Step by Step solution about How to resolve the algorithm Knuth shuffle step by step in the C programming language

The provided C code defines two shuffle functions: a generic one (shuffle) that can be used with any data type and a specialized one (shuffle_int) that is optimized for integers.

1. Generic Shuffle Function (shuffle):

   • The shuffle function takes three arguments: obj (a pointer to an array of elements), nmemb (number of elements in the array), and size (size of each element in bytes).
   • It uses the Fisher-Yates shuffle algorithm to randomly rearrange the elements in the array.
   • The algorithm maintains a pointer n to the unsorted portion of the array and repeatedly selects a random element from the unsorted portion to swap with the element at position n.
   • It uses the rrand function to generate random numbers.
   • The function allocates a temporary buffer temp to facilitate the swapping.

2. Specialized Shuffle Function (shuffle_int):

   • The shuffle_int function is a specialized version of the generic shuffle function that is optimized for integer data.
   • It uses the irand function to generate random indices and does not need to allocate a temporary buffer since it can directly swap integers using the tmp variable.
   • This optimization can lead to improved performance compared to the generic shuffle function for integer arrays.

3. Random Number Generation Functions:

   • The rrand function generates a random integer between 0 and m-1 using the rand function and a floating-point conversion.
   • The irand function generates a random integer between 0 and n-1 by filtering out values that fall outside the desired range and using integer division to map the generated random number to the desired range.

4. Main Function (main):

   • The main function demonstrates the usage of the shuffle_int function.
   • It initializes an array x with integers from 0 to 19, prints the initial order of elements, shuffles the array using shuffle_int, and then prints the shuffled order.

#include <stdlib.h>
#include <string.h>

int rrand(int m)
{
  return (int)((double)m * ( rand() / (RAND_MAX+1.0) ));
}

#define BYTE(X) ((unsigned char *)(X)) 
void shuffle(void *obj, size_t nmemb, size_t size)
{
  void *temp = malloc(size);
  size_t n = nmemb;
  while ( n > 1 ) {
    size_t k = rrand(n--);
    memcpy(temp, BYTE(obj) + n*size, size);
    memcpy(BYTE(obj) + n*size, BYTE(obj) + k*size, size);
    memcpy(BYTE(obj) + k*size, temp, size);
  }
  free(temp);
}


#include <stdio.h>
#include <stdlib.h>

/* define a shuffle function. e.g. decl_shuffle(double).
 * advantage: compiler is free to optimize the swap operation without
 *            indirection with pointers, which could be much faster.
 * disadvantage: each datatype needs a separate instance of the function.
 *            for a small funciton like this, it's not very big a deal.
 */
#define decl_shuffle(type)				\
void shuffle_##type(type *list, size_t len) {		\
	int j;						\
	type tmp;					\
	while(len) {					\
		j = irand(len);				\
		if (j != len - 1) {			\
			tmp = list[j];			\
			list[j] = list[len - 1];	\
			list[len - 1] = tmp;		\
		}					\
		len--;					\
	}						\
}							\

/* random integer from 0 to n-1 */
int irand(int n)
{
	int r, rand_max = RAND_MAX - (RAND_MAX % n);
	/* reroll until r falls in a range that can be evenly
	 * distributed in n bins.  Unless n is comparable to
	 * to RAND_MAX, it's not *that* important really. */
	while ((r = rand()) >= rand_max);
	return r / (rand_max / n);
}

/* declare and define int type shuffle function from macro */
decl_shuffle(int);

int main()
{
	int i, x[20];

	for (i = 0; i < 20; i++) x[i] = i;
	for (printf("before:"), i = 0; i < 20 || !printf("\n"); i++)
		printf(" %d", x[i]);

	shuffle_int(x, 20);

	for (printf("after: "), i = 0; i < 20 || !printf("\n"); i++)
		printf(" %d", x[i]);
	return 0;
}