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Two games, Crossway and Mincut, are believed to be functionally equivalent. That is, a win by Crossway rules will necessarily lead to a win by Mincut rules. And a win in Mincut will be a win in Crossway.

My question is, "Are these two games functionally equivalent?"

In particular, can you find a win for one player in Crossway that, if you then switched to Mincut rules, would not be a win for the same player in Mincut?

If not, can you prove that the two games are functionally equivalent?

Crossway rules

Crossway rules

Mincut rules

Mincut rules

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3 Answers 3

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The games are not functionally equivalent. Consider the following position, where O represents the left-right player, and # represents the top-bottom player:

OO#O
#  #
O#OO
###O

It is O to move. If this was a Crossway position, O would have no legal moves, and would have to pass, as all possible moves lead to a crosscut. # would therefore be guaranteed to win.

However, if this is a Mincut position, then O is allowed to move a piece closer to the center. The game could proceed as follows:

OO#O
#O #
 #OO
###O

OO#O
#O #
##OO
###O

OO#
#OO#
##OO
###O

O has now won the game. In fact, O wins under all possible moves for either player from this position under Mincut rules.


Note that I am assuming that the first move, from row 3 col 1 to row 2 col 2, qualifies as moving a piece "closer to the center", for the purpose of the Mincut rules. If that move does not count as "closer to the center", I have a different counterexample:

O#OO
O #O
#O#O
###O

Under Crossway rules, # has won. Under Mincut rules, there are no legal moves for either player, assuming that moving from row 3 col 1 to row 2 col 2 does not count as "closer to the center". I believe the other interpretation makes more sense, which is why I listed that example first.

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  • $\begingroup$ Not entirely what I was looking for but good answer. $\endgroup$ Aug 31, 2023 at 0:53
  • $\begingroup$ Yeah, distance to center is straight line distance to center point. So you got that right. $\endgroup$ Aug 31, 2023 at 0:55
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@isaacg has a good answer showing the outcome of a given position is game dependent.

The OP also asked if the game had been won in crossway, would it necessarily be won in mincut by the same player. The answer to that is:

No.

Consider this position:

red has won

Red has just won the crossway game, following which we switched to mincut and Blue played B6

Red then has to move B4->C3 or E5->C4. Blue fills in the space left behind. Red does the other move. And then Blue has an easy win.

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  • $\begingroup$ You have a typo: "Red has just won the mincut game." should be "Red has just won the crossway game." Also, you have misinterpreted the mincut rules. In mincut, red would be required to play at C3, as it does not create a crosscut. $\endgroup$
    – isaacg
    Aug 30, 2023 at 20:45
  • $\begingroup$ Thanks. Fixed... $\endgroup$
    – Dr Xorile
    Aug 30, 2023 at 20:47
  • $\begingroup$ See the other half of my comment - in the mincut game, red just plays at C3, rather than moving there. $\endgroup$
    – isaacg
    Aug 30, 2023 at 20:47
  • $\begingroup$ Ah. Okay. I think I corrected it. $\endgroup$
    – Dr Xorile
    Aug 30, 2023 at 21:50
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    $\begingroup$ Dr Xorile strikes again :) Nice solution. This is kind of a win-win for me. I would have preferred if Mincut was equivalent to Crossway just because it would have been interesting. But this way, Mincut is its own game, a genuine OOSCG. $\endgroup$ Aug 31, 2023 at 0:52
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I would like to debate both answers presented so far, but I don't have the rep points to comment yet. If this doesn't belong here, please delete it.

In isaacg's solution, the beginning is not a valid Crossway win by either player since nothing reaches from one end of the board to the other. This was specifically pointed out in the OP "In particular, can you find a win for one player in Crossway that, if you then switched to Mincut rules, would not be a win for the same player in Mincut?" The Crossway game needs to be finished first. Assuming it is O's turn as suggested in the post, they have no valid moves since both open positions create crosscut conditions. Play passes to # and they can play either open position for the win.

00#0 00#0 00#0 # # ## # # ## 0#00 0#00 0#00 ###0 ###0 ###0 start win 1 win 2

Play can now continue using Mincut rules. It is now 0's turn again. Regardless of which win condition we examine, 0 has no valid moves. They cannot place a piece or move a piece without creating a crosscut condition. In win condition 1, both of the 0s in R1C2 and R1C4 create crosscut conditions. In win condition 2, the 0 in R3C3 creates a crosscut. Play moves over to # who places the final piece and makes a valid Mincut win.

Moving on to Dr. Xorile's solution. As suggested by isaacg in the comments, red's only valid moves are A6->B5, B4->C3, E4->D3, and F6->E5. But are B4->C3 and E4->D3 really valid moves since they move over the vertical center line? Since there are an even number of squares, I think the closer to the center rule is too vague for this condition. Since Crossway is played on a 19x19 Go board, there is a defined center position that removes the confusion of what moves are allowed. Assuming the B4->C3 move is not legal, red takes the win again via Red A6->B5, Blue C3 (or D3), Red F6->E5#.

If the B4->C3 move is legal, this potentially changes the end result of the game. Red can move B4-C3. Blue places B4 and locks up the win because red cannot place at B5 or move A6->B5 without creating a crosscut. But if Red makes the A6->B5 move, which IMO is the more obvious move to make, the result still ends up with Red as the winner of both games. This is a case where Red wins Crossway but loses Mincut.

Dr. Xorile also postulates swapping the A6 and B4 colors. You can't swap A6 with B4 and E4 with F6 at the same time or Red will have won both games by default. It doesn't matter where blue plays their first turn, they can't play E5 to block the Red win. Blue cannot shift a piece because they can place on either C3 or D3. Blue places either of those, Red shifts F6->E5 and they win again.

My Answer:

It depends on how you interpret the rules.

In the case of Dr. Xorile's puzzle, it boils down to how you interpret the closer to the center rule on a board with an even number of squares. In all of the other tests I made, I could not find a condition in which either side won without having the crosscut advantage. That is to say that once a Crossway line is made, the opponent cannot play to block both a vertical and horizontal line without creating a crosscut of their own. If the Crossway winner wasn't immediately able to place the pertinent piece to win, they only had to bide their time until the opponent shifted a piece out of the way allowing the win. The crosscut rule always seems to favor the Crossway winner. My testing assumed that a piece could not move over a vertical or horizontal center line to be considered closer to the center.

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  • $\begingroup$ Hi @ibanezplyr. Thank you for your response. There has been some confusion over what "closer to the center of the board" means. I used that expression twice in the rule sheet, and the third time shortened it to "closer to center." I don't see what the point of confusion is. One definition of center is "the point within a regular polygon equally distant from the vertices." If the distance from your stone to the center of the board is shorter after your move than it was before, you moved closer to the center of the board. $\endgroup$ Sep 1, 2023 at 23:29
  • $\begingroup$ Absent any language about horizontally centering about vertical lines etc.. I think it's fair to assume that center of the board simply means the commonly accepted definition of the center of a polygon. In Dr. Xorile's example, both B4->C3 and E4->D3 clearly (to me anyway) move the stone closer to the center of the board. $\endgroup$ Sep 1, 2023 at 23:30
  • $\begingroup$ Moving on... I could have made the OP question more specific. I should have asked, "In particular, can you find a win for one player in Crossway that, if you then switched to Mincut rules, would not NECESSARILY be a win for the same player in Mincut?" I didn't want to presume any level of skill. Actually, even cooperative play to disprove the games' equivalence would have been fine. Also, good moves will be more evident on a 6x6 board than they would be on a 19x19 board. $\endgroup$ Sep 1, 2023 at 23:30
  • $\begingroup$ Almost all of my games are scalable. The reason for this is that as player skill advances, first move advantage can become an issue. FMA can usually be alleviated by switching up to a larger board. The Crossway rules specify a 19x19 board and black and white stones because at that time in 2007 it was important for me to make the game playable in real life. The specified equipment was commonly available for Go. But these dimensions and colors are arbitrary. And a board has a center recardless of whether the dimensions are even or odd. $\endgroup$ Sep 1, 2023 at 23:30
  • $\begingroup$ Yes, as I said to Isaacg, "Not entirely what I was looking for but good answer." He was addressing the "functionally equivalent" part of the OP, which was not defined. His answer was interesting and informative even though he didn't provide the type of example @drxorile did. Again, thank you for your response. There was a discussion about this in my forum discord.gg/feCAeKxyxD. My other games marksteeregames.com/MSG_game_index.html $\endgroup$ Sep 1, 2023 at 23:31

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