// d2mcpp: https://github.com/mcpp-community/d2mcpp

// license: Apache-2.0

// reference solution for: dslings/cpp11/07-constexpr-1.cpp

//

// 用途: 仅给 CI 与维护者参考使用，不是教程入口。

// 教程练习入口: dslings/cpp11/07-constexpr-1.cpp

//

import std;

import d2x;

template <int N>

struct Sum {

static constexpr int value = Sum<N - 1>::value + N;

};

template <>

struct Sum<1> { static constexpr int value = 1; };

constexpr int factorial(int n) {

return n <= 1 ? 1 : n * factorial(n - 1);

}

constexpr double pow(double base, int exp) {

return exp == 0 ? 1.0 : base * pow(base, exp - 1);

}

constexpr double mysin(double x) {

#define radius(x) (x * 3.14159265358979323846 / 180.0)

return radius(x)

- pow(radius(x), 3) / factorial(3)

+ pow(radius(x), 5) / factorial(5);

}

int main() {

// 1. 编译期-函数计算

constexpr int fact_10 = factorial(10);

std::cout << "1 * 2 * .. * 10 = " << fact_10 << std::endl;

// 2. 编译期-模板参数计算

constexpr int sum_4 = Sum<4>::value;

std::cout << "1 + 2 + 3 + 4 = " << sum_4 << std::endl;

// 3. 编译期计算示例:

// value是多少时? value! + (1 + 2 + .. + value) > 10000

// value=7: 5040 + 28 = 5068 (不够)

// value=8: 40320 + 36 = 40356 (满足)

constexpr int value = 8;

constexpr int f = factorial(value);

constexpr int s = Sum<value>::value;

constexpr int ans = f + s;

static_assert(ans > 10000, "ans should be > 10000");

// 4. 编译期计算sin值(自动打表) - 时间复杂度O(1)

constexpr double sin30 = mysin(30.0);

std::cout << "mysin(30): " << sin30 << " " << std::endl;

return 0;

}