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An entry in Fortnightly Topic Challenge #40: Retrograde Analysis!


I can't believe it! Last night we heard a bit of fighting... but we had no idea that... our King was in check, and my fellow Rook and Knight were dead! enter image description here

Of course, the Black Kingdom must have done this, but we have no idea how. The remaining White soldiers are calling foul play, and I'm certainly one of them. In fact, we're going to take it to one of the most prestigious courts in all of Chesslandia to make sure their checkmate was fair. We have four eyewitnesses: myself, the bishop on g1, the pawn on f2, and the pawn on e2. We each have our one piece of evidence, but... I think one of us is a spy who will lie about the evidence to make sure Black gets off scot-free!

Here are our statements. I'm obviously not the spy, so my statement is truthful.

Me (Rh2):

There was not a pawn on g2 on our turn 2 turns ago, but there was on our turn 3 turns ago, and I know for a fact it wasn't captured.

Bg1:

The knight on h1 is not a promoted piece.

Pf2:

The bishop on f1 is a promoted piece.

Pe2:

There was a piece on e1 on our last turn.

So, how did Black check our King? What were the last 3 moves? Who is the spy? And, most importantly, was Black cheating?

Please help. Our kingdom needs you!

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    $\begingroup$ Alright, time to go out of character for a bit. I just realized the position is not checkmate (Rg2), but the puzzle still stands. $\endgroup$ – Excited Raichu Oct 16 '18 at 13:11
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First notice that:

Black as exactly one promoted unit. No more, because he still has 7 pawns, and no less, since his last move can only be Pg2xf1:B+ or Pg2xh1:N+.

As consequence,

Bg1 and Pf2 are either both lying or both saying the truth. The first case is excluded because that would be too many spies. Hence, either every witness is truthful, either Pe2 is the only liar.

Hypothesis A:

Let's suppose that Pe2 is not lying: There was a piece on e1 on our last turn.

Then:

The only explanation for the disappearance of said piece is that it was white, moved, and got taken. Since your dead friends are a Rook and a Knight, it must be a wRe1 that moved to f1 and got taken by bPg2 (not a bishop, as it would check the Kh3), the pawn then promoting to a bishop.

Last move would then be:

-1.Re1-f1 Pg2xf1:B+

Going further,

We know that there was a pawn on g2 3 moves ago that didn't get taken. But not two moves ago. If that pawn was White, it would still be on the board. If it was Black, it would be the promoted Bf1, but it should have stayed on g2 on the previous move.

In both cases,

We reached a contradiction, so we are sure that Black cheated !

Hypothesis B:

Even if Pe2 lies, Black last move must have been Pg2xf1:B+ (taking either a rook or a knight) and the same contradiction about the pawn seen on g2 appears. Once again: Black has cheated, your Honor !

Side remark:

I have a feeling that this is not the expected answer, that you rather wanted to promote two pawns from g2 to f1 first and then to h1, but there are too many bP on the board for that to be possible.

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    $\begingroup$ Your honor: the white King was on g3, and we had a pawn on g2 three turns ago. We then did … gxf1=N+ and white moved Kh3. As the Rook testimony pretends us to believe, they supposedly had no idea his King was in check, and we didn't notice either. So we did. … Ra4. c4 e4. Then we noticed, and immediately announced, that the white king was in check. $\endgroup$ – Ángel Oct 16 '18 at 21:19
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    $\begingroup$ Now, not only did white cheat by "not noticing" they were in mate, but they are accusing us of cheating, something we would never do! and the testimony of three of their pieces (Bg1, Pf2 and Pe2) was clearly fabricated to cover up and attempt to win the game at the court, as they are unable to do so at the board. We humbly request the case to be dismissed. $\endgroup$ – Ángel Oct 16 '18 at 21:19

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