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This introduces variants of the divsteps-based GCD algorithm used for modular inverses to compute Jacobi symbols. Changes compared to the normal vartime divsteps: * Only positive matrices are used, guaranteeing that f and g remain positive. * An additional jac variable is updated to track sign changes during matrix computation. * There is (so far) no proof that this algorithm terminates within reasonable amount of time for every input, but experimentally it appears to almost always need less than 900 iterations. To account for that, only a bounded number of iterations is performed (1500), after which failure is returned. The field logic then falls back to using square roots to determining the result. * The algorithm converges to f=g=gcd(f0,g0) rather than g=0. To keep this test simple, the end condition is f=1, which won't be reached if started with g=0. That case is dealt with specially.
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