@@ -1,21 +1,23 @@
# Modular Division :
# An efficient algorithm for dividing b by a modulo n.
from typing import Tuple
# GCD ( Greatest Common Divisor ) or HCF ( Highest Common Factor )
# Given three integers a, b, and n, such that gcd(a,n)=1 and n>1, the algorithm should
# return an integer x such that 0≤x≤n−1, and b/a=x(modn) (that is, b=ax(modn)).
def modular_division (a : int , b : int , n : int ) -> int :
"""
Modular Division :
An efficient algorithm for dividing b by a modulo n.
# Theorem:
# a has a multiplicative inverse modulo n iff gcd(a,n) = 1
GCD ( Greatest Common Divisor ) or HCF ( Highest Common Factor )
Given three integers a, b, and n, such that gcd(a,n)=1 and n>1, the algorithm should
return an integer x such that 0≤x≤n−1, and b/a=x(modn) (that is, b=ax(modn)).
# This find x = b*a^(-1) mod n
# Uses ExtendedEuclid to find the inverse of a
Theorem:
a has a multiplicative inverse modulo n iff gcd(a,n) = 1
def modular_division (a : int , b : int , n : int ) -> int :
"""
This find x = b*a^(-1) mod n
Uses ExtendedEuclid to find the inverse of a
>>> modular_division(4,8,5)
2
@@ -32,9 +34,10 @@ def modular_division(a: int, b: int, n: int) -> int:
return x
# This function find the inverses of a i.e., a^(-1)
def invert_modulo (a : int , n : int ) -> int :
"""
This function find the inverses of a i.e., a^(-1)
>>> invert_modulo(2, 5)
3
@@ -50,9 +53,11 @@ def invert_modulo(a: int, n: int) -> int:
# ------------------ Finding Modular division using invert_modulo -------------------
# This function used the above inversion of a to find x = (b*a^(-1))mod n
def modular_division2 (a : int , b : int , n : int ) -> int :
"""
This function used the above inversion of a to find x = (b*a^(-1))mod n
>>> modular_division2(4,8,5)
2
@@ -68,17 +73,15 @@ def modular_division2(a: int, b: int, n: int) -> int:
return x
# Extended Euclid's Algorithm : If d divides a and b and d = a*x + b*y for integers x
# and y, then d = gcd(a,b)
def extended_gcd (a : int , b : int ) -> (int , int , int ):
def extended_gcd (a : int , b : int ) -> Tuple [int , int , int ]:
"""
>>> extended_gcd(10, 6)
(2, -1, 2)
Extended Euclid's Algorithm : If d divides a and b and d = a*x + b*y for integers x
and y, then d = gcd(a,b)
>>> extended_gcd(10, 6)
(2, -1, 2)
>>> extended_gcd(7, 5)
(1, -2, 3)
>>> extended_gcd(7, 5)
(1, -2, 3)
** extended_gcd function is used when d = gcd(a,b) is required in output
@@ -98,9 +101,9 @@ def extended_gcd(a: int, b: int) -> (int, int, int):
return (d , x , y )
# Extended Euclid
def extended_euclid (a : int , b : int ) -> (int , int ):
def extended_euclid (a : int , b : int ) -> Tuple [int , int ]:
"""
Extended Euclid
>>> extended_euclid(10, 6)
(-1, 2)
@@ -115,12 +118,11 @@ def extended_euclid(a: int, b: int) -> (int, int):
return (y , x - k * y )
# Euclid's Lemma : d divides a and b, if and only if d divides a-b and b
# Euclid's Algorithm
def greatest_common_divisor (a : int , b : int ) -> int :
"""
Euclid's Lemma : d divides a and b, if and only if d divides a-b and b
Euclid's Algorithm
>>> greatest_common_divisor(7,5)
1