decimal.hpp

#include <string>

#if defined(_MSC_VER) && _MSC_VER < 1920
#include <stdlib.h>
#define strtoll _strtoi64
#endif

namespace Traits{
	template<int digits>
	struct ScaleTraits
	{
		static const int Scale = ScaleTraits<digits-1>::Scale * 10;
	};

	template<>
	struct ScaleTraits<0>
	{
		static const int Scale = 1;
	};
}

template<int digits>
class decimal_templated
{
	long long fixed_float;

	static const int Scale = Traits::ScaleTraits<digits>::Scale;

private:
	void from_string(const std::string& string_reprent)
	{
		auto dot_pos = string_reprent.find('.');
		if (dot_pos == std::string::npos)
		{
			fixed_float = Scale * std::strtoll(string_reprent.c_str(), nullptr, 10);
		}
		else
		{
			std::string int_part = string_reprent.substr(0, dot_pos);
			std::string xiaoshu_part = string_reprent.substr( dot_pos + 1 );
			fixed_float = Scale * std::strtoll(int_part.c_str(), nullptr, 10);

			// 补齐 tailing 0
			if (xiaoshu_part.length() < digits)
				xiaoshu_part.insert(xiaoshu_part.end(), digits - xiaoshu_part.length(), '0');
			fixed_float += std::strtoll(xiaoshu_part.c_str(), nullptr, 10);
		}
	}

public:
	decimal_templated() : fixed_float(0) {}
	decimal_templated(int i) : fixed_float(i * Scale) {}
	decimal_templated(unsigned i) : fixed_float(i * Scale) {}
	decimal_templated(long i) : fixed_float(i * Scale) {}
	decimal_templated(unsigned long i) : fixed_float(i * Scale) {}
	decimal_templated(float i) : fixed_float( i * Scale + 0.5) {};
	decimal_templated(double i) : fixed_float( i * Scale + 0.5) {};
	decimal_templated(long double i) : fixed_float( i * Scale + 0.5) {};

	decimal_templated(const std::string& string_reprent) { from_string(string_reprent); }

	decimal_templated(char string_reprent[]){ from_string(string_reprent); }

public: // io
	operator std::string() const // as string
	{
		auto fixed_float_ = fixed_float;
		if (fixed_float_ >= 0)
		{
			auto int_part = std::to_string( (fixed_float_ / Scale) );

			if (fixed_float_ % Scale != 0)
			{
				std::string xiaoshu_part = std::to_string( (fixed_float_ % Scale) );
				if (xiaoshu_part.length() < digits)
					xiaoshu_part.insert(xiaoshu_part.begin(), digits - xiaoshu_part.length(), '0');
				// remove tailing 0
				while(*xiaoshu_part.rbegin() == '0')
					xiaoshu_part.pop_back();

				return int_part + "." + xiaoshu_part;
			}
			return int_part;
		}
		else
		{
			fixed_float_ = - fixed_float;
			auto int_part = std::to_string( (fixed_float_ / Scale) );

			if (fixed_float_ % Scale != 0)
			{
				std::string xiaoshu_part = std::to_string( (fixed_float_ % Scale) );
				if (xiaoshu_part.length() < digits)
					xiaoshu_part.insert(xiaoshu_part.begin(), digits - xiaoshu_part.length(), '0');
				// remove tailing 0
				while(*xiaoshu_part.rbegin() == '0')
					xiaoshu_part.pop_back();

				return "-" + int_part + "." + xiaoshu_part;
			}
			return "-" + int_part;
		}
	}

public: // operations
	decimal_templated<digits> operator + (const decimal_templated<digits>& b)
	{
		decimal_templated<digits> r;
		r.fixed_float = fixed_float + b.fixed_float;
		return r;
	}
	decimal_templated<digits> operator - (const decimal_templated<digits>& b)
	{
		decimal_templated<digits> r;
		r.fixed_float = fixed_float - b.fixed_float;
		return r;
	}

	decimal_templated<digits> operator * (const decimal_templated<digits>& b)
	{
		decimal_templated<digits> r;
		r.fixed_float = fixed_float * b.fixed_float / decimal_templated<digits>::Scale;
		return r;
	}
	decimal_templated<digits> operator / (const decimal_templated<digits>& b)
	{
		decimal_templated<digits> r;
		r.fixed_float = fixed_float * decimal_templated<digits>::Scale /  b.fixed_float;
		return r;
	}

	decimal_templated<digits> operator % (const decimal_templated<digits>& b)
	{
		decimal_templated<digits> r;
		r.fixed_float = fixed_float % b.fixed_float;
		return r;
	}

	decimal_templated& operator+= (const decimal_templated& o){ fixed_float += o.fixed_float; return *this; }
	decimal_templated& operator-= (const decimal_templated& o){ fixed_float -= o.fixed_float; return *this; }

	decimal_templated& operator*= (const decimal_templated& o){ fixed_float = fixed_float * o.fixed_float / Scale; return *this; }

	decimal_templated& operator/= (const decimal_templated& o){ fixed_float = fixed_float * Scale / fixed_float; return *this; }

	decimal_templated& operator%= (const decimal_templated& o){ fixed_float = fixed_float % fixed_float; return *this; }
};

typedef decimal_templated<4> decimal;

#include <iostream>

int main(int argc, char **argv)
{
	decimal a = std::string("2.1");

	decimal b = 4;

	auto c = a * b;
	auto d = a + b;
	auto e = a - b;
	auto f = b / a;

	std::cerr << (std::string) c << std::endl;
	std::cerr << (std::string) d << std::endl;
	std::cerr << (std::string) e << std::endl;
	std::cerr << (std::string) f << std::endl;

    return 0;
}