Author: Bruce Moreland
Date: 21:48:00 08/05/99
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On August 05, 1999 at 22:51:41, Scott Gasch wrote: >Both Bob and Bruce mentioned a small hash table specifically for detecting >repetition. I guess this would be implemented as an array of counts indexed by >a subset of the position's hash key -- say the last n bits. What is to prevent >false draw reading, however, if two (different) positions in the same search >yield the same key subset (and artificially inflate the count). In the hash element you store the whole hash key. So the odds of a bad draw score on any given node are the same as the odds of a full 64-bit hash collision. >Related question: I am initializing my transposition hash square-piece array >with just random numbers (rand()). This is not good for many reasons: 1) if the >random numbers I get (seeded off system clock) are bad (not very random) hashing >performs badly and we have many collisions (I have not really seen this happen >but in theory I guess it could). 2) there is no way I can predict what a key >for a given position will be beforehand which is okay now but when I write an >opening book I'd like to be able to do this. So definately I need some kind of >function to generate the seeds. Use the "srand" function to seed the random number generator with a known constant value, then seed it based upon the system clock when you are done, so that you get real random numbers after that. Alternately, you can write a program that will write some C that initializes the data, then just run this once and include the produced file in your souce. >My (next) question is this -- what do you guys do for said function? I am >computing two numbers for each position - a key and a checksum. The key is just >xors of seeds for piece/square and the checksum is adding and subtracting seeds. >I had considered using prime numbers for the seeds but I don't see how that >really will help produce a uniform hash distribution. I just use rand(). I don't think it matters what number you use for the seed to rand(). bruce
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