docs: public float[] getFeatureCounts() {
      float[] counts = new float[featureIndex.size()];
      for (int i = 0, m = size; i < m; i++) {
        for (int j = 0, n = data[i].length; j < n; j++) {
          counts[data[i][j]] += 1.0;
        }
      }
      return counts;
    }