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//! Calculates a Greatest Common Divisor (GCD).
//!
//! ```
//! assert_eq!(2, gcd::gcd(10, 2));
//! assert_eq!(1, gcd::gcd(10, 3));
//! assert_eq!(14, gcd::gcd(56, 42));
//! ```

/// Calculates a Greatest Common Divisor (GCD).
pub fn gcd(a: u64, b: u64) -> u64 {
    if a == b {
        return a;
    }
    if a == 0 || b == 0 {
        return a | b;
    }

    let common_zeros = (a | b).trailing_zeros();
    let mut a = a >> a.trailing_zeros();
    let mut b = b >> b.trailing_zeros();

    while a != b {
        if a > b {
            a -= b;
            a >>= a.trailing_zeros();
        } else {
            b -= a;
            b >>= b.trailing_zeros();
        }
    }
    a << common_zeros
}