Stockfish Game Phase Function

/// Position::game_phase() calculates the game phase interpolating total non-pawn
/// material between endgame and midgame limits.

Phase Position::game_phase() const {

  Value npm = st->nonPawnMaterial[WHITE] + st->nonPawnMaterial[BLACK];

  npm = std::max(EndgameLimit, std::min(npm, MidgameLimit));

  return Phase(((npm - EndgameLimit) * PHASE_MIDGAME) / (MidgameLimit - EndgameLimit));
}