pancake_sort.cpp

/**
 * @file
 * @brief pancake sort sorts a disordered stack of pancakes by flipping any
 * number of pancakes using a spatula using minimum number of flips.
 *
 * @details
 * Unlike a traditional sorting algorithm, which attempts to sort with the
 * fewest comparisons possible, the goal is to sort the sequence in as few
 * reversals as possible. Overall time complexity of pancake sort is O(n^2) For
 * example: example 1:- Disordered pancake sizes: {2,5,3,7,8} Sorted:
 * {2,3,5,7,8} For example: example 2:- Disordered pancake sizes:
 * {22,51,37,73,81} Sorted: {22,37,51,73,81}
 * @author [Divyansh Gupta](https://github.com/divyansh12323)
 * @see more on [Pancake sort](https://en.wikipedia.org/wiki/Pancake_sorting)
 * @see related problem at
 * [Leetcode](https://leetcode.com/problems/pancake-sorting/)
 */

#include <algorithm>  // for std::is_sorted
#include <cassert>    // for std::assert
#include <iostream>   // for io operations
#include <vector>     // for std::vector

/**
 * @namespace sorting
 * @brief Sorting algorithms
 */
namespace sorting {
/**
 * @namespace pancake_sort
 * @brief Functions for [Pancake
 * sort](https://en.wikipedia.org/wiki/Pancake_sorting) algorithm
 */
namespace pancake_sort {
/**
 * @brief This implementation is for reversing elements in a a C-style array .
 * @param [start,end] arr our vector of elements.
 * @param start starting index of array
 * @param end ending index of array
 * @returns void
 */
template <typename T>
void reverse(std::vector<T> &arr, int start, int end) {
    T temp;  // Temporary variable
    while (start <= end) {
        temp = arr[start];
        arr[start] = arr[end];
        arr[end] = temp;
        start++;
        end--;
    }
}
/**
 * @brief This implementation is for a C-style array input that gets modified in
 * place.
 * @param [start,end] arr our vector of elements.
 * @param size size of given array
 * @returns 0 on exit
 */
template <typename T>
int pancakeSort(std::vector<T> &arr, int size) {
    for (int i = size; i > 1; --i) {
        int max_index = 0, j = 0;  // intialize some variables.
        T max_value = 0;
        for (j = 0; j < i; j++) {
            if (arr[j] >= max_value) {
                max_value = arr[j];
                max_index = j;
            }
        }
        if (max_index != i - 1)  // check for reversing
        {
            reverse(arr, 0, max_index);
            reverse(arr, 0, i - 1);
        }
    }
    return 0;
}
}  // namespace pancake_sort
}  // namespace sorting

/**
 * @brief Test implementations
 * @returns void
 */
static void test() {
    // example 1: vector of int
    const int size1 = 7;
    std::cout << "\nTest 1- as std::vector<int>...";
    std::vector<int> arr1 = {23, 10, 20, 11, 12, 6, 7};
    sorting::pancake_sort::pancakeSort(arr1, size1);
    assert(std::is_sorted(arr1.begin(), arr1.end()));
    std::cout << "Passed\n";
    for (int i = 0; i < size1; i++) {
        std::cout << arr1[i] << " ,";
    }
    std::cout << std::endl;

    // example 2: vector of double
    const int size2 = 8;
    std::cout << "\nTest 2- as std::vector<double>...";
    std::vector<double> arr2 = {23.56, 10.62, 200.78, 111.484,
                                3.9,   1.2,   61.77,  79.6};
    sorting::pancake_sort::pancakeSort(arr2, size2);
    assert(std::is_sorted(arr2.begin(), arr2.end()));
    std::cout << "Passed\n";
    for (int i = 0; i < size2; i++) {
        std::cout << arr2[i] << ", ";
    }
    std::cout << std::endl;

    // example 3:vector of float
    const int size3 = 7;
    std::cout << "\nTest 3- as std::vector<float>...";
    std::vector<float> arr3 = {6.56, 12.62, 200.78, 768.484, 19.27, 68.87, 9.6};
    sorting::pancake_sort::pancakeSort(arr3, size3);
    assert(std::is_sorted(arr3.begin(), arr3.end()));
    std::cout << "Passed\n";
    for (int i = 0; i < size3; i++) {
        std::cout << arr3[i] << ", ";
    }
    std::cout << std::endl;
}
/**
 * @brief Main function
 * @returns 0 on exit
 */
int main() {
    test();
    return 0;
}