Python/maths/karatsuba.py

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""" Multiply two numbers using Karatsuba algorithm """
def karatsuba(a, b):
"""
>>> karatsuba(15463, 23489) == 15463 * 23489
True
>>> karatsuba(3, 9) == 3 * 9
True
"""
if len(str(a)) == 1 or len(str(b)) == 1:
return a * b
m1 = max(len(str(a)), len(str(b)))
m2 = m1 // 2
a1, a2 = divmod(a, 10**m2)
b1, b2 = divmod(b, 10**m2)
x = karatsuba(a2, b2)
y = karatsuba((a1 + a2), (b1 + b2))
z = karatsuba(a1, b1)
return (z * 10 ** (2 * m2)) + ((y - z - x) * 10 ** (m2)) + (x)
def main():
print(karatsuba(15463, 23489))
if __name__ == "__main__":
main()