#### Union of Two Arrays

```/**
* @file
* @brief Implementation for the [Union of two sorted
* Arrays](https://en.wikipedia.org/wiki/Union_(set_theory))
* algorithm.
* @details The Union of two arrays is the collection of all the unique elements
* in the first array, combined with all of the unique elements of a second
* array. This implementation uses ordered arrays, and an algorithm to correctly
* order them and return the result as a new array (vector).
* @see intersection_of_two_arrays.cpp
* @author [Alvin](https://github.com/polarvoid)
*/

#include <algorithm>  /// for std::sort
#include <cassert>    /// for assert
#include <iostream>   /// for IO operations
#include <vector>     /// for std::vector

/**
* @namespace operations_on_datastructures
* @brief Operations on Data Structures
*/
namespace operations_on_datastructures {

/**
* @brief Prints the values of a vector sequentially, ending with a newline
* character.
* @param array Reference to the array to be printed
* @returns void
*/
void print(const std::vector<int32_t> &array) {
for (int32_t i : array) {
std::cout << i << " ";  /// Print each value in the array
}
std::cout << "\n";  /// Print newline
}

/**
* @brief Gets the union of two sorted arrays, and returns them in a
* vector.
* @details An algorithm is used that compares the elements of the two vectors,
* appending the one that has a lower value, and incrementing the index for that
* array. If one of the arrays reaches its end, all the elements of the other
* are appended to the resultant vector.
* @param first A std::vector of sorted integer values
* @param second A std::vector of sorted integer values
* @returns A std::vector of the union of the two arrays, in ascending order
*/
std::vector<int32_t> get_union(const std::vector<int32_t> &first,
const std::vector<int32_t> &second) {
std::vector<int32_t> res;         ///< Vector to hold the union
size_t f_index = 0;               ///< Index for the first array
size_t s_index = 0;               ///< Index for the second array
size_t f_length = first.size();   ///< Length of first array
size_t s_length = second.size();  ///< Length of second array
int32_t next = 0;  ///< Integer to store value of the next element

while (f_index < f_length && s_index < s_length) {
if (first[f_index] < second[s_index]) {
next = first[f_index];  ///< Append from first array
f_index++;              ///< Increment index of second array
} else if (first[f_index] > second[s_index]) {
next = second[s_index];  ///< Append from second array
s_index++;               ///< Increment index of second array
} else {
next = first[f_index];  ///< Element is the same in both
f_index++;              ///< Increment index of first array
s_index++;              ///< Increment index of second array too
}
if ((res.size() == 0) || (next != res.back())) {
res.push_back(next);  ///< Add the element if it is unique
}
}
while (f_index < f_length) {
next = first[f_index];  ///< Add remaining elements
if ((res.size() == 0) || (next != res.back())) {
res.push_back(next);  ///< Add the element if it is unique
}
f_index++;
}
while (s_index < s_length) {
next = second[s_index];  ///< Add remaining elements
if ((res.size() == 0) || (next != res.back())) {
res.push_back(next);  ///< Add the element if it is unique
}
s_index++;
}
return res;
}

}  // namespace operations_on_datastructures

/**
* @namespace tests
* @brief Testcases to check Union of Two Arrays.
*/
namespace tests {
using operations_on_datastructures::get_union;
using operations_on_datastructures::print;
/**
* @brief A Test to check an edge case (two empty arrays)
* @returns void
*/
void test1() {
std::cout << "TEST CASE 1\n";
std::cout << "Intialized a = {} b = {}\n";
std::cout << "Expected result: {}\n";
std::vector<int32_t> a = {};
std::vector<int32_t> b = {};
std::vector<int32_t> result = get_union(a, b);
assert(result == a);  ///< Check if result is empty
print(result);        ///< Should only print newline
std::cout << "TEST PASSED!\n\n";
}
/**
* @brief A Test to check an edge case (one empty array)
* @returns void
*/
void test2() {
std::cout << "TEST CASE 2\n";
std::cout << "Intialized a = {} b = {2, 3}\n";
std::cout << "Expected result: {2, 3}\n";
std::vector<int32_t> a = {};
std::vector<int32_t> b = {2, 3};
std::vector<int32_t> result = get_union(a, b);
assert(result == b);  ///< Check if result is equal to b
print(result);        ///< Should print 2 3
std::cout << "TEST PASSED!\n\n";
}
/**
* @brief A Test to check correct functionality with a simple test case
* @returns void
*/
void test3() {
std::cout << "TEST CASE 3\n";
std::cout << "Intialized a = {4, 6} b = {2, 3}\n";
std::cout << "Expected result: {2, 3, 4, 6}\n";
std::vector<int32_t> a = {4, 6};
std::vector<int32_t> b = {2, 3};
std::vector<int32_t> result = get_union(a, b);
std::vector<int32_t> expected = {2, 3, 4, 6};
assert(result == expected);  ///< Check if result is correct
print(result);               ///< Should print 2 3 4 6
std::cout << "TEST PASSED!\n\n";
}
/**
* @brief A Test to check correct functionality with duplicate values
* @returns void
*/
void test4() {
std::cout << "TEST CASE 4\n";
std::cout << "Intialized a = {4, 6, 6, 7} b = {2, 3, 4}\n";
std::cout << "Expected result: {2, 3, 4, 6, 7}\n";
std::vector<int32_t> a = {4, 6, 6, 7};
std::vector<int32_t> b = {2, 3, 4};
std::vector<int32_t> result = get_union(a, b);
std::vector<int32_t> expected = {2, 3, 4, 6, 7};
assert(result == expected);  ///< Check if result is correct
print(result);               ///< Should print 2 3 4 6 7
std::cout << "TEST PASSED!\n\n";
}
/**
* @brief A Test to check correct functionality with a harder test case
* @returns void
*/
void test5() {
std::cout << "TEST CASE 5\n";
std::cout << "Intialized a = {1, 4, 6, 7, 9} b = {2, 3, 5}\n";
std::cout << "Expected result: {1, 2, 3, 4, 5, 6, 7, 9}\n";
std::vector<int32_t> a = {1, 4, 6, 7, 9};
std::vector<int32_t> b = {2, 3, 5};
std::vector<int32_t> result = get_union(a, b);
std::vector<int32_t> expected = {1, 2, 3, 4, 5, 6, 7, 9};
assert(result == expected);  ///< Check if result is correct
print(result);               ///< Should print 1 2 3 4 5 6 7 9
std::cout << "TEST PASSED!\n\n";
}
/**
* @brief A Test to check correct functionality with an array sorted using
* std::sort
* @returns void
*/
void test6() {
std::cout << "TEST CASE 6\n";
std::cout << "Intialized a = {1, 3, 3, 2, 5, 9, 4, 3, 2} ";
std::cout << "b = {11, 3, 7, 8, 6}\n";
std::cout << "Expected result: {1, 2, 3, 4, 5, 6, 7, 8, 9, 11}\n";
std::vector<int32_t> a = {1, 3, 3, 2, 5, 9, 4, 3, 2};
std::vector<int32_t> b = {11, 3, 7, 8, 6};
std::sort(a.begin(), a.end());  ///< Sort vector a
std::sort(b.begin(), b.end());  ///< Sort vector b
std::vector<int32_t> result = get_union(a, b);
std::vector<int32_t> expected = {1, 2, 3, 4, 5, 6, 7, 8, 9, 11};
assert(result == expected);  ///< Check if result is correct
print(result);               ///< Should print 1 2 3 4 5 6 7 8 9 11
std::cout << "TEST PASSED!\n\n";
}
}  // namespace tests

/**
* @brief Function to test the correctness of get_union() function
* @returns void
*/
static void test() {
tests::test1();
tests::test2();
tests::test3();
tests::test4();
tests::test5();
tests::test6();
}

/**
* @brief main function
* @returns 0 on exit
*/
int main() {
test();  // run self-test implementations
return 0;
}
```