PieceDiversity

Unweighted Piece Diversity Index (PDI): presence of Pawn, Knight, Bishop, Rook, and Queen for one side.

Each represented type contributes one, regardless of its number of pieces or their squares. Kings are excluded. This measures material diversity, not mobility or the number of dice a side can use: a present type may be blocked, and promotion can restore an absent type. The index is independent of the active color and dice pool.