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 * jdatasrc.c
 * Copyright (C) 1994-1996, Thomas G. Lane.
 * Modified 2009-2010 by Guido Vollbeding.
 * This file is part of the Independent JPEG Group's software.
 * For conditions of distribution and use, see the accompanying README 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"

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

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.

fill_mem_input_buffer (j_decompress_ptr cinfo)
  static JOCTET mybuffer[4];

  /* 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.
  /* Insert a fake EOI marker */
  mybuffer[0] = (JOCTET) 0xFF;
  mybuffer[1] = (JOCTET) JPEG_EOI;

  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.

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.

term_source (j_decompress_ptr cinfo)
  /* no work necessary here */

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

jpeg_mem_src_tj (j_decompress_ptr cinfo,
            unsigned char * inbuffer, unsigned long insize)
  struct jpeg_source_mgr * src;

  if (inbuffer == NULL || insize == 0)    /* Treat empty input as fatal error */

  /* 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));

  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 = (JOCTET *) inbuffer;

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