# Binary GCD (Stein’s Algorithm) in C

Binary GCD also known as Stein’s Algorithm is an algorithm that computes the greatest common divisor of two (positive) numbers . Discovered in 1967 by the Israeli programmer Josef Stein, it’s an alternative to the classical Euclid’s Algorithm, and is considered to be more efficient than this as it’s replacing divisions and multiplications with bitwise operations . The algorithm is recursive by nature, but loops can be used instead of recursion .

Note that by `B_GCD(num1, num2)` we will refer to a function that returns the greatest common divisor of two positive numbers (`num1` and `num2`).

Rules of the algorithm:

1. `B_GCD(0,0)` is not defined, but for convenience we will consider it `0`;
2. `B_GCD(num1,0) = num1` and `B_GCD(0,num2) = num2`;
3. If `num1` and `num2` are even, `B_GCD(num1, num2) = 2 * B_GCD(num1/2, num2/2)`, as 2 is a common divisor
4. If `num1` is even and `num2` is odd, `B_GCD(num1, num2) = B_GCD(num1 /2, num2)`, as 2 is not a common divisor . The steps are the same if `num1` is odd and `num2` is even : `B_GCD(num1, num2) = B_GCD(num1, num2/2)`
5. If both `num1` and `num2` are odd, then:
• if `num1 >= num2` -> `B_GCD(num1, num2) = B_GCD((num1-num2)/2, num2)`
• else `B_GCD(num1, num2) = B_GCD((num2-num1)/2, num1)`
6. Step 4 and 5 are repeated until `num1 = num2`, or `num1 = 0`

We can also use pseudo code to describe the above algorithm.

Recursive Version of Binary GCD (Stein Algorithm):

``````FUNCTION  B_GCD(num1, num2)
IF num1 = num2 THEN
RETURN num1
IF num1 = 0 AND num2 = 0 THEN
RETURN 0
IF num1 = 0 THEN
RETURN num2
IF num2 = 0 THEN
RETURN num1
IF num1 IS EVEN AND num2 IS EVEN THEN
RETURN (B_GCD(num1/2, num2/2) * 2)
IF num1 IS EVEN AND num2 IS ODD THEN
RETURN B_GCD(num1/2, num2)
IF num1 IS ODD AND num2 IS EVEN THEN
RETURN B_GCD(num1, num2/2)
IF num1 IS ODD AND num2 IS ODD THEN
IF num1 >= num2 THEN
RETURN B_GCD((num1-num2)/2, num2)
ELSE
RETURN B_GCD((num2-num1)/2, num1)
``````

The loop-version of the Binary GCD Algorithm

``````FUNCTION B_GCD(num1, num2)
power_of_two := 0
IF (num1 = 0 OR num2 = 0) THEN
RETURN num1 | num2
WHILE ((num1 IS EVEN) AND (num2 IS EVEN))
num1 := num1 / 2
num2 := num2 / 2
power_of_two := power_of_two + 1
DO
WHILE(num1 IS EVEN)
num1 := num1 / 2
WHILE(num2 IS EVEN)
num2 := num2 / 2
IF (num1 >= num2) THEN
num1 := (num1 - num2) / 2
ELSE
tmp  := num1
num1 := (num2 - num1) / 2
num2 := tmp
WHILE NOT ((num1 = num2) OR (num1 = 0))
RETURN num2 * power_of_two
``````

# Implementation in C

The code is available in this github repo:

``````gh repo clone nomemory/blog-stein-algorithm-c
``````

## Recursive Version of Binary GCD (Stein Algorithm):

``````#include <stdio.h>

/**
* Stein's Algorithm .
* @author Andrei Ciobanu
* @date DEC 12, 2010
*/

int b_gcd(int num1, int num2)
{
if (num1 == num2) {
return (num1);
}
if (!num1 && !num2) {
/* Convention: GCD(0, 0) = 0 */
return (0);
}
if (!num1 || !num2) {
// GCD(0, num2) = num2
// GCD(num1, 0) = num1
return (num1 | num2);
}
if ( !(num1 & 1) && !(num2 & 1)) {
// num1 and num2 are even,
// then gcd(num1, num2) = 2 * gcd(num1/2, num2/2)
return (b_gcd(num1 >> 1, num2 >> 1) << 1);
}
if ( !(num1 & 1) && (num2 & 1)) {
// num1 is even, and num2 is odd
// then gcd(num1, num2) = gcd(num1/2, num2)
return b_gcd(num1 >> 1, num2);
}
if ( (num1 & 1) && !(num2 & 1)) {
// num1 is odd, and num2 is even
// then gcd(num1, num2) = gcd(num1, num2/2)
return b_gcd(num1, num2 >> 1);
}
if ( (num1 & 1) && (num2 & 1)) {
// num1 and num2 are odd
if(num1 >= num2) {
return b_gcd((num1 - num2) >> 1, num1);
}
else {
return b_gcd((num2 - num1) >> 1, num1);
}
}
return (0);
}

int main(int argc, char *argv[])
{
printf("%dn", b_gcd(9 * 16, 3 * 32));
return (0);
}
``````

## The loop-version of the Binary GCD Algorithm:

``````#include <stdio.h>

/**
* Stein's Algorithm .
* @author Andrei Ciobanu
* @date DEC 12, 2010
*/

int b_gcd(int num1, int num2)
{
int pof2, tmp;
if (!num1 || !num2) {
return (num1 | num2);
}

// pof2 is the greatest power of 2 deviding both numbers .
// We will use pof2 to multiply the returning number .
pof2 = 0;
while(!(num1 & 1) && !(num2 & 1)) {
// gcd(even1, even1) = pof2 * gcd(even1/pof2, even2/pof2)
num1 >>=1;
num2 >>=1;
pof2++;
}

do {
while (!(num1 & 1)) {
num1 >>=1;
}
while (!(num2 & 1)) {
num2 >>=1;
}
// At this point we know for sure that
// num1 and num2 are odd
if (num1 >= num2) {
num1 = (num1 - num2) >> 1;
}
else {
tmp = num1;
num1 = (num2 - num1) >> 1;
num2 = tmp;
}
} while (!(num1 == num2 || num1 == 0));

return (num2 << pof2);
}

int main(int argc, char *argv[])
{
printf("%d", b_gcd(9 * 16, 3 * 32));
return (0);
}
``````

If both cases the output is 48, and if you look closely, in both cases we’ve used bitwise operations instead of the standard multiplication / division operators .

Note:

If you are interested in the classical Euclid’s Algorithm (for finding the greatest common divisor) + pseudocode and implementation please read this article: Euclid’s Algorithm .

Updated: