Difference between revisions of "Swap rule"

Revision as of 08:18, 21 March 2017

Because the first player has a considerable advantage if he is allowed to make his first move without restrictions, the swap rule was devised. It states that one player first makes a move, and then the second player decides who plays with which colour. The swap rule is also called the "Pie rule", since it resembles the You cut, I choose rule when sharing a pie between two children. The swap rule can be implemented in two ways, as follows. (Assume that the colours are red and blue, with red moving first.)

1. The first player places a red piece in any hex on the board. Then the other player can either make a move with the blue pieces, in which case he becomes blue, or he can state that he wants to be red. After this the game continues without any more swapping.
2. The first player makes a move with the red pieces. The other player either makes a move with the blue pieces, or makes a piece swap. This means to remove the red piece from the board, and place a blue piece at the hexagon which is the mirror image of the hexagon in which the red piece was placed, with respect to the board's long diagonal.

In face-to-face play the first option is most practical, since it is easier to change colours of the players that removing and adding pieces on the board. It is also less error-prone. On game sites on the Internet the second version is more common, presumably because the colours are determined at the start of the game, and it is easier to change the board position than the colour designation.

When playing with the swap rule, the second player theoretically has a forced win. However, the second player's advantage is much smaller than the advantage of being the first player when playing without swap. Moreover, in either cases, the first player can study the move that he'll play before the game even starts. This provides an incentive for playing as the first player even when the game is played with the swap rule.

How to find a "fair" first move

According to many strong hex players, everything else being equal, having an extra stone on the board makes the game easier to play. This means that whenever one is in doubt about a move, it's a good idea to swap it. The first player should also take this into account by playing a first move that is probably losing under perfect play. This fact has been popularized (and written here in hexwiki) by "lazplayer" at littlegolem/igg.

The generalized swap rule

Instead of placing just one piece, the first player can place any number of red and blue pieces, and state which colour has the next move. The second player then decides who has which colour. This version has the drawback that the first player can prepare in advance a position which he has studied extensively, which the second player doesn't see until the game starts, thereby giving the first player an advantage.