Binary GCD in Python

from bisect import bisect_left

# Binary GCD in Python
#
# 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
# Python 3.9+: automatic lengths, native lists, arbitrary-precision arithmetic, and
# bisect_left for LIS. Invalid scalar/shape inputs raise ValueError. Integer element types
# and sorted/tree invariants remain caller preconditions.
#
# EXAMPLE
# Expected return: 6.
# Run: python example.py
# Tests for this exact source artifact run before publication. The accompanying source
# archive includes unit cases, runtime results, and source hashes.

def alg_require(condition):
    if not condition:
        raise ValueError("invalid algorithm input; see contract")
def alg_size(n):
    alg_require(0 <= n <= 4000000)
    return n

def binary_gcd(a: int, b: int) -> int:
    alg_require(type(a) is int and -2147483648 <= a <= 2147483647)
    alg_require(type(b) is int and -2147483648 <= b <= 2147483647)
    alg_require(a >= 0 and b >= 0)
    if a == 0:
        return b
    if b == 0:
        return a
    factor: int = 1
    temp: int = 0
    while a % 2 == 0 and b % 2 == 0:
        a = a // 2
        b = b // 2
        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 -= a
    return a * factor

a = 48
b = 18
result = binary_gcd(a, b)
print(result)