For the 5 player variant specifically, the concept is as follows:
Out of the 8 moves each other player can make (play any of 4 cards, discard any of 4 cards), you figure out the best move, add them up mod 8, and make one of 8 hints (you have a color or number hint available for each player). Then each of the 4 other players deduces which move you wanted them to make by subtracting out everyone else's best move.
There's some room for improvement on figuring out how to avoid one player mistaking another's best play (because it might be predicated on them playing a card they don't know they have first), or how to hint in ways which avoid that.
Nice! Any intuition on whether this is practical for humans to implement in a game?
(Side note: in practice, it's almost impossible to avoid the illegal side-channel communication of watching the other players get nervous as you start to do something stupid, so really the only rules-faithful way is to play over a computer.)
(Another side note: there's a game called The Mind[1] which is the board-game version of SleepSport -- the goal is to, without most kinds of communication, play cards from each players hand in increasing order. One way to win is to synchronize everyone's internal clock and play each card when that many seconds have passed.)
Played The Mind today with workmates. It was uncanny how everyone independently arrived at the sleep sort strategy independently and without cooperation.
In the linked paper they actually run two bots that use such strategies (they call them "hat strategies") on their platform and show how they do against some AIs.
The WTFWThat bot achieves the rather remarkable average score of 24.89 and 91.5% perfect games in 5-player self-play (that is, all 5 seats are WTFWThat bots).
That's very clever but feels like breaking the game slightly.
In the game Bridge, which involves communication between competing pairs of players, you can communicate a suit and a number, but it is meant to have some kind of meaning within the game, so I think strategies like this would be illegal.
There's no requirement for the bidding to be natural, there are plenty of artificial bidding systems -- the only constraints are that the bidding system you are using must be known to your opponent (to enforce this, I believe that on a bid your opponent can ask your partner what this bid is supposed to communicate), and that you do eventually have to play your bid so it can't match your cards too badly.
This is why calling Hanabi a "game" is generous. It's largely just an exercise in mathematics disguised as a game. There is only one real player: the person declaring the rules of deduction for everyone to follow.
Well if you play differently from how everyone else expects, you all suffer for it. Most games reward you for thinking up something creative that the other players missed; Hanabi punishes you instead. You have to hew to how you played in previous games and play as boringly as possible.
For the 5 player variant specifically, the concept is as follows: Out of the 8 moves each other player can make (play any of 4 cards, discard any of 4 cards), you figure out the best move, add them up mod 8, and make one of 8 hints (you have a color or number hint available for each player). Then each of the 4 other players deduces which move you wanted them to make by subtracting out everyone else's best move. There's some room for improvement on figuring out how to avoid one player mistaking another's best play (because it might be predicated on them playing a card they don't know they have first), or how to hint in ways which avoid that.
A friend has put up an implementation up on github: https://github.com/chikinn/hanabi/blob/master/players/hat_pl...