Bug in threefold repetition detection with en passant
Posted: 21 August 2020 11:07 AM  
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Joined  2020-02-10

The threefold repetition detection seems wrong when en passant is involved.

In the following example, after 6… Bc8 and pressing the draw by either White or Black, the draw is automatically accepted. This automatic draw should however only be made, if there is a threefold repetition on the board, according to the threefold repetition rule.

But after 6… Bc8 there is only a twofold repetition on the board. Because for the en passant condition in the threefold repetition rule, the positions after 2… d5 and 4… Bc8 are not considered the same.

Example:

1. e4 Nf6
2. e5 d5 {Position arises, e5 can capture d5 en passant, repetition count #1}
3. Be2 Bd7
4. Bf1 Bc8 {Position arises, e5 cannot capture d5 en passant, so repetition count back to #1}
5. Bd3 Be6
6. Bf1 Bc8 {Position arises, e5 cannot capture d5 en passant, repetition count #2}

But as mentioned the draw claim by both sides after 6… Bc8 is accepted automatically, without the agreement of the opponent.

Has this situation ever be raised so far?

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