feat: Addressed comments for adding modular division algorithm

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shubhamamsa 2021-01-18 02:13:32 +05:30
parent 0450f6a179
commit 2ad5420a7c

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@ -1,7 +1,7 @@
/** /**
* @file * @file
* @brief Division of two numbers under modulo * @brief An algorithm to divide two numbers under modulo p [Modular
* * Division](https://www.geeksforgeeks.org/modular-division)
* @details To calculate division of two numbers under modulo p * @details To calculate division of two numbers under modulo p
* Modulo operator is not distributive under division, therefore * Modulo operator is not distributive under division, therefore
* we first have to calculate the inverse of divisor using * we first have to calculate the inverse of divisor using
@ -15,64 +15,67 @@
* NOTE: For the existence of inverse of 'b', 'b' and 'p' must be coprime * NOTE: For the existence of inverse of 'b', 'b' and 'p' must be coprime
* For simplicity we take p as prime * For simplicity we take p as prime
* Time Complexity: O(log(b)) * Time Complexity: O(log(b))
*
* Example: ( 24 / 3 ) % 5 => 8 % 5 = 3 --- (i) * Example: ( 24 / 3 ) % 5 => 8 % 5 = 3 --- (i)
Now the inverse of 3 is 2 Now the inverse of 3 is 2
(24 * 2) % 5 = (24 % 5) * (2 % 5) = (4 * 2) % 5 = 3 --- (ii) (24 * 2) % 5 = (24 % 5) * (2 % 5) = (4 * 2) % 5 = 3 --- (ii)
(i) and (ii) are equal hence the answer is correct. (i) and (ii) are equal hence the answer is correct.
*
* @see modular_inverse_fermat_little_theorem.cpp, modular_exponentiation.cpp * @see modular_inverse_fermat_little_theorem.cpp, modular_exponentiation.cpp
*
* @author [Shubham Yadav](https://github.com/shubhamamsa) * @author [Shubham Yadav](https://github.com/shubhamamsa)
*/ */
#include <cassert> #include <cassert> /// for assert
#include <iostream> #include <iostream> /// for IO operations
/** /**
* @namespace math * @namespace math
* @brief Mathematical algorithms
*/ */
namespace math { namespace math {
/** /**
* @brief This function calculates a raised to exponent b under modulo c using * @namespace modular_division
* modular exponentiation. * @brief Functions for Modular Division implementation
* @param a integer base
* @param b unsigned integer exponent
* @param c integer modulo
* @return a raised to power b modulo c
*/ */
int power(int a, int b, int c) { namespace modular_division {
int ans = 1; /// Initialize the answer to be returned /**
a = a % c; /// Update a if it is more than or equal to c * @brief This function calculates a raised to exponent b under modulo c using
if (a == 0) { * modular exponentiation.
return 0; /// In case a is divisible by c; * @param a integer base
} * @param b unsigned integer exponent
while (b > 0) { * @param c integer modulo
/// If b is odd, multiply a with answer * @return a raised to power b modulo c
if (b & 1) { */
ans = ((ans % c) * (a % c)) % c; uint64_t power(uint64_t a, uint64_t b, uint64_t c) {
} uint64_t ans = 1; /// Initialize the answer to be returned
/// b must be even now a = a % c; /// Update a if it is more than or equal to c
b = b >> 1; /// b = b/2 if (a == 0) {
a = ((a % c) * (a % c)) % c; return 0; /// In case a is divisible by c;
} }
return ans; while (b > 0) {
} /// If b is odd, multiply a with answer
if (b & 1) {
ans = ((ans % c) * (a % c)) % c;
}
/// b must be even now
b = b >> 1; /// b = b/2
a = ((a % c) * (a % c)) % c;
}
return ans;
}
/** /**
* @brief This function calculates modular division * @brief This function calculates modular division
* @param a integer dividend * @param a integer dividend
* @param b integer divisor * @param b integer divisor
* @param p integer modulo * @param p integer modulo
* @return a/b modulo c * @return a/b modulo c
*/ */
int mod_division(int a, int b, int p) { uint64_t mod_division(uint64_t a, uint64_t b, uint64_t p) {
int inverse = power(b, p-2, p)%p; /// Calculate the inverse of b uint64_t inverse = power(b, p-2, p)%p; /// Calculate the inverse of b
int result = ((a%p)*(inverse%p))%p; /// Calculate the final result uint64_t result = ((a%p)*(inverse%p))%p; /// Calculate the final result
return result; return result;
} }
} } // namespace modular_division
} // namespace math
/** /**
* Function for testing power function. * Function for testing power function.
@ -80,19 +83,19 @@ namespace math {
* @returns `void` * @returns `void`
*/ */
static void test() { static void test() {
int test_case_1 = math::mod_division(8, 2, 2); uint64_t test_case_1 = math::modular_division::mod_division(8, 2, 2);
assert(test_case_1 == 0); assert(test_case_1 == 0);
std::cout << "Test 1 Passed!" << std::endl; std::cout << "Test 1 Passed!" << std::endl;
int test_case_2 = math::mod_division(15, 3, 7); uint64_t test_case_2 = math::modular_division::mod_division(15, 3, 7);
assert(test_case_2 == 5); assert(test_case_2 == 5);
std::cout << "Test 2 Passed!" << std::endl; std::cout << "Test 2 Passed!" << std::endl;
int test_case_3 = math::mod_division(10, 5, 2); uint64_t test_case_3 = math::modular_division::mod_division(10, 5, 2);
assert(test_case_3 == 0); assert(test_case_3 == 0);
std::cout << "Test 3 Passed!" << std::endl; std::cout << "Test 3 Passed!" << std::endl;
int test_case_4 = math::mod_division(81, 3, 5); uint64_t test_case_4 = math::modular_division::mod_division(81, 3, 5);
assert(test_case_4 == 2); assert(test_case_4 == 2);
std::cout << "Test 4 Passed!" << std::endl; std::cout << "Test 4 Passed!" << std::endl;
int test_case_5 = math::mod_division(12848, 73, 29); uint64_t test_case_5 = math::modular_division::mod_division(12848, 73, 29);
assert(test_case_5 == 2); assert(test_case_5 == 2);
std::cout << "Test 5 Passed!" << std::endl; std::cout << "Test 5 Passed!" << std::endl;
} }