46 lines
1.6 KiB
C++
46 lines
1.6 KiB
C++
#ifndef _QSORT_H
|
|
#define _QSORT_H
|
|
//general quicksort suitable if we do not want to create extra class for the member type but prefer to encapsulate it into compare and swap soutines
|
|
//returns parity of the permutation
|
|
|
|
template<typename INDEX, typename COMPAR>
|
|
int genqsort(INDEX l, INDEX r,COMPAR (*cmp)(const INDEX, const INDEX), void (*swap)(const INDEX,const INDEX))
|
|
{
|
|
INDEX i,j,piv;
|
|
int parity=0;
|
|
|
|
if(r<=l) return parity; //1 element
|
|
if(cmp(r,l)<0) {parity^=1; swap(l,r);}
|
|
if(r-l==1) return parity; //2 elements and preparation for median
|
|
piv= (l+r)/2; //pivoting by median of 3 - safer
|
|
if(cmp(piv,l)<0) {parity^=1; swap(l,piv);} //and change the pivot element implicitly
|
|
if(cmp(r,piv)<0) {parity^=1; swap(r,piv);} //and change the pivot element implicitly
|
|
if(r-l==2) return parity; //in the case of 3 elements we are finished too
|
|
|
|
//general case , l-th r-th already processed
|
|
i=l+1; j=r-1;
|
|
do{
|
|
//important sharp inequality - stops at sentinel element for efficiency
|
|
// this is inefficient if all keys are equal - unnecessary n log n swaps are done, but we assume that it is atypical input
|
|
while(cmp(i++,piv)<0);
|
|
i--;
|
|
while(cmp(j--,piv)>0);
|
|
j++;
|
|
if(i<j)
|
|
{
|
|
// swap and keep track of position of pivoting element
|
|
parity^=1; swap(i,j);
|
|
if(i==piv) piv=j; else if(j==piv) piv=i;
|
|
}
|
|
if(i<=j) {i++; j--;}
|
|
}while(i<=j);
|
|
|
|
if(j-l < r-i) //because of the stack in bad case process first the shorter subarray
|
|
{if(l<j) parity ^=genqsort(l,j,cmp,swap); if(i<r) parity ^=genqsort(i,r,cmp,swap);}
|
|
else
|
|
{if(i<r) parity ^=genqsort(i,r,cmp,swap); if(l<j) parity ^=genqsort(l,j,cmp,swap);}
|
|
return parity;
|
|
}
|
|
|
|
#endif
|