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Best approach for pattern recognition in Gomoku

2 posts in this topic

I am working on a little Gomoku engine, this project s mainly for learning and practicing in programming and the C language, and it is also fun smile.png

I want to represent the board with Bit-Boards , so I can do pattern checking for the evalution function and checkWin function etc.   
first thing,  because the board is huge (15x15), I need to hold more than one bit word for each player representing the board.     one idea I got from a forum member here is to have 15  16-bit words for each player,every word represents a row.  the board will look like this:


29   28   27   26   25   24 23 22 21 20 19 18 17 16 15  ....

14   13   12   11   10   9   8   7   6   5   4   3   2   1   0


But here comes my dillema.     let's say I want to check a certain row for a threat pattern that looks like this:   011110 (1E Hex, 30 Dec),  than i will go through every 6 bits in that row (and shift the row >> by 1 in every iteration) and with a & operation check if there is a match.   And I will do this for every threat pattern I want to check. 


So far it sound good, and also pretty fast the way I see it.    But checking the rows is easy becuase I have them ready in bit sets.   but what about the columns and diagonals? (even harder then columns!)   How can check for a bit- pattern match if i don't have the columns and diagonals already in my disposal  (like the rows).  ?     

The way i see it, I can "generate" a new 16 bit word each time I want to check for a certain column/diagonal, by means of starting from an empty number ( unsigned short y = 0) and than construct it one by one (with | operation or +...) by going through the rows at the right indexes.   and than, after i have a ready to go bit-word, i can again do the bit pattern checking to see if i have a threat someplace in that line.   just like i do for the rows.   Hope i am understood smile.png   


But it seems like very ineffiecient and slow way to do this, because imagine that in the evaluation function i will have to do this "construction" operation for every vertical and diagonal line on the board  (unlike the check win function where i can just do this for the last played square!).

So my way to get around this, is to just have the diagonal and columns already in my disposal,(every player with have 15 rows,15 columns,21 left diagonals and another 21 right diagonals)  so actually when i make a move on the board, I will have to play it on the row,columns and both diagonals for the played square (for each player!).     But than i can just go through all the lines and do the pattern checking.  The penalty will be a slightly slower make and unmake move functions ,but the evaluation function and check win functions the operation will be much much faster.  

It seems like a good idea, But i wonder if you maybe can suggest me better approach,  Maybe there is a way to check for patterns on bit boards in a more clever way than this naive way.    

My first considertion is performance ofcourse, so ican search for a deeper depth in less time.  The search space in Gomoku is huge already!


Edited by patishi

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The technique you describe in the last paragraph is similar to rotated bitboards in computer chess. It's definitely worth exploring, and it's probably the fastest solution. For each row, column or diagonal you can quickly evaluate where a pattern happens by a single table lookup.

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