The longest possible normal game of chess has been calculated, but I want to know what the longest possible game of Horde Chess is. What makes this an interesting case is the crazy amount of pawn moves and captures there are that allow the game to be extended for a very long time.
For the sake of the question, as to avoid answers of "infinity," the 75-move will be in affect. Captures and pawn moves will be labeled as "marks" for simplicity
As for calculating the longest possible game, White needs to promote a pawn to gain waiting moves. This takes 4 moves at the least.
[FEN "rnbqkbnr/pppppppp/8/1PP2PP1/PPPPPPPP/PPPPPPPP/PPPPPPPP/PPPPPPPP w kq - 0 1"] 1. b6 a6 2. b5 Ra7 3. bxa7 Nc6 4. a8=Q
The promotion is our first mark. After this, White has 184 marks in pawn moves. All but one piece can be captured for 31 more marks. (184+31+1)*75 gives us 16200 moves for White piece alone.
Each Black pawn should be able to get at least two moves given that White captures properly, so that's 16 marks from all Black pawns. Additionally, each Black piece (minus a rook from the opening) is a mark, minus the king. (14+16)*75=2250. After all marks are done, the Black king and the last White piece can duke it out for another 75 moves.
16200+2250+75=18525 moves, give or take a ply or two for parity switches, as a lower bound. I'm sure that there are ways to give Black more pawn moves.
Can anyone refine my starter pack calculations and find the longest possible Horde Chess Game?