[Flashcoders] bitwise question
When i have a 8 bit int where only one of the bit can be 1, what is then
the quickest way to get that bit position with value 1?
Now I use this.
function getBit():int{
var tCount:int = 0;
for (var i:int=0; i 8; i++) {
if (8 (1 i)) return i;
}
return 0;
}
Can this be done faster?
Jiri
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Re: [Flashcoders] bitwise question
Hey,
not sure if it's faster, but this would work as well:
var i:Number = 32;
trace (Math.log(i)/Math.LN2);
returns bit number 5.
Lookuptable might be faster in this case.
greetz
JC
On Tue, Sep 15, 2009 at 10:04 AM, Jiri jiriheitla...@googlemail.com wrote:
When i have a 8 bit int where only one of the bit can be 1, what is then
the quickest way to get that bit position with value 1?
Now I use this.
function getBit():int{
var tCount:int = 0;
for (var i:int=0; i 8; i++) {
if (8 (1 i)) return i;
}
return 0;
}
Can this be done faster?
Jiri
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Re: [Flashcoders] bitwise question
Jiri wrote:
When i have a 8 bit int where only one of the bit can be 1, what is
then the quickest way to get that bit position with value 1?
Now I use this.
function getBit():int{
var tCount:int = 0;
for (var i:int=0; i 8; i++) {
if (8 (1 i))return i;
}
return 0;
}
Can this be done faster?
Yes. Use a lookup table;
var lookup:Array=[];
for (var n:int=0; n128;n++)
{
lookup.push(0);
}
lookup[2]=1;
lookup[4]=2;
lookup[8]=4;
//etc..
function getBit(n:int):int{
return lookup[n];
}
Paul
Jiri
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Re: [Flashcoders] bitwise question
Paul Andrews wrote:
Jiri wrote:
When i have a 8 bit int where only one of the bit can be 1, what is
then the quickest way to get that bit position with value 1?
Now I use this.
function getBit():int{
var tCount:int = 0;
for (var i:int=0; i 8; i++) {
if (8 (1 i))return i;
}
return 0;
}
Can this be done faster?
Yes. Use a lookup table;
var lookup:Array=[];
for (var n:int=0; n128;n++)
{
lookup.push(0);
}
lookup[2]=1;
lookup[4]=2;
lookup[8]=4;
//etc..
function getBit(n:int):int{
return lookup[n];
}
LOL, better still, just ditch the function altogether and use
lookup[somevariable] directly.
I might as well comment that this is lean and mean without any safety
net to make sure the int is in range and indeed has only one bit set.
Paul
Jiri
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Re: [Flashcoders] bitwise question
Thnx, i tought that there would be some kind of smart bitwise operation
to do that. This will do fine. Cheers.
Jiri
Paul Andrews wrote:
Paul Andrews wrote:
Jiri wrote:
When i have a 8 bit int where only one of the bit can be 1, what is
then the quickest way to get that bit position with value 1?
Now I use this.
function getBit():int{
var tCount:int = 0;
for (var i:int=0; i 8; i++) {
if (8 (1 i))return i;
}
return 0;
}
Can this be done faster?
Yes. Use a lookup table;
var lookup:Array=[];
for (var n:int=0; n128;n++)
{
lookup.push(0);
}
lookup[2]=1;
lookup[4]=2;
lookup[8]=4;
//etc..
function getBit(n:int):int{
return lookup[n];
}
LOL, better still, just ditch the function altogether and use
lookup[somevariable] directly.
I might as well comment that this is lean and mean without any safety
net to make sure the int is in range and indeed has only one bit set.
Paul
Jiri
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Re: [Flashcoders] bitwise question
Another option:
function test(value:int):int {
// just to be safe...
value = value 0xff;
switch(value) {
case 0x01: return 0;
case 0x02: return 1;
case 0x04: return 2;
case 0x08: return 3;
case 0x10: return 4;
case 0x20: return 5;
case 0x40: return 6;
case 0x80: return 7;
default: return -1;
}
}
Cheers
Juan Pablo Califano
2009/9/15 Paul Andrews p...@ipauland.com
Paul Andrews wrote:
Jiri wrote:
When i have a 8 bit int where only one of the bit can be 1, what is then
the quickest way to get that bit position with value 1?
Now I use this.
function getBit():int{
var tCount:int = 0;
for (var i:int=0; i 8; i++) {
if (8 (1 i))return i;
}
return 0;
}
Can this be done faster?
Yes. Use a lookup table;
var lookup:Array=[];
for (var n:int=0; n128;n++)
{
lookup.push(0);
}
lookup[2]=1;
lookup[4]=2;
lookup[8]=4;
//etc..
function getBit(n:int):int{
return lookup[n];
}
LOL, better still, just ditch the function altogether and use
lookup[somevariable] directly.
I might as well comment that this is lean and mean without any safety net
to make sure the int is in range and indeed has only one bit set.
Paul
Jiri
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Re: [Flashcoders] bitwise question
Juan Pablo Califano wrote:
Another option:
function test(value:int):int {
// just to be safe...
value = value 0xff;
switch(value) {
case 0x01: return 0;
case 0x02: return 1;
case 0x04: return 2;
case 0x08: return 3;
case 0x10: return 4;
case 0x20: return 5;
case 0x40: return 6;
case 0x80: return 7;
default: return -1;
}
}
Cheers
Juan Pablo Califano
Yes, this is the best way to do it.
Paul
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Re: [Flashcoders] bitwise question
Jiri,
I haven't done much bit twiddling yet in AS3 but I think you were fast
already, but your i was being incremented and shifted.
Also i8 would only get you bits 2,1,0
In binary that would be i1000
How about:
function getBit(var numb:int):int {
var bit:int=1;
var count:int=0;
if (numb==0 || numb0x80) return -1; // error
while (bit numb == 0) {
1 bit;
count++;
}
return count;
}
John
Jiri wrote:
When i have a 8 bit int where only one of the bit can be 1, what is
then the quickest way to get that bit position with value 1?
Now I use this.
function getBit():int{
var tCount:int = 0;
for (var i:int=0; i 8; i++) {
if (8 (1 i))return i;
}
return 0;
}
Can this be done faster?
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Re: [Flashcoders] bitwise question
I think his i 8 was valid as the i is used for the shift, not the value...
The only thing that might be faster is using i-- rather than i++ - for
some reason decrementing through a loop is suppsoed to be faster.
Glen :)
John McCormack wrote:
Jiri,
I haven't done much bit twiddling yet in AS3 but I think you were fast
already, but your i was being incremented and shifted.
Also i8 would only get you bits 2,1,0
In binary that would be i1000
How about:
function getBit(var numb:int):int {
var bit:int=1;
var count:int=0;
if (numb==0 || numb0x80) return -1; // error
while (bit numb == 0) {
1 bit;
count++;
}
return count;
}
John
Jiri wrote:
When i have a 8 bit int where only one of the bit can be 1, what is
then the quickest way to get that bit position with value 1?
Now I use this.
function getBit():int{
var tCount:int = 0;
for (var i:int=0; i 8; i++) {
if (8 (1 i))return i;
}
return 0;
}
Can this be done faster?
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Re: [Flashcoders] bitwise question
It's a little curious loop!
i=0
i shifted = 0
i++ gives = 1 binary 0001
i shifted = 2 binary 0010
i++ gives i=3 binary 0011
i shifted=6 binary 0110
i++ gives i= 7 binary 0111
i shifted=14 binary 1110
End of loop because i=8
i++ gives i= 15 binary
i shifted= 30 binary 0001 1110
John
Glen Pike wrote:
I think his i 8 was valid as the i is used for the shift, not the
value...
The only thing that might be faster is using i-- rather than i++ - for
some reason decrementing through a loop is suppsoed to be faster.
Glen :)
John McCormack wrote:
Jiri,
I haven't done much bit twiddling yet in AS3 but I think you were
fast already, but your i was being incremented and shifted.
Also i8 would only get you bits 2,1,0
In binary that would be i1000
How about:
function getBit(var numb:int):int {
var bit:int=1;
var count:int=0;
if (numb==0 || numb0x80) return -1; // error
while (bit numb == 0) {
1 bit;
count++;
}
return count;
}
John
Jiri wrote:
When i have a 8 bit int where only one of the bit can be 1, what is
then the quickest way to get that bit position with value 1?
Now I use this.
function getBit():int{
var tCount:int = 0;
for (var i:int=0; i 8; i++) {
if (8 (1 i))return i;
}
return 0;
}
Can this be done faster?
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Re: [Flashcoders] bitwise question
Can't confirm your findings right now, and also wonder which loop you
are talking about. My initially posted one?
function getBit():int{
var tCount:int = 0;
for (var i:int=0; i 8; i++) {
if (8 (1 i))return i;
}
return 0;
}
or the one suggested by John McCormack. I cant remember have faulty
result with my loop because like Glen Pike mentioned the i is used for
the shifting.
Also the 8bit to test will always only have one bit set to 1. Which
could be a potential dangerous assumption i guess.
0001
0010
0100
etc..
Will do testing tomorrow.
jiri
John McCormack wrote:
It's a little curious loop!
i=0
i shifted = 0
i++ gives = 1 binary 0001
i shifted = 2 binary 0010
i++ gives i=3 binary 0011
i shifted=6 binary 0110
i++ gives i= 7 binary 0111
i shifted=14 binary 1110
End of loop because i=8
i++ gives i= 15 binary
i shifted= 30 binary 0001 1110
John
Glen Pike wrote:
I think his i 8 was valid as the i is used for the shift, not the
value...
The only thing that might be faster is using i-- rather than i++ - for
some reason decrementing through a loop is suppsoed to be faster.
Glen :)
John McCormack wrote:
Jiri,
I haven't done much bit twiddling yet in AS3 but I think you were
fast already, but your i was being incremented and shifted.
Also i8 would only get you bits 2,1,0
In binary that would be i1000
How about:
function getBit(var numb:int):int {
var bit:int=1;
var count:int=0;
if (numb==0 || numb0x80) return -1; // error
while (bit numb == 0) {
1 bit;
count++;
}
return count;
}
John
Jiri wrote:
When i have a 8 bit int where only one of the bit can be 1, what is
then the quickest way to get that bit position with value 1?
Now I use this.
function getBit():int{
var tCount:int = 0;
for (var i:int=0; i 8; i++) {
if (8 (1 i))return i;
}
return 0;
}
Can this be done faster?
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Re: [Flashcoders] bitwise question
Cheers.
Jiri
Paul Andrews wrote:
Juan Pablo Califano wrote:
Another option:
function test(value:int):int {
// just to be safe...
value = value 0xff;
switch(value) {
case 0x01: return 0;
case 0x02: return 1;
case 0x04: return 2;
case 0x08: return 3;
case 0x10: return 4;
case 0x20: return 5;
case 0x40: return 6;
case 0x80: return 7;
default: return -1;
}
}
Cheers
Juan Pablo Califano
Yes, this is the best way to do it.
Paul
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