Author: Dann Corbit
Date: 09:06:19 02/11/02
Go up one level in this thread
On February 09, 2002 at 11:56:20, Heiner Marxen wrote:
>On February 09, 2002 at 03:07:47, Dann Corbit wrote:
>
>>On February 08, 2002 at 17:17:26, Oliver Roese wrote:
>>
>>>Hi!
>>>
>>>http://citeseer.nj.nec.com/338945.html
>>>Hope you find it interesting.
>>
>>Unless I've screwed it up a bit, it doesn't quite work right at the endpoints.
>>
>>#define debruijn32 0x077CB531UL
>>/* debruijn32 = 0000 0111 0111 1100 1011 0101 0011 0001 */
>>/* table to convert debruijn index to standard index */
>>static unsigned int index32[32];
>>/* routine to initialize index32 */
>>void setup(void)
>>{
>> unsigned i;
>> for (i = 0U; i < 32U; i++)
>> index32[(debruijn32 << i) >> 27U] = i;
>>}
>>/* compute index of rightmost 1 */
>>int rightmost_index(unsigned long b)
>>{
>> b &= -b;
>> b *= debruijn32;
>> b >>= 27U;
>> return index32[b];
>>}
>>#ifdef UNIT_TEST
>>#include <stdio.h>
>>#include <limits.h>
>>int main(void)
>>{
>> unsigned i;
>> int j;
>> setup();
>> j = rightmost_index(0);
>> printf("right bit of %u is %d\n", 0, j);
>> for (i = 1U; i < UINT_MAX && i > 0; i <<= 1U) {
>> int j = rightmost_index(i);
>> printf("right bit of %u is %d\n", i, j);
>> }
>> j = rightmost_index(UINT_MAX);
>> printf("right bit of %u is %d\n", UINT_MAX, j);
>> return 0;
>>}
>>#endif
>
>From the above code I get:
>
>right bit of 0 is 0
>right bit of 1 is 0
>right bit of 2 is 1
>right bit of 4 is 2
>right bit of 8 is 3
>right bit of 16 is 4
>right bit of 32 is 5
>right bit of 64 is 6
>right bit of 128 is 7
>right bit of 256 is 8
>right bit of 512 is 9
>right bit of 1024 is 10
>right bit of 2048 is 11
>right bit of 4096 is 12
>right bit of 8192 is 13
>right bit of 16384 is 14
>right bit of 32768 is 15
>right bit of 65536 is 16
>right bit of 131072 is 17
>right bit of 262144 is 18
>right bit of 524288 is 19
>right bit of 1048576 is 20
>right bit of 2097152 is 21
>right bit of 4194304 is 22
>right bit of 8388608 is 23
>right bit of 16777216 is 24
>right bit of 33554432 is 25
>right bit of 67108864 is 26
>right bit of 134217728 is 27
>right bit of 268435456 is 28
>right bit of 536870912 is 29
>right bit of 1073741824 is 30
>right bit of 2147483648 is 31
>right bit of 4294967295 is 0
>
>which appears to be perfect IMO.
>The last number (UINT_MAX) has the low bit set, so 0 is ok.
>
>What exactly don't you like, Dann?
right bit of 0 is 0
right bit of 1 is 0
But I guess it does not matter for chess, since you will always have at least
two kings on the board. Of course, you might be looking for a bishop position
or something from a list of bishops.
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