fast_power.cpp

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#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdlib>
#include <ctime>
#include <iostream>
 
template <typename T>
double fast_power_recursive(T a, T b) {
    // negative power. a^b = 1 / (a^-b)
    if (b < 0)
        return 1.0 / fast_power_recursive(a, -b);
 
    if (b == 0)
        return 1;
    T bottom = fast_power_recursive(a, b >> 1);
    // Since it is integer division b/2 = (b-1)/2 where b is odd.
    // Therefore, case2 is easily solved by integer division.
 
    double result;
    if ((b & 1) == 0)  // case1
        result = bottom * bottom;
    else  // case2
        result = bottom * bottom * a;
    return result;
}
 
template <typename T>
double fast_power_linear(T a, T b) {
    // negative power. a^b = 1 / (a^-b)
    if (b < 0)
        return 1.0 / fast_power_linear(a, -b);
 
    double result = 1;
    while (b) {
        if (b & 1)
            result = result * a;
        a = a * a;
        b = b >> 1;
    }
    return result;
}
 
int main() {
    std::srand(std::time(nullptr));
    std::ios_base::sync_with_stdio(false);
 
    std::cout << "Testing..." << std::endl;
    for (int i = 0; i < 20; i++) {
        int a = std::rand() % 20 - 10;
        int b = std::rand() % 20 - 10;
        std::cout << std::endl << "Calculating " << a << "^" << b << std::endl;
        assert(fast_power_recursive(a, b) == std::pow(a, b));
        assert(fast_power_linear(a, b) == std::pow(a, b));
 
        std::cout << "------ " << a << "^" << b << " = "
                  << fast_power_recursive(a, b) << std::endl;
    }
 
    int64_t a, b;
    std::cin >> a >> b;
 
    std::cout << a << "^" << b << " = " << fast_power_recursive(a, b)
              << std::endl;
 
    std::cout << a << "^" << b << " = " << fast_power_linear(a, b) << std::endl;
 
    return 0;
}