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

// When calling this funcion, please be aware that input must be twice as big as the string it is storing
// Like this: ['b', 'e', 'd', 0, '', '', '']
// If not your program will crash or maybe do something weird
void double_letters(char *input){
	// An in-place solution.  Will it be slow?
	int final_length = strlen(input) * 2;
	for(int i = 0; i < final_length; i+=2){
		// shuffle everything over
		for(int j = strlen(input); j > i; j--)
			input[j]  = input[j-1];
		input[i+1] = input[i];
	}
}

char* allocate_and_double_letters(char *input){
	char *return_location = malloc(strlen(input) * 2);
	strcpy(return_location, input);
	double_letters(return_location);
	return return_location;
}

int main(){

	char bug[19] = "cockroach";
	double_letters(bug);
	printf("bug = %s\n", bug);

	char *another_bug = "bedbug";
	char *tmp = allocate_and_double_letters(another_bug);
	printf("another_bug modifed is:  %s\n", tmp);
	free(tmp);

	return 0;
}