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/*
 * jdatasrc.c
 *
 * This file was part of the Independent JPEG Group's software:
 * Copyright (C) 1994-1996, Thomas G. Lane.
 * Modified 2009-2011 by Guido Vollbeding.
 * libjpeg-turbo Modifications:
 * Copyright (C) 2013, 2016, 2022, D. R. Commander.
 * For conditions of distribution and use, see the accompanying README.ijg
 * file.
 *
 * This file contains decompression data source routines for the case of
 * reading JPEG data from memory or from a file (or any stdio stream).
 * While these routines are sufficient for most applications,
 * some will want to use a different source manager.
 * IMPORTANT: we assume that fread() will correctly transcribe an array of
 * JOCTETs from 8-bit-wide elements on external storage.  If char is wider
 * than 8 bits on your machine, you may need to do some tweaking.
 */

/* this is not a core library module, so it doesn't define JPEG_INTERNALS */
#include "jinclude.h"
#include "jpeglib.h"
#include "jerror.h"


/* Expanded data source object for stdio input */

typedef struct {
  struct jpeg_source_mgr pub;   /* public fields */

  FILE *infile;                 /* source stream */
  JOCTET *buffer;               /* start of buffer */
  boolean start_of_file;        /* have we gotten any data yet? */
} my_source_mgr;

typedef my_source_mgr *my_src_ptr;

#define INPUT_BUF_SIZE  4096    /* choose an efficiently fread'able size */


/*
 * Initialize source --- called by jpeg_read_header
 * before any data is actually read.
 */

METHODDEF(void)
init_source(j_decompress_ptr cinfo)
{
  my_src_ptr src = (my_src_ptr)cinfo->src;

  /* We reset the empty-input-file flag for each image,
   * but we don't clear the input buffer.
   * This is correct behavior for reading a series of images from one source.
   */
  src->start_of_file = TRUE;
}

METHODDEF(void)
init_mem_source(j_decompress_ptr cinfo)
{
  /* no work necessary here */
}


/*
 * Fill the input buffer --- called whenever buffer is emptied.
 *
 * In typical applications, this should read fresh data into the buffer
 * (ignoring the current state of next_input_byte & bytes_in_buffer),
 * reset the pointer & count to the start of the buffer, and return TRUE
 * indicating that the buffer has been reloaded.  It is not necessary to
 * fill the buffer entirely, only to obtain at least one more byte.
 *
 * There is no such thing as an EOF return.  If the end of the file has been
 * reached, the routine has a choice of ERREXIT() or inserting fake data into
 * the buffer.  In most cases, generating a warning message and inserting a
 * fake EOI marker is the best course of action --- this will allow the
 * decompressor to output however much of the image is there.  However,
 * the resulting error message is misleading if the real problem is an empty
 * input file, so we handle that case specially.
 *
 * In applications that need to be able to suspend compression due to input
 * not being available yet, a FALSE return indicates that no more data can be
 * obtained right now, but more may be forthcoming later.  In this situation,
 * the decompressor will return to its caller (with an indication of the
 * number of scanlines it has read, if any).  The application should resume
 * decompression after it has loaded more data into the input buffer.  Note
 * that there are substantial restrictions on the use of suspension --- see
 * the documentation.
 *
 * When suspending, the decompressor will back up to a convenient restart point
 * (typically the start of the current MCU). next_input_byte & bytes_in_buffer
 * indicate where the restart point will be if the current call returns FALSE.
 * Data beyond this point must be rescanned after resumption, so move it to
 * the front of the buffer rather than discarding it.
 */

METHODDEF(boolean)
fill_input_buffer(j_decompress_ptr cinfo)
{
  my_src_ptr src = (my_src_ptr)cinfo->src;
  size_t nbytes;

  nbytes = fread(src->buffer, 1, INPUT_BUF_SIZE, src->infile);

  if (nbytes <= 0) {
    if (src->start_of_file)     /* Treat empty input file as fatal error */
      ERREXIT(cinfo, JERR_INPUT_EMPTY);
    WARNMS(cinfo, JWRN_JPEG_EOF);
    /* Insert a fake EOI marker */
    src->buffer[0] = (JOCTET)0xFF;
    src->buffer[1] = (JOCTET)JPEG_EOI;
    nbytes = 2;
  }

  src->pub.next_input_byte = src->buffer;
  src->pub.bytes_in_buffer = nbytes;
  src->start_of_file = FALSE;

  return TRUE;
}

METHODDEF(boolean)
fill_mem_input_buffer(j_decompress_ptr cinfo)
{
  static const JOCTET mybuffer[4] = {
    (JOCTET)0xFF, (JOCTET)JPEG_EOI, 0, 0
  };

  /* The whole JPEG data is expected to reside in the supplied memory
   * buffer, so any request for more data beyond the given buffer size
   * is treated as an error.
   */
  WARNMS(cinfo, JWRN_JPEG_EOF);

  /* Insert a fake EOI marker */

  cinfo->src->next_input_byte = mybuffer;
  cinfo->src->bytes_in_buffer = 2;

  return TRUE;
}


/*
 * Skip data --- used to skip over a potentially large amount of
 * uninteresting data (such as an APPn marker).
 *
 * Writers of suspendable-input applications must note that skip_input_data
 * is not granted the right to give a suspension return.  If the skip extends
 * beyond the data currently in the buffer, the buffer can be marked empty so
 * that the next read will cause a fill_input_buffer call that can suspend.
 * Arranging for additional bytes to be discarded before reloading the input
 * buffer is the application writer's problem.
 */

METHODDEF(void)
skip_input_data(j_decompress_ptr cinfo, long num_bytes)
{
  struct jpeg_source_mgr *src = cinfo->src;

  /* Just a dumb implementation for now.  Could use fseek() except
   * it doesn't work on pipes.  Not clear that being smart is worth
   * any trouble anyway --- large skips are infrequent.
   */
  if (num_bytes > 0) {
    while (num_bytes > (long)src->bytes_in_buffer) {
      num_bytes -= (long)src->bytes_in_buffer;
      (void)(*src->fill_input_buffer) (cinfo);
      /* note we assume that fill_input_buffer will never return FALSE,
       * so suspension need not be handled.
       */
    }
    src->next_input_byte += (size_t)num_bytes;
    src->bytes_in_buffer -= (size_t)num_bytes;
  }
}


/*
 * An additional method that can be provided by data source modules is the
 * resync_to_restart method for error recovery in the presence of RST markers.
 * For the moment, this source module just uses the default resync method
 * provided by the JPEG library.  That method assumes that no backtracking
 * is possible.
 */


/*
 * Terminate source --- called by jpeg_finish_decompress
 * after all data has been read.  Often a no-op.
 *
 * NB: *not* called by jpeg_abort or jpeg_destroy; surrounding
 * application must deal with any cleanup that should happen even
 * for error exit.
 */

METHODDEF(void)
term_source(j_decompress_ptr cinfo)
{
  /* no work necessary here */
}


/*
 * Prepare for input from a stdio stream.
 * The caller must have already opened the stream, and is responsible
 * for closing it after finishing decompression.
 */

GLOBAL(void)
jpeg_stdio_src(j_decompress_ptr cinfo, FILE *infile)
{
  my_src_ptr src;

  /* The source object and input buffer are made permanent so that a series
   * of JPEG images can be read from the same file by calling jpeg_stdio_src
   * only before the first one.  (If we discarded the buffer at the end of
   * one image, we'd likely lose the start of the next one.)
   */
  if (cinfo->src == NULL) {     /* first time for this JPEG object? */
    cinfo->src = (struct jpeg_source_mgr *)
      (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_PERMANENT,
                                  sizeof(my_source_mgr));
    src = (my_src_ptr)cinfo->src;
    src->buffer = (JOCTET *)
      (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_PERMANENT,
                                  INPUT_BUF_SIZE * sizeof(JOCTET));
  } else if (cinfo->src->init_source != init_source) {
    /* It is unsafe to reuse the existing source manager unless it was created
     * by this function.  Otherwise, there is no guarantee that the opaque
     * structure is the right size.  Note that we could just create a new
     * structure, but the old structure would not be freed until
     * jpeg_destroy_decompress() was called.
     */
    ERREXIT(cinfo, JERR_BUFFER_SIZE);
  }

  src = (my_src_ptr)cinfo->src;
  src->pub.init_source = init_source;
  src->pub.fill_input_buffer = fill_input_buffer;
  src->pub.skip_input_data = skip_input_data;
  src->pub.resync_to_restart = jpeg_resync_to_restart; /* use default method */
  src->pub.term_source = term_source;
  src->infile = infile;
  src->pub.bytes_in_buffer = 0; /* forces fill_input_buffer on first read */
  src->pub.next_input_byte = NULL; /* until buffer loaded */
}


/*
 * Prepare for input from a supplied memory buffer.
 * The buffer must contain the whole JPEG data.
 */

GLOBAL(void)
jpeg_mem_src(j_decompress_ptr cinfo, const unsigned char *inbuffer,
             unsigned long insize)
{
  struct jpeg_source_mgr *src;

  if (inbuffer == NULL || insize == 0)  /* Treat empty input as fatal error */
    ERREXIT(cinfo, JERR_INPUT_EMPTY);

  /* The source object is made permanent so that a series of JPEG images
   * can be read from the same buffer by calling jpeg_mem_src only before
   * the first one.
   */
  if (cinfo->src == NULL) {     /* first time for this JPEG object? */
    cinfo->src = (struct jpeg_source_mgr *)
      (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_PERMANENT,
                                  sizeof(struct jpeg_source_mgr));
  } else if (cinfo->src->init_source != init_mem_source) {
    /* It is unsafe to reuse the existing source manager unless it was created
     * by this function.
     */
    ERREXIT(cinfo, JERR_BUFFER_SIZE);
  }

  src = cinfo->src;
  src->init_source = init_mem_source;
  src->fill_input_buffer = fill_mem_input_buffer;
  src->skip_input_data = skip_input_data;
  src->resync_to_restart = jpeg_resync_to_restart; /* use default method */
  src->term_source = term_source;
  src->bytes_in_buffer = (size_t)insize;
  src->next_input_byte = (const JOCTET *)inbuffer;
}
kc3-lang/libjpeg-turbo/src/jdatasrc.c

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src/jdatasrc.c
