tim_sort.cpp

// C++ program to perform TimSort.
#include <algorithm>
#include <cassert>
#include <iostream>
#include <numeric>

const int RUN = 32;

// this function sorts array from left index to to right index which is of size
// atmost RUN
void insertionSort(int arr[], int left, int right) {
    for (int i = left + 1; i <= right; i++) {
        const int temp = arr[i];
        int j = i - 1;
        while (j >= left && arr[j] > temp) {
            arr[j + 1] = arr[j];
            j--;
        }
        arr[j + 1] = temp;
    }
}

// merge function merges the sorted runs
void merge(int arr[], int l, int m, int r) {
    // original array is broken in two parts, left and right array
    const int len1 = m - l + 1, len2 = r - m;
    int *left = new int[len1], *right = new int[len2];
    for (int i = 0; i < len1; i++) left[i] = arr[l + i];
    for (int i = 0; i < len2; i++) right[i] = arr[m + 1 + i];

    int i = 0;
    int j = 0;
    int k = l;

    // after comparing, we merge those two array in larger sub array
    while (i < len1 && j < len2) {
        if (left[i] <= right[j]) {
            arr[k] = left[i];
            i++;
        } else {
            arr[k] = right[j];
            j++;
        }
        k++;
    }

    // copy remaining elements of left, if any
    while (i < len1) {
        arr[k] = left[i];
        k++;
        i++;
    }

    // copy remaining element of right, if any
    while (j < len2) {
        arr[k] = right[j];
        k++;
        j++;
    }
    delete[] left;
    delete[] right;
}

// iterative Timsort function to sort the array[0...n-1] (similar to merge sort)
void timSort(int arr[], int n) {
    // Sort individual subarrays of size RUN
    for (int i = 0; i < n; i += RUN)
        insertionSort(arr, i, std::min((i + 31), (n - 1)));

    // start merging from size RUN (or 32). It will merge to form size 64, then
    // 128, 256 and so on ....
    for (int size = RUN; size < n; size = 2 * size) {
        // pick starting point of left sub array. We are going to merge
        // arr[left..left+size-1] and arr[left+size, left+2*size-1] After every
        // merge, we increase left by 2*size
        for (int left = 0; left < n; left += 2 * size) {
            // find ending point of left sub array
            // mid+1 is starting point of right sub array
            const int mid = std::min((left + size - 1), (n - 1));
            const int right = std::min((left + 2 * size - 1), (n - 1));

            // merge sub array arr[left.....mid] & arr[mid+1....right]
            merge(arr, left, mid, right);
        }
    }
}

// utility function to print the Array
void printArray(int arr[], int n) {
    for (int i = 0; i < n; i++) printf("%d  ", arr[i]);
    std::cout << std::endl;
}

/**
 * @brief self-test implementation
 * @returns void
 */
void tests() {
    // Case: array of length 65
    constexpr int N = 65;
    int arr[N];

    std::iota(arr, arr + N, 0);
    std::reverse(arr, arr + N);
    assert(!std::is_sorted(arr, arr + N));

    timSort(arr, N);
    assert(std::is_sorted(arr, arr + N));
}

// Driver program to test above function
int main() {
    tests();  // run self test implementations

    int arr[] = {5, 21, 7, 23, 19};
    const int n = sizeof(arr) / sizeof(arr[0]);
    printf("Given Array is\n");
    printArray(arr, n);

    timSort(arr, n);

    printf("After Sorting Array is\n");
    printArray(arr, n);
    return 0;
}