Function tiles 

pub fn tiles(
    width: u32,
    height: u32,
    tile_w: u32,
    tile_h: u32,
    worker_index: u16,
    worker_count: u16,
) -> impl Iterator<Item = Tile>

Iterate the tiles assigned to worker_index. Tiles are produced in row-major order across the (width, height) image with cell size (tile_w, tile_h). Worker i receives every worker_count-th tile starting at index i, so the union of all workers covers the image exactly once.