double_factorial.cpp

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#include <cassert>
#include <cstdint>
#include <iostream>
 
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;
}
 
uint64_t double_factorial_recursive(uint64_t n) {
    if (n <= 1)
        return 1;
    return n * double_factorial_recursive(n - 2);
}
 
void test(uint64_t n, uint64_t expected) {
    assert(double_factorial_iterative(n) == expected);
    assert(double_factorial_recursive(n) == expected);
}
 
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";
}
 
int main() {
    tests();
    return 0;
}