Block_swap_algorithm_for array rotation.cpp

#include <bits/stdc++.h>
using namespace std;

/*
Initialize A = arr[0..d-1] and B = arr[d..n-1]
1) Do following until size of A is equal to size of B

  a)  If A is shorter, divide B into Bl and Br such that Br is of same
       length as A. Swap A and Br to change ABlBr into BrBlA. Now A
       is at its final place, so recur on pieces of B.

   b)  If A is longer, divide A into Al and Ar such that Al is of same
       length as B Swap Al and B to change AlArB into BArAl. Now B
       is at its final place, so recur on pieces of A.

2)  Finally when A and B are of equal size, block swap them
*/
/*Prototype for utility functions */
void printArray(int arr[], int size);
void swap(int arr[], int fi, int si, int d);

void leftRotate(int arr[], int d, int n)
{
	/* Return If number of elements to be rotated
	is zero or equal to array size */
	if(d == 0 || d == n)
		return;

	/*If number of elements to be rotated
	is exactly half of array size */
	if(n - d == d)
	{
		swap(arr, 0, n - d, d);
		return;
	}

	/* If A is shorter*/
	if(d < n - d)
	{
		swap(arr, 0, n - d, d);
		leftRotate(arr, d, n - d);
	}
	else /* If B is shorter*/
	{
		swap(arr, 0, d, n - d);
		leftRotate(arr + n - d, 2 * d - n, d); /*if 12345678, then became 67845123 . so
		after the first trial of swapping u have to rotate 45123 then u consider ur array begin from 4
		so adjust the beginning of array by its address*/
	}
}

/*UTILITY FUNCTIONS*/
/* function to print an array */
void printArray(int arr[], int size)
{
	int i;
	for(i = 0; i < size; i++)
		cout << arr[i] << " ";
	cout << endl;
}

/*This function swaps d elements starting at index fi
with d elements starting at index si */
void swap(int arr[], int fi, int si, int d) /*
1234567 if u want to rotate 12 then swap them first
*/
{
	int i, temp;
	for(i = 0; i < d; i++)
	{
		temp = arr[fi + i];
		arr[fi + i] = arr[si + i];
		arr[si + i] = temp;
	}
}

// Driver Code
int main()
{
	int arr[] = {1, 2, 3, 4, 5, 6, 7};
	leftRotate(arr, 2, 7);
	printArray(arr, 7);
	return 0;
}

// This code is contributed by rathbhupendra