/*
 ******************************************************************************
 * file name  : better_rqs.cpp
 * author     : Hung Q. Ngo
 * description: a minor twist to the randomized quick sort algorithm which
 *              makes it run faster on inputs where the item values belong
 *              to a small finite class
 ******************************************************************************
 */
#include <iostream>
#include <vector>
#include <cstdlib>  // for rand() and srand()
#include <ctime>    // for time()

using namespace std;

/**
 *
 * -----------------------------------------------------------------------------
 * print a vector of Item_Type, finish with a new line
 * assumes compiler knows how to cout << Item_Type 
 * 
 * -----------------------------------------------------------------------------
 */
template <typename Item_Type>
void print_vec(vector<Item_Type>& vec) {
    for (int i=0; i<vec.size(); i++) {
        cout << vec[i] << " ";
    }
    cout << endl;
}

/**
 * -----------------------------------------------------------------------------
 * -1 if a < b, 0 if a==b, 1 if a>b
 * -----------------------------------------------------------------------------
 */
template <typename Item_Type>
int default_cmp(Item_Type a, Item_Type b) {
    return a < b? -1 : a == b ? 0 : 1;
}

/**
 * -----------------------------------------------------------------------------
 * The Randomized Quick Sort algorithm
 * - pick a random pivot, that's all
 * -----------------------------------------------------------------------------
 */
template <typename Item_Type>
void rqs_with_range(std::vector<Item_Type> &vec, int p, int q,
                    int (*cmp)(Item_Type, Item_Type))
{
    if (p >= q) return;

    // pick a random pivot
    int m = std::rand() % (q-p+1);
    std::swap(vec[q], vec[p+m]); 

    // partition
    int r=p-1;
    for (int j=p; j<q; j++) {
        if (rand() % 2 == 0) {
            if (cmp(vec[j],vec[q])<0)
                std::swap(vec[++r], vec[j]);
        } else {
        if (cmp(vec[j],vec[q])<=0)
                std::swap(vec[++r], vec[j]);
        }
    }
    std::swap(vec[++r], vec[q]);

    // sort the left & the right parts
    rqs_with_range(vec, p, r-1, cmp);
    rqs_with_range(vec, r+1, q, cmp);
}

/**
 * -----------------------------------------------------------------------------
 * Wrapper to the randomized quick sort algorithm
 * -----------------------------------------------------------------------------
 */
template <typename Item_Type>
void randomized_quick_sort(std::vector<Item_Type> &vec,
                    int (*cmp)(Item_Type, Item_Type) = default_cmp) 
{
    std::srand(static_cast<unsigned int>(std::time(0)));
    rqs_with_range(vec, 0, vec.size()-1, cmp);
}

/**
 * -----------------------------------------------------------------------------
 * returns a random vector of n elements, each from 0 to k-1
 * -----------------------------------------------------------------------------
 */
vector<int> random_vector(size_t n, size_t k) {
    srand(static_cast<unsigned int>(time(0)));
    vector<int> ret(n);
    for (int i=0; i<n; i++) 
        ret[i] = rand() % k;
    return ret;
}

// *****************************************************************************
int main() {
    // to test a specific sorting algorithm you can do
    vector<int> vec = random_vector(50000, 2);
    cout << "Before: "; print_vec(vec);
    randomized_quick_sort(vec);
    cout << "After: ";  print_vec(vec);
}