Python/maths/modular_exponential.py

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"""
Modular Exponential.
Modular exponentiation is a type of exponentiation performed over a modulus.
For more explanation, please check https://en.wikipedia.org/wiki/Modular_exponentiation
"""
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"""Calculate Modular Exponential."""
def modular_exponential(base: int, power: int, mod: int):
"""
>>> modular_exponential(5, 0, 10)
1
>>> modular_exponential(2, 8, 7)
4
>>> modular_exponential(3, -2, 9)
-1
"""
if power < 0:
return -1
base %= mod
result = 1
while power > 0:
if power & 1:
result = (result * base) % mod
power = power >> 1
base = (base * base) % mod
return result
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def main():
"""Call Modular Exponential Function."""
print(modular_exponential(3, 200, 13))
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if __name__ == "__main__":
import doctest
doctest.testmod()
main()