In theory, chess is a game of perfect information.
But in practice, this is only true for ~6-piece endgames: every outcome has been / can be computed. But for any middle- or early-game position, the amount of finite information greatly exceeds any person's or computer's capacity, practically speaking.
In concrete terms, the tablebase for 6-piece endgames is ~150 GiB. For 7-piece, it's 16 TiB.
One thing that I wondered about when looking at the longest recorded professional games that occurred in real play was at what point they reached a position that is actually solved in a tablebase. In particular, chess endgames involving only 7 pieces were explicitly solved by computer search in 2012. (But I also wonder if bringing those solutions to bear in a real game would involve ignoring the 50-move rule, since tablebase solution for these positions may often require more than 50 moves to complete.)
On tablebase-informed endgame theory, some of the professionals in these ultra-long games should probably have resigned long before the actual end of the games, except that they can also assume that their opponents don't know the full solutions.
The article itself doesn't arrive at the correct number of 5898 moves, but some of the commenters do. At my own page http://tromp.github.io/chess/longest.html you can play through a 3-moves-shorter game.
Indeed; endgame tablebases have been constructed for all positions with up to 7 pieces (including kings), and at some point in the not too distant future, we expect to have all 8 piece ones as well.
For what it's worth, endgame table databases exist that give perfect play for when there are a limited number of pieces left on the board.
As of now, we have databases for perfect play when 7 pieces remain on the board (including the two kings). The 8-piece tablebase is computationally possible, but I don't believe a comprehensive release has come out yet. Even the current 7-piece tables are incomplete because situations like lone king vs. six opposing pieces haven't been explicitly calculated due to their obviousness.
The longest 2x2 game possible seems to be 46 [1].
I can't find the source for the number given by tromp either. But considering that log_2(386,356,909,593) = ~ 38 and log_3(386,356,909,593) = ~24. If the board can have a branching factor of 2.5 on average for a game length of 30 moves, the number seems reasonable. There are 81 possible states (counting illegal states), and seems to be about 50 legal states. And I'd imagine any given state on a 2x2 board would easily have more than 2 possible moves :-).
Seems like longest chess game is about less than 300 turns, there are 64 cells, and even if you don't know chess, and even without efficiently coded there are max of 128bits per turn (a lot of them not valid, and probably space that can be explored better) - e.g. it feels like not only the current position, but the history can be recorded in small amount of data in an URL.
> A maximum of 8 pawns (4 white and 4 black, for example) can possibly be advanced over the course of the game to the opposite side (6 squares ahead from their home square).
Not so! Pawns move diagonally via capturing, which theoretically could allow white to promote all 8 pawns just in the d column, and black to promote all 8 in the e column (at the expense of various other pieces).
edit: of course, you'd run out of pieces first, but you can accomplish something similar by using multiple columns.
Sample game start that accomplishes a subset of this (2 pawns promoted from each side, 6 pawns remaining from each side):
[1]: https://syzygy-tables.info/?fen=QN4n1/6r1/3k4/8/b2K4/8/8/8_b...
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