// This is based on the FILE source manager in libjpeg

const static JOCTET EOI_BUFFER[1] = { JPEG_EOI };
struct my_source_mgr {
  struct jpeg_source_mgr pub;
  const JOCTET *data;
  size_t       len;
};

static void my_init_source(j_decompress_ptr cinfo) {}

static boolean my_fill_input_buffer(j_decompress_ptr cinfo) {
  my_source_mgr* src = (my_source_mgr*)cinfo->src;
  // No more data.  Probably an incomplete image;  just output EOI.
  src->pub.next_input_byte = EOI_BUFFER;
  src->pub.bytes_in_buffer = 1;
  return TRUE;
}
static void my_skip_input_data(j_decompress_ptr cinfo, long num_bytes) {
  my_source_mgr* src = (my_source_mgr*)cinfo->src;
  if (src->pub.bytes_in_buffer < num_bytes) {
    // Skipping over all of remaining data;  output EOI.
    src->pub.next_input_byte = EOI_BUFFER;
    src->pub.bytes_in_buffer = 1;
  } else {
    // Skipping over only some of the remaining data.
    src->pub.next_input_byte += num_bytes;
    src->pub.bytes_in_buffer -= num_bytes;
  }
}
static void my_term_source(j_decompress_ptr cinfo) {}

static void my_set_source_mgr(j_decompress_ptr cinfo, const char* data, size_t len) {
  my_source_mgr* src;
  if (cinfo->src == 0) { // if this is first time;  allocate memory
    cinfo->src = (struct jpeg_source_mgr *)(*cinfo->mem->alloc_small)
      ((j_common_ptr) cinfo, JPOOL_PERMANENT, sizeof(my_source_mgr));
  }
  src = (my_source_mgr*) cinfo->src;
  src->pub.init_source = my_init_source;
  src->pub.fill_input_buffer = my_fill_input_buffer;
  src->pub.skip_input_data = my_skip_input_data;
  src->pub.resync_to_restart = jpeg_resync_to_restart; // default
  src->pub.term_source = my_term_source;
  // fill the buffers
  src->data = (const JOCTET *)data;
  src->len = len;
  src->pub.bytes_in_buffer = len;
  src->pub.next_input_byte = src->data;
}

void decompressJpeg(const char* data, size_t len) {
  struct jpeg_decompress_struct cinfo; 
  struct jpeg_error_mgr jerr;

  // Setup decompression structure
  cinfo.err = jpeg_std_error(&jerr); 
  jpeg_create_decompress(&cinfo); 
  my_set_source_mgr(&cinfo, data,len);

  // read info from header.
  int r = jpeg_read_header(&cinfo, TRUE);
  jpeg_start_decompress(&cinfo);

  // ..<snip>..
  // .. decompress the JPEG here as usual
  // ..<snip>..

  jpeg_destroy_decompress(&cinfo); 
}