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https://issues.apache.org/jira/browse/MATH-841?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13428849#comment-13428849
]
Sebastien Riou commented on MATH-841:
-------------------------------------
Here it is:
public static int gcd(final int p, final int q) {
int a = p;
int b = q;
if ((a == 0) || (b == 0)) {
if ((a == Integer.MIN_VALUE) || (b == Integer.MIN_VALUE)){
throw new
MathArithmeticException(LocalizedFormats.GCD_OVERFLOW_32_BITS,p, q);
}
return FastMath.abs(a+b);
}
long al=a;
long bl=b;
boolean useLong=false;
if(a<0){
if(Integer.MIN_VALUE==a) useLong=true;
else a=-a;
al=-al;
}
if(b<0){
if(Integer.MIN_VALUE==b) useLong=true;
else b=-b;
bl=-bl;
}
if(useLong){
if(al==bl) throw new
MathArithmeticException(LocalizedFormats.GCD_OVERFLOW_32_BITS,p, q);
long blbu=bl;
bl=al;
al=blbu % al;
if(0L==al){
if(bl>Integer.MAX_VALUE) throw new
MathArithmeticException(LocalizedFormats.GCD_OVERFLOW_32_BITS,p, q);
return (int)bl;
}
blbu=bl;
//now al and bl can fit in int, so we use the int agorithm for
speed matters
b=(int)al;
a=(int)(blbu % al);
}
return gcdEuclideIterative(a,b);
}
//direct translation of the gcd definition into an iterative form
//rule 1: gcd(a,0)=a
//rule 2: gcd(a,b)=gcd(b,a mod b)
static int gcdEuclideIterative(int x,int y){
assert(x>=0);
assert(y>=0);
while(0!=x && 0!=y){
int xbu=x;
x=y;//gcd(a,b)=gcd(b,a mod b)
y=xbu%y;
}
return x+y;//gcd(a,0)=a
}
> gcd speed up
> ------------
>
> Key: MATH-841
> URL: https://issues.apache.org/jira/browse/MATH-841
> Project: Commons Math
> Issue Type: Improvement
> Affects Versions: 3.0
> Environment: ubuntu 32bits/intel-i5/java6
> Reporter: Sebastien Riou
> Priority: Trivial
> Labels: patch, performance
> Fix For: 3.1
>
> Attachments: patchGcdInt.txt
>
> Original Estimate: 1h
> Remaining Estimate: 1h
>
> The gcd(int,int) method of ArithmeticUtils seems 2 times slower than the
> naive approach using modulo operator. The following test code runs in 11s
> with current version and in 6s with the patch.
> public void testApache(){
> Random rng=new Random(0);
> long checksum=0;
> long start=System.nanoTime();
> checksum+=gcd(0,Integer.MAX_VALUE);
> checksum+=gcd(Integer.MAX_VALUE,0);
> checksum+=gcd(Integer.MAX_VALUE,rng.nextInt());
> for(int i=0;i<10000;i++)
> checksum+=gcd(rng.nextInt(),Integer.MAX_VALUE);
> checksum+=gcd(Integer.MAX_VALUE,Integer.MAX_VALUE);
> checksum+=gcd(Integer.MIN_VALUE,1<<30);
> checksum+=gcd(1<<30,1<<30);
> checksum+=gcd(3 * (1<<20),9 * (1<<15));
> for(int i=0;i<30000000;i++)
> checksum+=gcd(rng.nextInt(),rng.nextInt());
> long end=System.nanoTime();
> long tns=end-start;
> long tms=(tns+500000)/1000000;
> long ts=(tms+500)/1000;
> System.out.println("exec time="+ts+"s, ("+tms+"ms),
> checksum="+checksum);
> assertEquals(9023314441L,checksum);
> }
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