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Copy pathselection_gen.cpp
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executable file
·242 lines (211 loc) · 6.66 KB
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#include <iostream>
#include <algorithm>
#include <vector>
#include <utility>
using namespace std;
// After pivoting, we have:
// vec[0], vec[2], ..., vec[l-1] < pivot
// vec[l], vec[l+1], ..., vec[l+m-1] = pivot
// vec[l+m], vec[l+m+1], vec[-1] > pivot
// return the position (l,m)
// start+distance refers to the last position, exclusive
// Note: the return val might out of the index bound
/*
* Return a true pivot index such that minimalize the recursive call
*/
template <class Iterator, class T=typename Iterator::value_type> inline size_t finalize_pivot(Iterator start, size_t l, size_t m, size_t len, const T& v) {
// fill pivot values
for (size_t i=0; i<m; ++i) {
*(start+l+i) = v;
}
if (m==0 || l >= len/2) {
return l;
} else if (l+m < len/2) {
return l+m;
} else {
return len/2;
}
}
template <class Iterator, class T=typename Iterator::value_type> size_t pivot(const T& pivot_value, Iterator start, size_t distance) {
size_t l = 0;
size_t l_cur = 1;
size_t r_cur = distance - 1;
size_t m = 0;
T val = *start;
while (l_cur <= r_cur) { // ending condition: m = r-l
if (val == pivot_value) {
m++;
while (l_cur<=r_cur) {
T v = *(start+l_cur++);
if (v != pivot_value) {
*(start+l) = v;
val = v;
break;
} else { // (v == pivot) {
m++;
}
}
}
else if ( val > pivot_value) {
//scan from r_cur to left until find element < pivot
while (l_cur<=r_cur) {
T v = *(start+r_cur);
if (v>pivot_value) {
if (r_cur>0) {
--r_cur;
} else {
return finalize_pivot<Iterator, T>(start, l, m, distance, pivot_value);
}
} else if (v == pivot_value) {
m++;
while (l_cur<=r_cur) {
T v = *(start+l_cur++);
if (v != pivot_value) {
*(start+r_cur) = v;
break;
} else { // (v == pivot) {
m++;
}
}
} else {
*(start+(l++)) = v;
*(start+r_cur) = val;
if (l_cur == r_cur) {
++l_cur;
} else {
val = *(start+l_cur++);
*(start + l) = val; // read next value
}
if (r_cur>0) {
--r_cur;
break;
} else {
return finalize_pivot<Iterator, T>(start, l, m, distance, pivot_value);
}
}
}
}
else {
++l;
if (l_cur == r_cur) {
++l_cur;
} else {
val = *(start+l_cur++);
*(start + l) = val; // read next value
}
}
}
return finalize_pivot<Iterator, T>(start, l, m, distance, pivot_value);
}
template <class Iterator> class IndexComparator {
Iterator start;
static unsigned index_table[5]; // for 5 group
public:
IndexComparator(Iterator newStart) {
start = newStart;
}
unsigned getMedian(Iterator newStart) {
start = newStart;
sort(index_table, index_table+5, *this);
return index_table[2];
}
bool operator()(unsigned a, unsigned b){
return (*(start+a) > *(start+b)) ? true : false;
}
};
template <class Iterator> unsigned IndexComparator<Iterator>::index_table[5] = {0, 1, 2, 3, 4};
template <class Iterator, class T=typename Iterator::value_type> T median_medians(Iterator start, size_t num);
// return the kth minimal element in the range start, start + num (not inclusive)
// ASSUME: 1<=k<=num
template <class Iterator, class T=typename Iterator::value_type> T find_k(Iterator start, size_t num, size_t k) {
if ((k==0) || (k>num)) throw "k should be greater than 0 and less or equal than num";
if (num <=5){
sort(start, start+num);
return *(start+k-1);
}
T m = median_medians<Iterator, T>(start, num);
size_t p = pivot(m, start, num); // partition the range with the median of medians
if (p>=k) // how about p == num or p == 0?
return find_k<Iterator, T>(start, p, k);
else
return find_k<Iterator, T>(start+p, num-p, k-p);
}
template <class Iterator, class T=typename Iterator::value_type>
T median_medians(Iterator start, size_t num) {
if (num == 0) {
throw "0 size error in medians_medians";
}
if (num <=10){
sort(start, start+num);
return *(start+num/2);
}
static IndexComparator<Iterator> median_in_5(start);
size_t num_m = num/5;
// medians of groups of 5
for(size_t i=0; i<num_m; i++) {
unsigned m = median_in_5.getMedian(start+5*i);
// swap median of the i-th 5 group to i-th position
T val = *(start+i);
*(start+i) = *(start+5*i+m);
*(start+5*i+m) = val;
}
return find_k<Iterator, T>(start, num_m, num_m/2);
}
int main(int argc, char** argv) {
size_t m = 0;
int *M = NULL;
if (argc>2) {
m = argc - 1;
M = new int[m];
for (size_t i = 1; i < argc; ++i) {
M[i-1] = atoi(argv[i]);
}
} else {
m = static_cast<size_t>(atoi(argv[1]));
M = new int[m];
srand (time(NULL));
for (size_t i = 0; i < m; ++i) {
M[i] = rand() % 100;
// cout << ' ' << M[i];
}
cout << endl;
}
int mm = median_medians<int*, int>(M, m);
cout << "First medians of medians: " << mm << endl;
/*
for (size_t i = 0; i < m; ++i) {
cout << ' ' << M[i];
}
cout << endl;
*/
size_t p = pivot<int*, int>(mm, M, m);
cout << "Pivoting with mm return: " << p << endl;
/*
for (size_t i = 0; i < m; ++i) {
cout << ' ' << M[i];
}
cout << endl;
*/
cout << "Median: " << find_k<int*, int>(M, m, m/2) << endl;
/*
for (size_t i = 0; i < m; ++i) {
cout << ' ' << M[i];
}
cout << endl;
*/
vector<int> vec(m);
for (size_t i = 0; i < m; ++i) {
vec[i] = M[i];
// cout << ' ' << vec[i];
}
cout << endl;
cout << "Median of vec: " << find_k(vec.begin(), m, m/2) << endl;
/*
for (size_t i = 0; i < m; ++i) {
cout << ' ' << vec[i];
}
cout << endl;
*/
delete [] M;
return 0;
}