Opening/Middlegame/Endgame division

[guidopetri]

How often do you blunder away your game in the middlegame? How good is your endgame play? Do you give up the advantage in the opening and never recover?

Being able to identify when a game is in each section of opening/middlegame/endgame would be useful to answer these questions. Lichess/Stockfish(?) use some heuristics to identify when the middlegame/endgame start. From the Lichess source, it seems they use number of majors/minors on the board, as well as "mixedness" of the board, or if the back rank is sparse. Stockfish seems to use some major/minor piece calc, too.

This probably needs some more discussion/thought before implementation.