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=gives around 18525 moves, give a take a ply or two for parity switches, as a lower bound for the longest possible Horde Chess Game. 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?

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    Unless there is a special rule in Horde-Chess I am not aware of, White can actually promote on move 3: 1.b6 a6 2.dc7 a5 3.cb8Q – Evargalo Nov 25 '19 at 13:22
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    @Evargalo While that is true, that wastes many moves by taking out a Black pawn. – Rewan Demontay Nov 25 '19 at 16:20
  • Hi Rewan - can I post this in PDB with joint authorship you and me? Ok to use your name here which I guess is a pseudonym – Laska Dec 16 '19 at 6:09
  • @No problem Mr.DeadReckoningMan (;D)!See P1370058 for my real name. – Rewan Demontay Dec 16 '19 at 12:13
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    @Rewan Demontay in a couple of days – Laska Dec 16 '19 at 20:12

EDIT: Final calculation matches actual proof game exactly.

That’s a nice new chess-math question, which deserves wider circulation.

Begin by promoting a White pawn as quickly as possible, because until that happens, White is just haemorrhaging pawn moves. Now can promote a second pawn more slowly. This promoted piece is captured by Black pawn to open a file, and the first file's promotions can happen.

Scheme: axb to let all wPs from the a-file promote, then bxa+bxa to let all remaining wPs from the b-file promote, then cxb to let all wPs from the c-file promote, and so on.

One player is the active one, moving pawns. This player can also capture spare officers with officers. But pawns can never be captured before they promote (hugely expensive) and there are only 2 captures by wPs and 9 captures by bPs.

There are in principle 188+36+48+15=287 marks (captures & pawn moves). However, we lose 9 for Black pawn captures, 2 for White pawn captures, and 5 for pawn moves leading to the first White promotion. So there are really just 271 marks. So if the count begins after 4. a8=?, we have a total length of 20328.5 moves minus 0.5 for each parity switch.

So how many parity switches must there be? White is the first active player, then need two switches to promote the pawns from any file. So that's 16 switches, and White has promoted all his h-file pawns. A 17th switch is required for Black to capture these.

What impact does dead position have? If it applies literally, then Black cannot win if he has lost all his units, because he can't capture the last White unit. So let's suppose that Black winning "counts" as checkmate for DP purposes. That gives a total of 20320.0 moves.

The final proposed total is hence: 20320.0 moves.

An actual animated proof game has been constructed which achieves this total: https://pdb.dieschwalbe.de/search.jsp. See probid='P1370701'.

  • Well do note that there will never be a dead rule in effect-Black can win with a lone king if it captures the last White piece, which is just Horde's version of checkmate. The laws of chess just say that a position is dead if there is no checkmating sequence for either side-it isn't one-sided (ahahahaha, :-D). Since Black can always have the possibility of checkmating White, the dead rule never applies. – Rewan Demontay Nov 25 '19 at 0:37
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    And nice calculations! Also, what do you mean by "Now that's a real question"? – Rewan Demontay Nov 25 '19 at 0:38
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    Hi @RewanDemontay well it's a semantic thing, but Black's win condition is a win condition. Checkmate is a win condition. But that doesn't mean that Black's win condition here is a "version of checkmate". Depends how you interpret the rules, but I am a fan of reading rules literally if there are otherwise multiple interpretations, in order to truncate arguments. And by "real question" I meant that is apparently a good and difficult question to resolve. But I wrote that at the beginning and then I found that I resolved it anyway :D – Laska Nov 25 '19 at 2:13
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    Makes sense. And thanks! – Rewan Demontay Nov 25 '19 at 2:21
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    On reflection have edited first sentence since it suggested that other questions aren’t “real” which is not at all my meaning – Laska Nov 25 '19 at 3:15

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