def test__v_isub(self):
f1 = bigint([lobj.MASK] + [0] * 9 + [1], 1)
f2 = bigint([1], 1)
borrow = lobj._v_isub(f1, 1, len(f1._digits)-1, f2, 1)
assert borrow == 0
assert f1.tolong() == (1 << lobj.SHIFT) ** 10 - 1
def gcd(u, v):
from rpython.rlib.rbigint import _v_isub, _v_rshift, SHIFT
# binary gcd from https://en.wikipedia.org/wiki/Binary_GCD_algorithm
if not u.tobool():
return v.abs()
if not v.tobool():
return u.abs()
# The result is negative iff both inputs have a common -1 factor
if v.sign == -1 and u.sign == -1:
sign = -1
else:
sign = 1
if u.size == 1 and v.size == 1:
result = gcd1(u.digit(0), v.digit(0))
return rbigint([result], sign, 1)
# Compute the factors of 2 for u and v and record the number of
# common 2 factors in shift.
shiftu = count_trailing_zeros(u)
shiftv = count_trailing_zeros(v)
shift = min(shiftu, shiftv)
# Perform shift on each number, but guarantee that we will end up with a new
# digit array for each rbigint. They will be mutated
if shiftu:
u = u.rshift(shiftu, dont_invert=True)
if u.sign == -1:
u.sign = 1
else:
u = rbigint(u._digits[:], 1, u.numdigits())
if shiftv:
v = v.rshift(shiftv, dont_invert=True)
if v.sign == -1:
v.sign = 1
else:
v = rbigint(v._digits[:], 1, v.numdigits())
# From here on, u is always odd.
while True:
if u.size == 1 and v.size == 1:
digit = gcd1(u.digit(0), v.digit(0))
u = rbigint([digit], 1, 1)
break
# Now u and v are both odd. Swap if necessary so u <= v,
# then set v = v - u (which is even).
if u.gt(v):
u, v, = v, u
assert _v_isub(v, 0, v.numdigits(), u, u.numdigits()) == 0
v._normalize()
if not v.tobool():
break
# remove all factors of 2 in v -- they are not common
# note: v is not zero, so while will terminate
# XXX: Better to perform multiple inplace shifts, or one
# shift which allocates a new array?
rshift = count_trailing_zeros(v)
while rshift >= SHIFT:
assert _v_rshift(v, v, v.numdigits(), SHIFT - 1) == 0
rshift -= SHIFT - 1
assert _v_rshift(v, v, v.numdigits(), rshift) == 0
v._normalize()
# restore common factors of 2 and sign
result = u.lshift(shift)
result.sign = sign
return result
