    /*
     * jdatasrc.c
     *
     * Copyright (C) 1994-1996, Thomas G. Lane.
     * 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 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 */
      int 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;
    }

    /*
     * 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(int)
    fill_input_buffer (j_decompress_ptr cinfo)
    {
      my_src_ptr src = (my_src_ptr) cinfo->src;
      size_t nbytes;

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

      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;
    }

    /*
     * 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)
    {
      my_src_ptr src = (my_src_ptr) 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->pub.bytes_in_buffer) {
          num_bytes -= (long) src->pub.bytes_in_buffer;
          (void) fill_input_buffer(cinfo);
          /* note we assume that fill_input_buffer will never return FALSE,
           * so suspension need not be handled.
           */
        }
        src->pub.next_input_byte += (size_t) num_bytes;
        src->pub.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 )
    {
      /* 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.)
       * This makes it unsafe to use this manager and a different source
       * manager serially with the same JPEG object.  Caveat programmer.
       */
      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));
      }

      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 */
    }