompressor.

 *

 * The return value will be the number of lines actually read.

 * This may be less than the number requested in several cases,

 * including bottom of image, data source suspension, and operating

 * modes that emit multiple scanlines at a time.

 *

 * Note: we warn about excess calls to _jpeg_read_scanlines() since

 * this likely signals an application programmer error.  However,

 * an oversize buffer (max_lines > scanlines remaining) is not an error.

 */

GLOBAL(JDIMENSION)

_jpeg_read_scanlines(j_decompress_ptr cinfo, _JSAMPARRAY scanlines,

                     JDIMENSION max_lines)

{

#if BITS_IN_JSAMPLE != 16 || defined(D_LOSSLESS_SUPPORTED)

  JDIMENSION row_ctr;

  if (cinfo->data_precision != BITS_IN_JSAMPLE)

    ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);

  if (cinfo->global_state != DSTATE_SCANNING)

    ERREXIT1(cinfo, JERR_BAD_STATE, cinfo->global_state);

  if (cinfo->output_scanline >= cinfo->output_height) {

    WARNMS(cinfo, JWRN_TOO_MUCH_DATA);

    return 0;

  }

  /* Call progress monitor hook if present */

  if (cinfo->progress != NULL) {

    cinfo->progress->pass_counter = (long)cinfo->output_scanline;

    cinfo->progress->pass_limit = (long)cinfo->output_height;

    (*cinfo->progress->progress_monitor) ((j_common_ptr)cinfo);

  }

  /* Process some data */

  row_ctr = 0;

  (*cinfo->main->_process_data) (cinfo, scanlines, &row_ctr, max_lines);

  cinfo->output_scanline += row_ctr;

  return row_ctr;

#else

  ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);

  return 0;

#endif

}

#if BITS_IN_JSAMPLE != 16

/* Dummy color convert function used by _jpeg_skip_scanlines() */

LOCAL(void)

noop_convert(j_decompress_ptr cinfo, _JSAMPIMAGE input_buf,

             JDIMENSION input_row, _JSAMPARRAY output_buf, int num_rows)

{

}

/* Dummy quantize function used by _jpeg_skip_scanlines() */

LOCAL(void)

noop_quantize(j_decompress_ptr cinfo, _JSAMPARRAY input_buf,

              _JSAMPARRAY output_buf, int num_rows)

{

}

/*

 * In some cases, it is best to call _jpeg_read_scanlines() and discard the

 * output, rather than skipping the scanlines, because this allows us to

 * maintain the internal state of the context-based upsampler.  In these cases,

 * we set up and tear down a dummy color converter in order to avoid valgrind

 * errors and to achieve the best possible performance.

 */

LOCAL(void)

read_and_discard_scanlines(j_decompress_ptr cinfo, JDIMENSION num_lines)

{

  JDIMENSION n;

#ifdef UPSAMPLE_MERGING_SUPPORTED

  my_master_ptr master = (my_master_ptr)cinfo->master;

#endif

  _JSAMPLE dummy_sample[1] = { 0 };

  _JSAMPROW dummy_row = dummy_sample;

  _JSAMPARRAY scanlines = NULL;

  void (*color_convert) (j_decompress_ptr cinfo, _JSAMPIMAGE input_buf,

                         JDIMENSION input_row, _JSAMPARRAY output_buf,

                         int num_rows) = NULL;

  void (*color_quantize) (j_decompress_ptr cinfo, _JSAMPARRAY input_buf,

                          _JSAMPARRAY output_buf, int num_rows) = NULL;

  if (cinfo->cconvert && cinfo->cconvert->_color_convert) {

    color_convert = cinfo->cconvert->_color_convert;

    cinfo->cconvert->_color_convert = noop_convert;

    /* This just prevents UBSan from complaining about adding 0 to a NULL

     * pointer.  The pointer isn't actually used.

     */

    scanlines = &dummy_row;

  }

  if (cinfo->cquantize && cinfo->cquantize->_color_quantize) {

    color_quantize = cinfo->cquantize->_color_quantize;

    cinfo->cquantize->_color_quantize = noop_quantize;

  }

#ifdef UPSAMPLE_MERGING_SUPPORTED

  if (master->using_merged_upsample && cinfo->max_v_samp_factor == 2) {

    my_merged_upsample_ptr upsample = (my_merged_upsample_ptr)cinfo->upsample;

    scanlines = &upsample->spare_row;

  }

#endif

  for (n = 0; n < num_lines; n++)

    _jpeg_read_scanlines(cinfo, scanlines, 1);

  if (color_convert)

    cinfo->cconvert->_color_convert = color_convert;

  if (color_quantize)

    cinfo->cquantize->_color_quantize = color_quantize;

}

/*

 * Called by _jpeg_skip_scanlines().  This partially skips a decompress block

 * by incrementing the rowgroup counter.

 */

LOCAL(void)

increment_simple_rowgroup_ctr(j_decompress_ptr cinfo, JDIMENSION rows)

{

  JDIMENSION rows_left;

  my_main_ptr main_ptr = (my_main_ptr)cinfo->main;

  my_master_ptr master = (my_master_ptr)cinfo->master;

  if (master->using_merged_upsample && cinfo->max_v_samp_factor == 2) {

    read_and_discard_scanlines(cinfo, rows);

    return;

  }

  /* Increment the counter to the next row group after the skipped rows. */

  main_ptr->rowgroup_ctr += rows / cinfo->max_v_samp_factor;

  /* Partially skipping a row group would involve modifying the internal state

   * of the upsampler, so read the remaining rows into a dummy buffer instead.

   */

  rows_left = rows % cinfo->max_v_samp_factor;

  cinfo->output_scanline += rows - rows_left;

  read_and_discard_scanlines(cinfo, rows_left);

}

/*

 * Skips some scanlines of data from the JPEG decompressor.

 *

 * The return value will be the number of lines actually skipped.  If skipping

 * num_lines would move beyond the end of the image, then the actual number of

 * lines remaining in the image is returned.  Otherwise, the return value will

 * be equal to num_lines.

 *

 * Refer to libjpeg.txt for more information.

 */

GLOBAL(JDIMENSION)

_jpeg_skip_scanlines(j_decompress_ptr cinfo, JDIMENSION num_lines)

{

  my_main_ptr main_ptr = (my_main_ptr)cinfo->main;

  my_coef_ptr coef = (my_coef_ptr)cinfo->coef;

  my_master_ptr master = (my_master_ptr)cinfo->master;

  my_upsample_ptr upsample = (my_upsample_ptr)cinfo->upsample;

  JDIMENSION i, x;

  int y;

  JDIMENSION lines_per_iMCU_row, lines_left_in_iMCU_row, lines_after_iMCU_row;

  JDIMENSION lines_to_skip, lines_to_read;

  if (cinfo->data_precision != BITS_IN_JSAMPLE)

    ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);

  if (cinfo->master->lossless)

    ERREXIT(cinfo, JERR_NOTIMPL);

  /* Two-pass color quantization is not supported. */

  if (cinfo->quantize_colors && cinfo->two_pass_quantize)

    ERREXIT(cinfo, JERR_NOTIMPL);

  if (cinfo->global_state != DSTATE_SCANNING)

    ERREXIT1(cinfo, JERR_BAD_STATE, cinfo->global_state);

  /* Do not skip past the bottom of the image. */

  if ((unsigned long long)cinfo->output_scanline + num_lines >=

      cinfo->output_height) {

    num_lines = cinfo->output_height - cinfo->output_scanline;

    cinfo->output_scanline = cinfo->output_height;

    (*cinfo->inputctl->finish_input_pass) (cinfo);

    cinfo->inputctl->eoi_reached = TRUE;

    return num_lines;

  }

  if (num_lines == 0)

    return 0;

  lines_per_iMCU_row = cinfo->_min_DCT_scaled_size * cinfo->max_v_samp_factor;

  lines_left_in_iMCU_row =

    (lines_per_iMCU_row - (cinfo->output_scanline % lines_per_iMCU_row)) %

    lines_per_iMCU_row;

  lines_after_iMCU_row = num_lines - lines_left_in_iMCU_row;

  /* Skip the lines remaining in the current iMCU row.  When upsampling

   * requires context rows, we need the previous and next rows in order to read

   * the current row.  This adds some complexity.

   */

  if (cinfo->upsample->need_context_rows) {

    /* If the skipped lines would not move us past the current iMCU row, we

     * read the lines and ignore them.  There might be a faster way of doing

     * this, but we are facing increasing complexity for diminishing returns.

     * The increasing complexity would be a by-product of meddling with the

     * state machine used to skip context rows.  Near the end of an iMCU row,

     * the next iMCU row may have already been entropy-decoded.  In this unique

     * case, we will read the next iMCU row if we cannot skip past it as well.

     */

    if ((num_lines < lines_left_in_iMCU_row + 1) ||

        (lines_left_in_iMCU_row <= 1 && main_ptr->buffer_full &&

         lines_after_iMCU_row < lines_per_iMCU_row + 1)) {

      read_and_discard_scanlines(cinfo, num_lines);

      return num_lines;

    }

    /* If the next iMCU row has already been entropy-decoded, make sure that

     * we do not skip too far.

     */

    if (lines_left_in_iMCU_row <= 1 && main_ptr->buffer_full) {

      cinfo->output_scanline += lines_left_in_iMCU_row + lines_per_iMCU_row;

      lines_after_iMCU_row -= lines_per_iMCU_row;

    } else {

      cinfo->output_scanline += lines_left_in_iMCU_row;

    }

    /* If we have just completed the first block, adjust the buffer pointers */

    if (main_ptr->iMCU_row_ctr == 0 ||

        (main_ptr->iMCU_row_ctr == 1 && lines_left_in_iMCU_row > 2))

      set_wraparound_pointers(cinfo);

    main_ptr->buffer_full = FALSE;

    main_ptr->rowgroup_ctr = 0;

    main_ptr->context_state = CTX_PREPARE_FOR_IMCU;

    if (!master->using_merged_upsample) {

      upsample->next_row_out = cinfo->max_v_samp_factor;

      upsample->rows_to_go = cinfo->output_height - cinfo->output_scanline;

    }

  }

  /* Skipping is much simpler when context rows are not required. */

  else {

    if (num_lines < lines_left_in_iMCU_row) {

      increment_simple_rowgroup_ctr(cinfo, num_lines);

      return num_lines;

    } else {

      cinfo->output_scanline += lines_left_in_iMCU_row;

      main_ptr->buffer_full = FALSE;

      main_ptr->rowgroup_ctr = 0;

      if (!master->using_merged_upsample) {

        upsample->next_row_out = cinfo->max_v_samp_factor;

        upsample->rows_to_go = cinfo->output_height - cinfo->output_scanline;

      }

    }

  }

  /* Calculate how many full iMCU rows we can skip. */

  if (cinfo->upsample->need_context_rows)

    lines_to_skip = ((lines_after_iMCU_row - 1) / lines_per_iMCU_row) *

                    lines_per_iMCU_row;

  else

    lines_to_skip = (lines_after_iMCU_row / lines_per_iMCU_row) *

                    lines_per_iMCU_row;

  /* Calculate the number of lines that remain to be skipped after skipping all

   * of the full iMCU rows that we can.  We will not read these lines unless we

   * have to.

   */

  lines_to_read = lines_after_iMCU_row - lines_to_skip;

  /* For images requiring multiple scans (progressive, non-interleaved, etc.),

   * all of the entropy decoding occurs in jpeg_start_decompress(), assuming

   * that the input data source is non-suspending.  This makes skipping easy.

   */

  if (cinfo->inputctl->has_multiple_scans || cinfo->buffered_image) {

    if (cinfo->upsample->need_context_rows) {

      cinfo->output_scanline += lines_to_skip;

      cinfo->output_iMCU_row += lines_to_skip / lines_per_iMCU_row;

      main_ptr->iMCU_row_ctr += lines_to_skip / lines_per_iMCU_row;

      /* It is complex to properly move to the middle of a context block, so

       * read the remaining lines instead of skipping them.

       */

      read_and_discard_scanlines(cinfo, lines_to_read);

    } else {

      cinfo->output_scanline += lines_to_skip;

      cinfo->output_iMCU_row += lines_to_skip / lines_per_iMCU_row;

      increment_simple_rowgroup_ctr(cinfo, lines_to_read);

    }

    if (!master->using_merged_upsample)

      upsample->rows_to_go = cinfo->output_height - cinfo->output_scanline;

    return num_lines;

  }

  /* Skip the iMCU rows that we can safely skip. */

  for (i = 0; i < lines_to_skip; i += lines_per_iMCU_row) {

    for (y = 0; y < coef->MCU_rows_per_iMCU_row; y++) {

      for (x = 0; x < cinfo->MCUs_per_row; x++) {

        /* Calling decode_mcu() with a NULL pointer causes it to discard the

         * decoded coefficients.  This is ~5% faster for large subsets, but

         * it's tough to tell a difference for smaller images.

         */

        if (!cinfo->entropy->insufficient_data)

          cinfo->master->last_good_iMCU_row = cinfo->input_iMCU_row;

        (*cinfo->entropy->decode_mcu) (cinfo, NULL);

      }

    }

    cinfo->input_iMCU_row++;

    cinfo->output_iMCU_row++;

    if (cinfo->input_iMCU_row < cinfo->total_iMCU_rows)

      start_iMCU_row(cinfo);

    else

      (*cinfo->inputctl->finish_input_pass) (cinfo);

  }

  cinfo->output_scanline += lines_to_skip;

  if (cinfo->upsample->need_context_rows) {

    /* Context-based upsampling keeps track of iMCU rows. */

    main_ptr->iMCU_row_ctr += lines_to_skip / lines_per_iMCU_row;

    /* It is complex to properly move to the middle of a context block, so

     * read the remaining lines instead of skipping them.

     */

    read_and_discard_scanlines(cinfo, lines_to_read);

  } else {

    increment_simple_rowgroup_ctr(cinfo, lines_to_read);

  }

  /* Since skipping lines involves skipping the upsampling step, the value of

   * "rows_to_go" will become invalid unless we set it here.  NOTE: This is a

   * bit odd, since "rows_to_go" seems to be redundantly keeping track of

   * output_scanline.

   */

  if (!master->using_merged_upsample)

    upsample->rows_to_go = cinfo->output_height - cinfo->output_scanline;

  /* Always skip the requested number of lines. */

  return num_lines;

}

/*

 * Alternate entry point to read raw data.

 * Processes exactly one iMCU row per call, unless suspended.

 */

GLOBAL(JDIMENSION)

_jpeg_read_raw_data(j_decompress_ptr cinfo, _JSAMPIMAGE data,

                    JDIMENSION max_lines)

{

  JDIMENSION lines_per_iMCU_row;

  if (cinfo->data_precision != BITS_IN_JSAMPLE)

    ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);

  if (cinfo->master->lossless)

    ERREXIT(cinfo, JERR_NOTIMPL);

  if (cinfo->global_state != DSTATE_RAW_OK)

    ERREXIT1(cinfo, JERR_BAD_STATE, cinfo->global_state);

  if (cinfo->output_scanline >= cinfo->output_height) {

    WARNMS(cinfo, JWRN_TOO_MUCH_DATA);

    return 0;

  }

  /* Call progress monitor hook if present */

  if (cinfo->progress != NULL) {

    cinfo->progress->pass_counter = (long)cinfo->output_scanline;

    cinfo->progress->pass_limit = (long)cinfo->output_height;

    (*cinfo->progress->progress_monitor) ((j_common_ptr)cinfo);

  }

  /* Verify that at least one iMCU row can be returned. */

  lines_per_iMCU_row = cinfo->max_v_samp_factor * cinfo->_min_DCT_scaled_size;

  if (max_lines < lines_per_iMCU_row)

    ERREXIT(cinfo, JERR_BUFFER_SIZE);

  /* Decompress directly into user's buffer. */

  if (!(*cinfo->coef->_decompress_data) (cinfo, data))

    return 0;                   /* suspension forced, can do nothing more */

  /* OK, we processed one iMCU row. */

  cinfo->output_scanline += lines_per_iMCU_row;

  return lines_per_iMCU_row;

}

#endif /* BITS_IN_JSAMPLE != 16 */

#if BITS_IN_JSAMPLE == 8

/* Additional entry points for buffered-image mode. */

#ifdef D_MULTISCAN_FILES_SUPPORTED

/*

 * Initialize for an output pass in buffered-image mode.

 */

GLOBAL(boolean)

jpeg_start_output(j_decompress_ptr cinfo, int scan_number)

{

  if (cinfo->global_state != DSTATE_BUFIMAGE &&

      cinfo->global_state != DSTATE_PRESCAN)

    ERREXIT1(cinfo, JERR_BAD_STATE, cinfo->global_state);

  /* Limit scan number to valid range */

  if (scan_number <= 0)

    scan_number = 1;

  if (cinfo->inputctl->eoi_reached && scan_number > cinfo->input_scan_number)

    scan_number = cinfo->input_scan_number;

  cinfo->output_scan_number = scan_number;

  /* Perform any dummy output passes, and set up for the real pass */

  return output_pass_setup(cinfo);

}

/*

 * Finish up after an output pass in buffered-image mode.

 *

 * Returns FALSE if suspended.  The return value need be inspected only if

 * a suspending data source is used.

 */

GLOBAL(boolean)

jpeg_finish_output(j_decompress_ptr cinfo)

{

  if ((cinfo->global_state == DSTATE_SCANNING ||

       cinfo->global_state == DSTATE_RAW_OK) && cinfo->buffered_image) {

    /* Terminate this pass. */

    /* We do not require the whole pass to have been completed. */

    (*cinfo->master->finish_output_pass) (cinfo);

    cinfo->global_state = DSTATE_BUFPOST;

  } else if (cinfo->global_state != DSTATE_BUFPOST) {

    /* BUFPOST = repeat call after a suspension, anything else is error */

    ERREXIT1(cinfo, JERR_BAD_STATE, cinfo->global_state);

  }

  /* Read markers looking for SOS or EOI */

  while (cinfo->input_scan_number <= cinfo->output_scan_number &&

         !cinfo->inputctl->eoi_reached) {

    if ((*cinfo->inputctl->consume_input) (cinfo) == JPEG_SUSPENDED)

      return FALSE;             /* Suspend, come back later */

  }

  cinfo->global_state = DSTATE_BUFIMAGE;

  return TRUE;

}

#endif /* D_MULTISCAN_FILES_SUPPORTED */

#endif /* BITS_IN_JSAMPLE == 8 */