void web_demo::search(int xlo, int xhi, int ylo, int yhi,
                      const char* png_file_name)
{
    // Locate the Geophile index for the data set.
    initialize_for_search();

    // Create a box representing the query region.
    box* query_region = new box(xlo, xhi, ylo, yhi, 0, 0, 0);

    // Do the spatial search.
    GF_search_result& result =
        geophile::spatial_join_1_n(*query_region, *_space, refine);

    // Create image from query result.
    create_png(xlo, xhi, ylo, yhi, result, png_file_name);
}

void web_demo::initialize_for_search()
{
    
    // Create the database, overwriting an existing database
    // if necessary.
    
    _index = disk_array::open_for_read(_db_name);
    
    // Describe the coordinate space.
    int lo[] = { 0, 0 };
    int hi[] = { _space_width - 1, _space_height - 1 };
    GF_space_description* space_description =
        new GF_space_description(2, lo, hi);
    
    
    // Create the Geophile index by supplying 1) the data
    // structure to be used for recording index entries, and
    // 2) a description of the space. 
    
    _space = new GF_space(*_index, *space_description);
}

// This function is used to refine query results.
// It simply tests whether the query region and a box 
// from the data set really overlap.
int web_demo::refine(const void* p1, const void* p2)
{
    const colored_box& b1 = **(const colored_box**) p1;
    const colored_box& b2 = *(const colored_box*) p2;
    return b1.overlaps(b2);
}