/*
 * Nick Pafundi
 * Questions or comments on this code should be
 * submitted at http://www.static-zero.com.
 * 
 * This is a sample program that uses a decToBin
 * function to convert a decimal number to 2's
 * complement binary.
 */

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

/* Function Prototype */
int *decToBin(int decimal, int numBits, int binary[]);

int main(int argc, char *argv[]) {
	/* Initialize all values */
	int numBits = 10;
	int *binary;
	int num1 = 49;
	int num2 = -93;
	int i;
	
	/* Dynamically allocate array */
	binary = malloc(sizeof(int)*numBits);
	
	/* Try with 49 (should be 0000110001) */
	binary = decToBin(num1, numBits, binary);
	printf("Decimal %d in 2's complement binary is ", num1);
	for (i=0; i<numBits; i++)
		printf("%d", binary[i]);
	printf("\n");
	
	/* Try with -93 (should be 1110100011)  */
	binary = decToBin(num2, numBits, binary);
	printf("Decimal %d in 2's complement binary is ", num2);
	for (i=0; i<numBits; i++)
		printf("%d", binary[i]);
	printf("\n");
	
	return 0;
}

/* This function will convert base 10 decimal to 2's
 * complement binary.
 */
int *decToBin(int decimal, int numBits, int binary[]) {
	int tmp, i;
	int carry = 1;
	int flag = 0;
	
	/* Indicies are 0 - numBits-1, so subtract 1 */
	tmp = --numBits;
	
	/* Initialize array to 0 */
	for(i=tmp; i>=0; i--)
		binary[i] = 0;
	
	/* If the number is negative, set the
	 * flag and invert the number
	 */
	if (decimal < 0) {
		flag = 1;
		decimal = -decimal;
	}
	
	/* decimal%2 will give the current binary bit.
	 * Divide by 2 to get the next bit.  Do this while
	 * decimal is larger than 0. (Note - integer division
	 * will throw away the remainder, which must happen.
	 */
	while (decimal > 0) {
		binary[numBits--] = decimal%2;
		decimal/=2;
	}
	
	/* Deal with the negative number.  First, take the
	 * complement of the current bit.  This number will
	 * be 2 less than the bit we want, so add 2.  Then
	 * add the carry.  If the result is > 1, it's no
	 * longer binary, so set it to 0 and try the next
	 * bit.  Continue this until we can successfully add
	 * the carry bit. Once it is added, or once we've
	 * finished the loop, we're done.
	 */
	if (flag) {
		for(i=tmp; i>=0; i--){
			binary[i] = ~(binary[i])+2;
			binary[i]+=carry;
			carry = 0;
			if (binary[i] > 1) {
				binary[i] = 0;
				carry = 1;
			}
		}
	}
	
	/* Return the array */
	return binary;
}