loadPieceLayerRow
Loads a single piece-layer row for this file: the contiguous run of piece roots pieces, the first of which is piece number startPiece. Each supplied piece root is written into its piece-layer slot. The interior nodes are not recomputed (an enclosing proof or a later full loadPieceLayer establishes verification); this is the building-block libtorrent uses when filling the tree row-by-row from get_hashes responses.
Parameters
startPiece
index of the first supplied piece (>= 0).
pieces
the run of piece roots, in order. startPiece + pieces.size must not exceed numPieces.