Steven G. Johnson
Here is a little algorithm I came across whose implementation is
amusingly obscure: what simple function does the following C code
compute, and why?
#include <stdint.h>
unsigned foo(uint32_t n)
{
const uint32_t a = 0x05f66a47;
static const unsigned bar[32] =
{0,1,2,26,23,3, 15,27,24,21,19, 4,12,16,28,6,31 ,25,22,14,20,18 ,11,5,30,13,17, 10,29,9,8,7};
n = ~n;
return bar[(a * (n & (-n))) >27];
}
To save you the trouble of compiling and running it yourself, here is
what it produces for n = 0,1,2,...,31:
0 -0, 1 -1, 2 -0, 3 -2, 4 -0, 5 -1, 6 -0, 7 -3, 8 ->
0, 9 -1, 10 -0, 11 -2, 12 -0, 13 -1, 14 -0, 15 -4, 16 ->
0, 17 -1, 18 -0, 19 -2, 20 -0, 21 -1, 22 -0, 23 -3, 24 -
0, 25 -1, 26 -0, 27 -2, 28 -0, 29 -1, 30 -0, 31 -5
Mar 9 '08 #1
Subscribe Reply
7 
1878 
CBFalconer
"Steven G. Johnson" wrote:
>
Here is a little algorithm I came across whose implementation is
amusingly obscure: what simple function does the following C code
compute, and why?#include <stdint.h>
unsigned foo(uint32_t n)
{
const uint32_t a = 0x05f66a47;
static const unsigned bar[32] =
{0,1,2,26,23,3, 15,27,24,21,19, 4,12,16,28,6,31 ,25,22,14,20,18 ,11,5,30,13,17, 10,29,9,8,7};
n = ~n;
return bar[(a * (n & (-n))) >27];
}
Doesn't seem to work very well on a machine with 16 bit integers.
--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section.
--
Posted via a free Usenet account from http://www.teranews.com
Mar 10 '08 #2
Keith Thompson
CBFalconer <cb********@yah oo.comwrites:
"Steven G. Johnson" wrote:>Here is a little algorithm I came across whose implementation is
amusingly obscure: what simple function does the following C code
compute, and why?#include <stdint.h>
unsigned foo(uint32_t n)
{
const uint32_t a = 0x05f66a47;
static const unsigned bar[32] =
{0,1,2,26,23,3 ,15,27,24,21,19 ,4,12,16,28,6,3 1,25,22,14,20,1 8,11,5,30,13,17 ,10,29,9,8,7};
n = ~n;
return bar[(a * (n & (-n))) >27];
}
Doesn't seem to work very well on a machine with 16 bit integers.
Most machines have 16-bit integers; often the 16-bit integer type is
called "short".
If you mean 16-bit ints, I don't see how that would cause a problem,
since the fucntion's argument is of type uint32_t. (The unsigned
result shouldn't be a problem unless you're worried about integers
with more than 32767 bits.)
Or am I missing something?
--
Keith Thompson (The_Other_Keit h) <ks***@mib.or g>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Mar 10 '08 #3
Joachim Schmitz
CBFalconer wrote:
"Steven G. Johnson" wrote:>>
Here is a little algorithm I came across whose implementation is
amusingly obscure: what simple function does the following C code
compute, and why?#include <stdint.h>
unsigned foo(uint32_t n)
{
const uint32_t a = 0x05f66a47;
static const unsigned bar[32] =
{0,1,2,26,23,3 ,15,27,24,21,19 ,4,12,16,28,6,3 1,25,22,14,20,1 8,11,5,30,13,17 ,10,29,9,8,7};
n = ~n;
return bar[(a * (n & (-n))) >27];
}
Doesn't seem to work very well on a machine with 16 bit integers.
Mind to enlighten me why?
Bye, Jojo
Mar 10 '08 #4
user923005
On Mar 9, 10:40*am, "Steven G. Johnson" <stev...@alum.m it.eduwrote:
Here is a little algorithm I came across whose implementation is
amusingly obscure: what simple function does the following C code
compute, and why?#include <stdint.h>
unsigned foo(uint32_t n)
{
* * *const uint32_t a = 0x05f66a47;
* * *static const unsigned bar[32] =
{0,1,2,26,23,3, 15,27,24,21,19, 4,12,16,28,6,31 ,25,22,14,20,18 ,11,5,30,13,17, *10,29,9,8,7};
* * *n = ~n;
* * *return bar[(a * (n & (-n))) >27];}
To save you the trouble of compiling and running it yourself, here is
what it produces for n = 0,1,2,...,31:0 -0, 1 -1, 2 -0, 3 -2, 4 -0, 5 -1, 6 -0, 7 -3, 8 ->
0, 9 -1, 10 -0, 11 -2, 12 -0, 13 -1, 14 -0, 15 -4, 16 ->
0, 17 -1, 18 -0, 19 -2, 20 -0, 21 -1, 22 -0, 23 -3, 24 -0, 25 -1, 26 -0, 27 -2, 28 -0, 29 -1, 30 -0, 31 -5- Hide quoted text -
Here's a 64 bit version of something very similar:
const int lsz64_tbl[64] =
{
0, 31, 4, 33, 60, 15, 12, 34,
61, 25, 51, 10, 56, 20, 22, 35,
62, 30, 3, 54, 52, 24, 42, 19,
57, 29, 2, 44, 47, 28, 1, 36,
63, 32, 59, 5, 6, 50, 55, 7,
16, 53, 13, 41, 8, 43, 46, 17,
26, 58, 49, 14, 11, 40, 9, 45,
21, 48, 39, 23, 18, 38, 37, 27,
};
//Gerd Isenberg's implementation of bitscan:
int GerdBitScan(Bit board bb)
{
const Bitboard lsb = (bb & -(long long) bb) - 1;
const unsigned int foldedLSB = ((int) lsb) ^ ((int) (lsb >32));
return lsz64_tbl[foldedLSB * 0x78291ACF >26];
}
//Gerd Isenberg's implementation of bitscan with clear:
int GerdBitScanRese t(Bitboard *bb)
{
const Bitboard lsb = (bb[0] & -(long long) bb[0]) - 1;
const unsigned int foldedLSB = ((int) lsb) ^ ((int) (lsb >32));
bb[0] &= (bb[0] - 1);
return lsz64_tbl[foldedLSB * 0x78291ACF >26];
}
Chess programmers will recognize DeBrun's sequences. It was
popularized by Matthew Henry, IIRC.
See, for instance:
http://chessprogramming.wikispaces.com/BitScan
Mar 10 '08 #5
user923005
On Mar 10, 3:02*pm, user923005 <dcor...@connx. comwrote:
On Mar 9, 10:40*am, "Steven G. Johnson" <stev...@alum.m it.eduwrote:Here is a little algorithm I came across whose implementation is
amusingly obscure: what simple function does the following C code
compute, and why?#include <stdint.h>
unsigned foo(uint32_t n)
{
* * *const uint32_t a = 0x05f66a47;
* * *static const unsigned bar[32] =
{0,1,2,26,23,3, 15,27,24,21,19, 4,12,16,28,6,31 ,25,22,14,20,18 ,11,5,30,13,17, **10,29,9,8,7};
* * *n = ~n;
* * *return bar[(a * (n & (-n))) >27];}To save you the trouble of compiling and running it yourself, here is
what it produces for n = 0,1,2,...,31:0 -0, 1 -1, 2 -0, 3 -2, 4 -0, 5 -1, 6 -0, 7 -3, 8 ->
0, 9 -1, 10 -0, 11 -2, 12 -0, 13 -1, 14 -0, 15 -4, 16 ->
0, 17 -1, 18 -0, 19 -2, 20 -0, 21 -1, 22 -0, 23 -3, 24 -0, 25 -1, 26 -0, 27 -2, 28 -0, 29 -1, 30 -0, 31 -5- Hidequoted text -
Here's a 64 bit version of something very similar:
const int * * * lsz64_tbl[64] =
{
* * 0, 31, 4, 33, 60, 15, 12, 34,
* * 61, 25, 51, 10, 56, 20, 22, 35,
* * 62, 30, 3, 54, 52, 24, 42, 19,
* * 57, 29, 2, 44, 47, 28, 1, 36,
* * 63, 32, 59, 5, 6, 50, 55, 7,
* * 16, 53, 13, 41, 8, 43, 46, 17,
* * 26, 58, 49, 14, 11, 40, 9, 45,
* * 21, 48, 39, 23, 18, 38, 37, 27,};//Gerd Isenberg's implementation of bitscan:
int * * * * * * GerdBitScan(Bit board bb)
{
* * const Bitboard *lsb = (bb & -(long long) bb) - 1;
* * const unsigned int foldedLSB = ((int) lsb) ^ ((int) (lsb >32));
* * return lsz64_tbl[foldedLSB * 0x78291ACF >26];}
//Gerd Isenberg's implementation of bitscan with clear:
int * * * * * * GerdBitScanRese t(Bitboard *bb)
{
* * const Bitboard *lsb = (bb[0] & -(long long) bb[0]) - 1;
* * const unsigned int foldedLSB = ((int) lsb) ^ ((int) (lsb >32));
* * bb[0] &= (bb[0] - 1);
* * return lsz64_tbl[foldedLSB * 0x78291ACF >26];}
Chess programmers will recognize DeBrun's sequences. *It was
popularized by Matthew Henry, IIRC.
See, for instance:http://chessprogramming.wikispaces.com/BitScan- Hide quoted text -- Show quoted text -
Oops... Left out the typedef necessary to grok this code:
typedef unsigned long long Bitboard;
Mar 10 '08 #6
Richard
CBFalconer <cb********@yah oo.comwrites:
"Steven G. Johnson" wrote:>>
Here is a little algorithm I came across whose implementation is
amusingly obscure: what simple function does the following C code
compute, and why?#include <stdint.h>
unsigned foo(uint32_t n)
{
const uint32_t a = 0x05f66a47;
static const unsigned bar[32] =
{0,1,2,26,23,3 ,15,27,24,21,19 ,4,12,16,28,6,3 1,25,22,14,20,1 8,11,5,30,13,17 ,10,29,9,8,7};
n = ~n;
return bar[(a * (n & (-n))) >27];
}
Doesn't seem to work very well on a machine with 16 bit integers.--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section.
Why? Please explain.
Mar 10 '08 #7
CBFalconer
Keith Thompson wrote:
CBFalconer <cb********@yah oo.comwrites:>"Steven G. Johnson" wrote:>>Here is a little algorithm I came across whose implementation is
amusingly obscure: what simple function does the following C code
compute, and why?#include <stdint.h>
unsigned foo(uint32_t n) {
const uint32_t a = 0x05f66a47;
static const unsigned bar[32] =
{0,1,2,26,23, 3,15,27,24,21,1 9,4,12,16,28,6, 31,25,22,14,20, 18,11,5,30,13,1 7,10,29,9,8,7};
n = ~n;
return bar[(a * (n & (-n))) >27];
}
Doesn't seem to work very well on a machine with 16 bit integers.
Most machines have 16-bit integers; often the 16-bit integer type is
called "short".If you mean 16-bit ints, I don't see how that would cause a problem,
since the fucntion's argument is of type uint32_t. (The unsigned
result shouldn't be a problem unless you're worried about integers
with more than 32767 bits.)Or am I missing something?
Yeah, I meant int, and was sloppy. To me, uint32_t doesn't exist,
since it is not guaranteed. Lets make the complaint about a
machine with an 18 bit int.
--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section.
--
Posted via a free Usenet account from http://www.teranews.com
Mar 13 '08 #8
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