Backgammon, Part 1


I was first introduced to backgammon when I was a part of the Minnesota SF group in Minneapolis in the 80’s. I was never very good, relying more on luck than skill, but I still liked playing whenever I had the chance. I’d win maybe a third of my games, but when I went against a better player who only played for money, my ratio dropped to at best 1 out of 4 (if not less).

Backgammon is virtually unheard of in Japan. The boards aren’t sold in book or department stores (you can easily get Othello and Reversi boards here, though). So, it’s been decades since I’ve been able to find a human opponent. A few days ago, I gave up and tried out the backgammon page on Yahoo games. I know I shouldn’t expect much from something that’s free, but the AI in the yahoo game is pretty pathetic – I have about an 80% win rate.

I’ve long wanted to write a role-playing game that revolves around backgammon, but until I can afford to buy a copy of Adobe Flash ($400 USD?) that probably won’t happen soon. In the meantime, I think I’ll write a blog series on what would be necessary for a good game AI.

Starting out

A typical backgammon board is divided up into quadrants, with alternating light and dark triangles, called “points”. There’s a total of 24 points, 12 on each side. (The alternating colors don’t mean anything, they’re just there to make counting them easier.) There are 2 players, with 15 checkers, called “stones”. The stones are generally placed on the board as shown (image taken from the wiki page).

The players take turns rolling 2 dice, and moving their stones in a horseshoe direction in order to get all of their stones into the quadrant directly in front of them (points 6-1 for black, 19-24 for red or white). When they reach this goal, they can start removing the stones from the board to the tray; the first one to take off all of their stones wins. You move a stone exactly the number of points given on each die, and you can either move two stones one die each, or one stone twice. If you roll doubles, you get 4 moves.

A single stone on a point is called a “blot”. You can land on your opponent’s blot and send it to the bar (the area in the middle of the board). If a point has 2 or more stones on it, the other player can’t land on it. You CAN land on a point that has your own stones on it, a point that has no stones on it, or your opponent’s blot. If a blot is hit and put on the bar, that player has to roll the dice and try to get the blot back onto the board at the far end of their horseshoe pattern (points 6-1 for red or white, 19-24 for black) before proceeding.

Play starts by laying out the stones as shown in the image. Then each player rolls one die, and the player with the higher number goes first. If both players roll the same number, then you advance the doubling cube by one, and roll again. (The yahoo game doesn’t use the doubling cube).

Right now, you have enough information to try out the yahoo game, if you want.

Doubling Cube

If you play for money, or just want to keep track of your score, then the doubling cube becomes more important. This looks like a 6-sided die, but it’s printed with the values 2, 4, 8, 16, 32 and 64. When the cube is on the bar, it has a value of 1. If both players roll the same number when determining who goes first, then the cube goes to 2. Roll doubles again, it goes to 4, etc. Then, during the course of the game, one player can challenge the other by “doubling the game”. If the challengee accepts, then he/she gets control of the cube and the game is now worth twice as many points. If the challengee declines, he/she loses the game and it’s worth whatever value the cube had been standing at (1, 2, 4, etc.) Whoever has control of the cube can then double the game again, if they think they can win, or if they just want to end a game that they’re obviously going to win.

I really wish the yahoo game had a doubling cube to end boring games, rather than having to play them all the way to the end all the time. On the other hand, I think the game invokes a double – occasinally when I’m winning, yahoo games crashes Firefox. I hate that.

Games are normally worth 1 point, unless they’ve been doubled. Since the games are fairly quick – 5-10 minutes – players can decide to play until one person reaches 10 points, or whatever. Or, play for money, with 1 point = $1.

To be continued.

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