feat: added extended Euclidean algorithm (#1238)

* chore: made it so math directory gets built

* feat: added extended Euclidean algorithm

* docs: added details qualifier

Co-authored-by: David Leal <[email protected]>

* docs: added param qualifiers to functions that needed them

* docs: added details qualifier

Co-authored-by: David Leal <[email protected]>

* docs: small cleanup

---------

Co-authored-by: David Leal <[email protected]>

Author: dsmurrow

math/euclidean_algorithm_extended.c

@@ -0,0 +1,154 @@
+/**
+ * @{
+ * @file
+ * @brief Program to perform the [extended Euclidean
+ * algorithm](https://en.wikipedia.org/wiki/Extended_Euclidean_algorithm)
+ *
+ * @details The extended Euclidean algorithm, on top of finding the GCD (greatest common
+ * divisor) of two integers a and b, also finds the values x and y such that
+ * ax+by = gcd(a, b)
+ */
+
+#include <assert.h>  /// for tests
+#include <stdio.h>   /// for IO
+#include <stdlib.h>  /// for div function and corresponding div_t struct
+
+/**
+ * @brief a structure holding the values resulting from the extended Euclidean
+ * algorithm
+ */
+typedef struct euclidean_result
+{
+    int gcd;   ///< the greatest common divisor calculated with the Euclidean
+               ///< algorithm
+    int x, y;  ///< the values x and y such that ax + by = gcd(a, b)
+} euclidean_result_t;
+
+/**
+ * @brief gives queue-like behavior to an array of two ints, pushing an element
+ * onto the end and pushing one off the front
+ *
+ * @param arr an array of ints acting as a queue
+ * @param newval the value being pushed into arr
+ *
+ * @returns void
+ */
+static inline void xy_push(int arr[2], int newval)
+{
+    arr[1] = arr[0];
+    arr[0] = newval;
+}
+
+/**
+ * @brief calculates the value of x or y and push those into the small 'queues'
+ *
+ * @details Both x and y are found by taking their value from 2 iterations ago minus the
+ * product of their value from 1 iteration ago and the most recent quotient.
+ *
+ * @param quotient the quotient from the latest iteration of the Euclidean
+ * algorithm
+ * @param prev the 'queue' holding the values of the two previous iterations
+ *
+ * @returns void
+ */
+static inline void calculate_next_xy(int quotient, int prev[2])
+{
+    int next = prev[1] - (prev[0] * quotient);
+    xy_push(prev, next);
+}
+
+/**
+ * @brief performs the extended Euclidean algorithm on integer inputs a and b
+ *
+ * @param a first integer input
+ * @param b second integer input
+ *
+ * @returns euclidean_result_t containing the gcd, and values x and y such that
+ * ax + by = gcd
+ */
+euclidean_result_t extended_euclidean_algorithm(int a, int b)
+{
+    int previous_remainder = 1;
+    int previous_x_values[2] = {0, 1};
+    int previous_y_values[2] = {1, 0};
+    div_t div_result;
+    euclidean_result_t result;
+
+    /* swap values of a and b */
+    if (abs(a) < abs(b))
+    {
+        a ^= b;
+        b ^= a;
+        a ^= b;
+    }
+
+    div_result.rem = b;
+
+    while (div_result.rem > 0)
+    {
+        div_result = div(a, b);
+
+        previous_remainder = b;
+
+        a = b;
+        b = div_result.rem;
+
+        calculate_next_xy(div_result.quot, previous_x_values);
+        calculate_next_xy(div_result.quot, previous_y_values);
+    }
+
+    result.gcd = previous_remainder;
+    result.x = previous_x_values[1];
+    result.y = previous_y_values[1];
+
+    return result;
+}
+
+/** @} */
+
+/**
+ * @brief perform one single check on the result of the algorithm with provided
+ * parameters and expected output
+ *
+ * @param a first paramater for Euclidean algorithm
+ * @param b second parameter for Euclidean algorithm
+ * @param gcd expected value of result.gcd
+ * @param x expected value of result.x
+ * @param y expected value of result.y
+ *
+ * @returns void
+ */
+static inline void single_test(int a, int b, int gcd, int x, int y)
+{
+    euclidean_result_t result;
+
+    result = extended_euclidean_algorithm(a, b);
+    assert(result.gcd == gcd);
+    assert(result.x == x);
+    assert(result.y == y);
+}
+
+/**
+ * @brief Perform tests on known results
+ * @returns void
+ */
+static void test()
+{
+    single_test(40, 27, 1, -2, 3);
+    single_test(71, 41, 1, -15, 26);
+    single_test(48, 18, 6, -1, 3);
+    single_test(99, 303, 3, -16, 49);
+    single_test(14005, 3507, 1, -305, 1218);
+
+    printf("All tests have successfully passed!\n");
+}
+
+/**
+ * @brief Main Function
+ * @returns 0 upon successful program exit
+ */
+int main()
+{
+    test();  // run self-test implementations
+    return 0;
+}