Method iteration

src/kmeans.rs:54–85

(hist: &mut HistogramInternal, palette: &mut PalF, adjust_weight: bool)

    #[inline(never)]
    pub(crate) fn iteration(hist: &mut HistogramInternal, palette: &mut PalF, adjust_weight: bool) -> Result<f64, Error> {
        if hist.items.is_empty() {
            return Ok(0.);
        }

        let n = Nearest::new(palette)?;
        let colors = palette.as_slice();
        let len = colors.len();

        let tls = ThreadLocal::new();
        let total = hist.total_perceptual_weight;

        // chunk size is a trade-off between parallelization and overhead
        hist.items.par_chunks_mut(256).for_each({
            let tls = &tls;
            move |batch| {
                let kmeans = tls.get_or(move || CacheLineAlign(RefCell::new(Self::new(len))));
                if let Ok(ref mut kmeans) = *kmeans.0.borrow_mut() {
                    kmeans.iterate_batch(batch, &n, colors, adjust_weight);
                }
            }
        });

        let diff = tls.into_iter()
            .map(|c| c.0.into_inner())
            .reduce(Self::try_merge)
            .transpose()?
            .map_or(0., |kmeans| kmeans.finalize(palette) / total);

        replace_unused_colors(palette, hist)?;
        Ok(diff)
    }

    fn iterate_batch(&mut self, batch: &mut [HistItem], n: &Nearest, colors: &[f_pixel], adjust_weight: bool) {
        self.weighed_diff_sum += batch.iter_mut().map(|item| {