quick_sort_3.cpp

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#include <algorithm>
#include <cassert>
#include <ctime>
#include <iostream>
#include <vector>
 
namespace {
template <typename T>
std::ostream &operator<<(std::ostream &out, const std::vector<T> &arr) {
    for (size_t i = 0; i < arr.size(); ++i) {
        out << arr[i];
        if (i < arr.size() - 1) {
            out << ", ";
        }
    }
    return out;
}
 
}  // namespace

namespace sorting {
namespace {  // using un-named namespace here to prevent partition function
             // being visible to end-users
template <typename T>
void partition3(std::vector<T> *arr, int32_t low, int32_t high, int32_t *i,
                int32_t *j) {
    // To handle 2 elements
    if (high - low <= 1) {
        if ((*arr)[high] < (*arr)[low]) {
            std::swap((*arr)[high], (*arr)[low]);
        }
        *i = low;
        *j = high;
        return;
    }
 
    int32_t mid = low;
    T pivot = (*arr)[high];
    while (mid <= high) {
        if ((*arr)[mid] < pivot) {
            std::swap((*arr)[low++], (*arr)[mid++]);
        } else if ((*arr)[mid] == pivot) {
            mid++;
        } else if ((*arr)[mid] > pivot) {
            std::swap((*arr)[mid], (*arr)[high--]);
        }
    }
 
    // update i and j
    *i = low - 1;
    *j = mid;  // or high-1
}
}  // namespace

template <typename T>
void quicksort(std::vector<T> *arr, int32_t low, int32_t high) {
    if (low >= high) {  // 1 or 0 elements
        return;
    }
 
    int32_t i = 0, j = 0;
 
    // i and j are passed as reference
    partition3(arr, low, high, &i, &j);
 
    // Recur two halves
    quicksort(arr, low, i);
    quicksort(arr, j, high);
}

template <typename T>
std::vector<T> quicksort(std::vector<T> arr, int32_t low, int32_t high) {
    if (low >= high) {  // 1 or 0 elements
        return arr;
    }
 
    int32_t i = 0, j = 0;
 
    // i and j are passed as reference
    partition3(&arr, low, high, &i, &j);
 
    // Recur two halves
    quicksort(&arr, low, i);
    quicksort(&arr, j, high);
 
    return arr;
}
}  // namespace sorting

static void test_int() {
    std::cout << "\nTesting integer type arrays\n";
 
    for (int num_tests = 1; num_tests < 21; num_tests++) {
        size_t size = std::rand() % 500;
        std::vector<int> arr(size);
        for (auto &a : arr) {
            a = std::rand() % 500 - 250;  // random numbers between -250, 249
        }
 
        std::cout << "Test " << num_tests << "\t Array size:" << size << "\t ";
        std::vector<int> sorted = sorting::quicksort(arr, 0, int32_t(size) - 1);
        if (size < 20) {
            std::cout << "\t Sorted Array is:\n\t";
            std::cout << sorted << "\n";
        }
        assert(std::is_sorted(std::begin(sorted), std::end(sorted)));
        std::cout << "\t Passed\n";
    }
}

static void test_double() {
    std::cout << "\nTesting Double type arrays\n";
    for (int num_tests = 1; num_tests < 21; num_tests++) {
        size_t size = std::rand() % 500;
        std::vector<double> arr(size);
        for (auto &a : arr) {
            a = double(std::rand() % 500) -
                250.f;   // random numbers between -250, 249
            a /= 100.f;  // convert to -2.5 to 2.49
        }
 
        std::cout << "Test " << num_tests << "\t Array size:" << size << "\t ";
        std::vector<double> sorted =
            sorting::quicksort(arr, 0, int32_t(size) - 1);
        if (size < 20) {
            std::cout << "\t Sorted Array is:\n\t";
            std::cout << sorted << "\n";
        }
        assert(std::is_sorted(std::begin(sorted), std::end(sorted)));
        std::cout << "\t Passed\n";
    }
}

int main() {
    std::srand(std::time(nullptr));
    test_int();
    test_double();
    return 0;
}