Binary GCD in Java

// Binary GCD in Java
//
// PURPOSE
// Binary GCD replaces general remainders with parity tests, division by two, and
// subtraction.
//
// METHOD
// Remove common factors of two, make the first operand odd, repeatedly make the second odd
// and subtract the smaller odd operand, then restore the common power of two.
//
// COMPLEXITY
// O(log(max(a,b))) iterations under fixed-width arithmetic, O(1) space.
//
// CONTRACT
// Nonnegative integers. gcd(0,0)=0.
// Public APIs infer array lengths except in C, which accepts pointer/length pairs. Vertex
// counts and matrix dimensions remain explicit. Arrays must contain signed 32-bit integers.
// Sorted order, correct tree topology, and caller-owned data-structure invariants are
// preconditions; checking them on every query would change query complexity.
// Scalar and shape checks are included. Value-range checks are included where needed to
// prevent invalid indexing or nontermination. Allocation sizes are capped at 4,000,000
// workspace entries. Arithmetic outside Python raises an error on 32-bit overflow; Python
// uses arbitrary precision. Native allocation failures propagate. In-place outputs may be
// partly updated after arithmetic failure. These are single-threaded, in-memory
// implementations.
//
// LANGUAGE DETAILS
// Java 17+: primitive int arrays, Arrays.binarySearch for LIS, and Math exact arithmetic.
// Invalid input throws IllegalArgumentException; arithmetic overflow throws
// ArithmeticException.
//
// EXAMPLE
// Expected return: 6.
// Run: javac AlgorithmExample.java && java AlgorithmExample
// Tests for this exact source artifact run before publication. The accompanying source
// archive includes unit cases, runtime results, and source hashes.

class AlgorithmExample {
    static void alg_require(boolean valid) { if (!valid) throw new IllegalArgumentException("invalid algorithm input"); }
    static int alg_size(int n) { alg_require(n>=0 && n<=4000000); return n; }

    static int binary_gcd(int a, int b) {
        alg_require(((a >= 0) && (b >= 0)));
        if ((a == 0)) {
            return b;
        }
        if ((b == 0)) {
            return a;
        }
        int factor = 1;
        int temp = 0;
        while ((((a % 2) == 0) && ((b % 2) == 0))) {
            a = (a / 2);
            b = (b / 2);
            factor = Math.multiplyExact(factor, 2);
        }
        while (((a % 2) == 0)) {
            a = (a / 2);
        }
        while ((b != 0)) {
            while (((b % 2) == 0)) {
                b = (b / 2);
            }
            if ((a > b)) {
                temp = a;
                a = b;
                b = temp;
            }
            b = Math.subtractExact(b, a);
        }
        return Math.multiplyExact(a, factor);
    }

    public static void main(String[] args) {
        int a = 48;
        int b = 18;
        int result = binary_gcd(a, b);
        System.out.println(result);
    }
}