Function score_pieces 

pub fn score_pieces(
    pieces: &[RecordPiece],
    params: &CurveParams,
) -> Vec<PieceCurves>

Score every piece, in parallel, preserving input order.

Pieces vary enormously in length – a record may hold one piece covering most of a chromosome alongside many short ones – so this splits by piece and lets rayon’s work stealing handle the imbalance, rather than dividing the work evenly up front.