08_constexpr.cpp

// 08 - Exact source constants composed with checked compile-time arithmetic.
#include <fixedwide/all.hpp>
#include <cstdio>

using namespace fixedwide;
using Money = Fixed64<4>;

// Arithmetic still returns expected. A failed .value() in a constant expression
// is a compile error; at runtime callers must check results before using them.
constexpr Money apply_rate(Money amount, Money rate) {
    return mul(amount, rate, Rounding::nearest_even).value();
}

constexpr Money hundred = "100";
constexpr Money rate = "0.1075";
constexpr auto amount = apply_rate(hundred, rate);
static_assert(amount == "10.75");
static_assert(apply_rate("100", "0.1075") == amount);
static_assert(add(hundred, hundred).value() == "200");
static_assert(div(hundred, Money{"3"}, Rounding::toward_zero).value() == "33.3333");
static_assert(quantize(Money{"1.2345"}, 2, Rounding::nearest_even).value() == "1.23");
static_assert(remainder(Money{"10"}, Money{"3"}).value() == "1");

// A constant is exact or rejected; there is deliberately no rounding argument.
// constexpr Money bad = "0.00001"; // does not compile
constexpr Money unit = "0.0001";
static_assert(!add(Money::max(), unit).has_value());
static_assert(add(Money::max(), unit).error() == ArithmeticError::overflow);
static_assert(!div(hundred, Money{}).has_value());
static_assert(hundred == Fixed128<8>{"100"});

int main() {
    std::printf("100.0000 * 0.1075 = %s (computed at compile time)\n", to_string(amount).value().c_str());
    std::puts("OK");
    return 0;
}