repetition detection using the TT

[tcusr]

hi everyone, after a 3 months break i'm back on the development of my engine.
i'm adding repetition detection and i want to use this method.
basically i'm setting the flag in make move and unsetting it when undoing the move. i check if a position was seen in the probing section, before looking for a cutoff.
it doesn't work but beside that does anyone use this method? to my understanding it only works for repetitions in the tree, not the game history, and also there's the problem of entry collisions (mentioned in the wiki).

[lithander]

to my understanding it only works for repetitions in the tree, not the game history, and also there's the problem of entry collisions (mentioned in the wiki).

I use the TT for repetitions from the game history. Basically I enter all positions from the history at an incredibly high depth and with a score of 0 before starting the search. The depth protects it against being overwritten.

Maybe you can also just not overwrite entries that have the flag set?

[hgm]

In Fairy-Max I use both these methods: upgrading the root entry to max depth and give it a draw score (inherited from micro-Max), and set a flag for open nodes in the tree (added in version 5.0). The latter is only important to plan for repetitions (e.g. sacrifice something to deliver a perpetual, when already behind, or prevent the opponont from doing this when ahead). It doesn't bring much Elo. Not repeating positions from the game history is essential; without that it becomes almost impossible to win, and very hard to test the effect of improvements.

Strictly speaking only positions that have occured twice in the game history should be put in the TT as max-depth draws.

In an SMP engine you would need one flag per thread, though. Which kind of spoils the idea, as it would require a larger TT entry. (While a single spare bit usually exists in those.)

[tcusr]

i really like this method, i thought that repetitions in the tree were as important.
thank you very much.

[tcusr]

to my understanding it only works for repetitions in the tree, not the game history, and also there's the problem of entry collisions (mentioned in the wiki).

I use the TT for repetitions from the game history. Basically I enter all positions from the history at an incredibly high depth and with a score of 0 before starting the search. The depth protects it against being overwritten.

Maybe you can also just not overwrite entries that have the flag set?

wait actually you don't even need to keep the history of the game.
just add a counter in the entry which is increased every time a "position" command is sent. if it equals to 2 you save the entry as a max depth draw.
meanwhile, in the tree, you don't overwrite the entries which have a count different than 0.