chore: import upstream snapshot with attribution

math/double_factorial.cpp

@@ -0,0 +1,71 @@
+/**
+ * @file
+ * @brief Compute [double
+ * factorial](https://en.wikipedia.org/wiki/Double_factorial): \f$n!!\f$
+ *
+ * Double factorial of a non-negative integer `n`, is defined as the product of
+ * all the integers from 1 to n that have the same parity (odd or even) as n.
+ * <br/>It is also called as semifactorial of a number and is denoted by
+ * \f$n!!\f$
+ */
+
+#include <cassert>
+#include <cstdint>
+#include <iostream>
+
+/** Compute double factorial using iterative method
+ */
+uint64_t double_factorial_iterative(uint64_t n) {
+    uint64_t res = 1;
+    for (uint64_t i = n;; i -= 2) {
+        if (i == 0 || i == 1)
+            return res;
+        res *= i;
+    }
+    return res;
+}
+
+/** Compute double factorial using resursive method.
+ * <br/>Recursion can be costly for large numbers.
+ */
+uint64_t double_factorial_recursive(uint64_t n) {
+    if (n <= 1)
+        return 1;
+    return n * double_factorial_recursive(n - 2);
+}
+
+/** Wrapper to run tests using both recursive and iterative implementations.
+ * The checks are only valid in debug builds due to the use of `assert()`
+ * statements.
+ * \param [in] n number to check double factorial for
+ * \param [in] expected expected result
+ */
+void test(uint64_t n, uint64_t expected) {
+    assert(double_factorial_iterative(n) == expected);
+    assert(double_factorial_recursive(n) == expected);
+}
+
+/**
+ * Test implementations
+ */
+void tests() {
+    std::cout << "Test 1:\t n=5\t...";
+    test(5, 15);
+    std::cout << "passed\n";
+
+    std::cout << "Test 2:\t n=15\t...";
+    test(15, 2027025);
+    std::cout << "passed\n";
+
+    std::cout << "Test 3:\t n=0\t...";
+    test(0, 1);
+    std::cout << "passed\n";
+}
+
+/**
+ * Main function
+ */
+int main() {
+    tests();
+    return 0;
+}