過于離譜，我實現(xiàn)憋不住了！

大家好，我是小林

在知乎，看到了一個問題：

這就是一個求階乘的問題，大家剛剛開始學(xué)編程的時候應(yīng)該都寫過這樣的程序。

一個求階乘的問題，還能玩出什么樣的花兒來？

我在回答區(qū)看到了一個非常有趣的回答，把各種版本的代碼都舉例了出來，一個比一個還離譜，后面的代碼，甚至讓懷疑是我學(xué)過的代碼嗎？

數(shù)學(xué)家版本：

（簡單利索，深藏功與名）

#include?
#include?
int?main()
{
????std::cout?<std::tgamma(20?+?1)?<std::endl;
}

語言學(xué)家版本：

（語言學(xué)家，你懂得，恨不得把所有語法特性都派上用場）

#include?
#include?

template<std::size_t...I>?constexpr?auto?foo(std::index_sequence)?{?return?((I+1)?*?...);?}

int?main()?
{
????std::cout?<std::make_index_sequence<20>())?<std::endl;
}

歷史學(xué)家版本：

（void main() 有沒有嗅到濃厚的歷史氣息？）

#include?
void?main(void)?{
????int?i;
????long?long?j;
????for(i?=?1,?j?=?1;i?<=?20;?j?*=?i++);
????????printf("%lld",?j);?
}

敏捷開發(fā)上線1.0版本：

（可以說是非常敏捷了）

#include?
int?main()?{
????//printf("%d",?1*2*3*4*5*6*7*8*9*10);
????printf("%lld",?(long?long)1*2*3*4*5*6*7*8*9*10*11*12*13*14*15*16*17*18*19*20);
}

面向?qū)ο髮＜野姹荆?/span>

（好家伙，一個簡單的問題，愣是祭出了接口、繼承、虛函數(shù)、虛繼承、智能指針等一大堆東西出來，這很面向?qū)ο螅?/p>

#include?
#include?
#include?

struct?IBaseInterface?{
????virtual?~IBaseInterface()?=?0;
};
inline?IBaseInterface::~IBaseInterface()?=?default;

struct?IDataProvider?:?virtual?public?IBaseInterface?{
????virtual?int?first()?=?0;
????virtual?int?last()?=?0;
????virtual?int?next(int?v)?=?0;
};

struct?ICalculator?:?virtual?public?IBaseInterface?{
????virtual?long?long?calc(IDataProvider?*)?=?0;
};

struct?IPrinter?:?virtual?public?IBaseInterface?{
????virtual?void?print(const?std::string?&)?=?0;
};

struct?ISerializer?:?virtual?public?IBaseInterface?{
????virtual?std::string?serialize(long?long?value)?=?0;
};

struct?IRunnable?:?virtual?public?IBaseInterface?{
????virtual?void?run()?=?0;
};

class?Foo?:?virtual?public?IRunnable?{
????std::shared_ptr?m_dp;
????std::shared_ptr?m_c;
????std::shared_ptr?m_s;
????std::shared_ptr?m_p;
public:
????Foo(std::shared_ptr?dp,?std::shared_ptr?c,?std::shared_ptr?s,?std::shared_ptr?p)?:?m_dp(std::move(dp)),?m_c(std::move(c)),?m_s(std::move(s)),m_p(std::move(p))?{}
????void?run()?override?{?return??m_p->print(m_s->serialize(m_c->calc(m_dp.get())));?}
};

class?DefaultSerializer?:?virtual?public?ISerializer?{
public:
????std::string?serialize(long?long?value)?override?{?return?std::to_string(value);?}
};

class?StreamPrinter?:?virtual?public?IPrinter?{
????std::ostream?&m_os;
public:
????explicit?StreamPrinter?(std::ostream?&os)?:?m_os(os)?{}
????void?print(const?std::string?&s)??override?{?m_os?<};

class?MultiplyAccumulateCalculator?:?virtual?public?ICalculator?{
public:
????long?long?calc(IDataProvider?*dp)?override?{
????????int?i?=?dp->first();
????????long?long?j?=?i;
????????do
????????????j?*=?(i?=?dp->next(i));
????????while(i?!=?dp->last());
????????return?j;
????}
};

int?main()?{
????struct?MyDataProvider?:?virtual?public?IDataProvider?{
????????int?first()?override?{?return?1;?}
????????int?last()?override?{?return?20;?}
????????int?next(int?v)?override?{?return?v+1;?}
????};
????Foo?foo(std::make_shared(),?std::make_shared(),?std::make_shared(),?std::make_shared(std::cout));
????foo.run();
}

提前優(yōu)化的并行版本：

（一看就是精通底層技術(shù)的大佬，把CPU拿捏得死死的）

#include?
#include?

double?foo(int?x)?{
????__m128?a?=?{1.0f,?2.0f,?3.0f,?4.0f};
????__m128?b?=?{4.0f,?4.0f,?4.0f,?4.0f};
????__m128?c?=?{1.0f,?1.0f,?1.0f,?1.0f};
????for(int?i?=?0;?i?4;?++i,?a?=?_mm_add_ps(a,?b))
????????c?=?_mm_mul_ps(c,?a);
????for(int?i?=?x?%?4;?i?4;?++i)
????????a[i]?=?1.0f;
????c?=?_mm_mul_ps(c,?a);
????return?(double)c[0]?*?(double)c[1]?*?(double)c[2]?*?(double)c[3];
}

int?main()?{
????std::cout?<20)?<std::endl;
}

黑魔法版本：

（能看懂這段代碼的，都不是普通人！）

#include?
#include?
#include?
#include?
int?main()?{
?std::vector<int>?v(std::atoi(std::end(__DATE__)?-?(__LINE__)?/?2)?-?1);?//?2021年，第六行
?std::iota(v.begin(),?v.end(),?1);
?std::cout?<std::accumulate(v.begin(),?v.end(),?1ull,?std::multiplies<>())?<std::endl;
}

“宏孩兒”元編程版：

（當(dāng)年看各種C++框架中，排山倒海一樣的宏定義，簡直令人發(fā)指）

#include?

//?由于boost.preprocessor僅提供255以下的整數(shù)運(yùn)算
//?所以使用sequence來?(十位個位)(千位百位)(十萬位萬位)?的方式來表示大整數(shù)。

//?不進(jìn)位加法：(77)(66)(55)?+?(44)(33)(22)?=?(121)(99)(77)
#define?PP_ADD_N_N_CARRY_OP(R,?DATA,?I,?ELEM)?(BOOST_PP_ADD(BOOST_PP_SEQ_ELEM(I,?DATA),?ELEM))
#define?PP_ADD_N_N_CARRY(SEQ_A,?SEQ_B)?BOOST_PP_SEQ_FOR_EACH_I(PP_ADD_N_N_CARRY_OP,?SEQ_A,?SEQ_B)

//?進(jìn)位加法：(121)(99)(77)?=?（21)(0)(78)
//?注意SEQ_A的長度要比SEQ_B長
#define?PP_ADD_N_N_OP(S,?STATE,?ELEM_CARRY)?\
????BOOST_PP_SEQ_PUSH_FRONT(?\
????????????BOOST_PP_SEQ_REPLACE(STATE,?0,?BOOST_PP_MOD(BOOST_PP_ADD(BOOST_PP_SEQ_HEAD(STATE),?ELEM_CARRY),?100)),?\
????????????BOOST_PP_DIV(BOOST_PP_ADD(BOOST_PP_SEQ_HEAD(STATE),?ELEM_CARRY),?100)?\
????????????)
#define?PP_ADD_N_N(SEQ_A,?SEQ_B)?BOOST_PP_SEQ_REVERSE(BOOST_PP_SEQ_FOLD_LEFT(PP_ADD_N_N_OP,?BOOST_PP_SEQ_NIL(0),?PP_ADD_N_N_CARRY(SEQ_A,?SEQ_B)))

//?沒什么好說的，X*N?=?X+X+X+X+X+...+X
#define?PP_MUL_N_1_EXP_OP(Z,?I,?DATA)?(DATA)
#define?PP_MUL_N_1_EXP(SEQ_N,?N)?BOOST_PP_REPEAT(N,?PP_MUL_N_1_EXP_OP,?SEQ_N)
#define?PP_MUL_N_1_MYOP(S,?STATE,?ITEM)?PP_ADD_N_N(STATE,?ITEM)
#define?PP_MUL_N_1_FWD(EXP)?BOOST_PP_SEQ_FOLD_LEFT(PP_MUL_N_1_MYOP,?BOOST_PP_SEQ_HEAD(EXP),?BOOST_PP_SEQ_TAIL(EXP))
#define?PP_MUL_N_1(SEQ_N,?N)?PP_MUL_N_1_FWD(PP_MUL_N_1_EXP(SEQ_N,?N))

#define?FACT5?PP_MUL_N_1(PP_MUL_N_1(PP_MUL_N_1(PP_MUL_N_1((1),?2),?3),?4),?5)
#define?FACT10?PP_MUL_N_1(PP_MUL_N_1(PP_MUL_N_1(PP_MUL_N_1(PP_MUL_N_1(FACT5,?6),?7),?8),?9),?10)
#define?FACT15?PP_MUL_N_1(PP_MUL_N_1(PP_MUL_N_1(PP_MUL_N_1(PP_MUL_N_1(FACT10,?11),?12),?13),?14),?15)
#define?FACT20?PP_MUL_N_1(PP_MUL_N_1(PP_MUL_N_1(PP_MUL_N_1(PP_MUL_N_1(FACT15,?16),?17),?18),?19),?20)
#define?FACT25?PP_MUL_N_1(PP_MUL_N_1(PP_MUL_N_1(PP_MUL_N_1(PP_MUL_N_1(FACT20,?21),?22),?23),?24),?25)

static_assert(false,?BOOST_PP_STRINGIZE(FACT10));

真·模板元編程版本

（泛型編程，碼不驚人死不休）

#include?
#include?
#include?

using?BaseType_t?=?long?long;
constexpr?BaseType_t?lgBase?=?9;?//?注意10000*10000剛剛好小于int的取值范圍
constexpr?BaseType_t?Base?=?1000000000;?//?注意10000*10000剛剛好小于int的取值范圍

//?大整數(shù)的表示
template?struct?BigInteger?{
????using?type?=?BigInteger;
};

//?連接
template<class?T1,?class?T2>?struct?BI_Cat;
template?struct?BI_Cat?,?BigInteger>?:?BigInteger?{};

//?左移一個單元（即*Base）
template<class?T>?struct?BI_SHL;
template?struct?BI_SHL>?:?BigInteger0>?{};

//?去除開頭的0
template<class?T>?struct?BI_Remove_Zeros?:?T?{};
template?struct?BI_Remove_Zeros>?:?BI_Remove_Zeros>?{};

//?填充0到N個單元
template<int?X,?class?IS>?struct?BI_Fill_Impl;
template<int?X,?class?T,?T...I>?struct?BI_Fill_Impl>?:?BigInteger<(I,?X)...>?{};
template<int?Size>?struct?BI_Fill_Zeros?:?BI_Fill_Impl<0,?std::make_index_sequence>?{};

template<class?T,?int?N>?struct?BI_Resize;
templateint?N>?struct?BI_Resize,?N>?:?BI_Cat<typename?BI_Fill_Zerossizeof...(I)>::type,?BigInteger>?{};

//?返回較大的數(shù)值
template<int?A,?int?B>?struct?int_min?:?std::integral_constant<int,?(A?{};

//?非進(jìn)位加法：先把兩個數(shù)的位數(shù)改成一樣的然后依次相加
template<class?A,?class?B,?class?ShouldResize>?struct?BI_AddNotCarry_Impl;
template?struct?BI_AddNotCarry_Impl?,?BigInteger,?std::true_type>?:?BigInteger<(I1?+?I2)...>?{};

template?struct?BI_AddNotCarry_Impl?,?BigInteger,?std::false_type>
?????:?BI_AddNotCarry_Impl<
?????????typename?BI_Resize,?int_min<sizeof...(I1),?sizeof...(I2)>::value>::type,
?????????typename?BI_Resize,?int_min<sizeof...(I1),?sizeof...(I2)>::value>::type,
?????????std::true_type
?????>{};

template<class?A,?class?B>?struct?BI_AddNotCarry;
template?struct?BI_AddNotCarry?,?BigInteger>
????:?BI_AddNotCarry_Impl,?BigInteger,?std::bool_constant<sizeof...(I1)?==?sizeof...(I2)>>?{};

//?判斷是否為0
template<class?Y>?struct?BI_IsZero;
template?struct?BI_IsZero>?:?std::bool_constant<((I?==?0)?&&?...)>?{};

//?自動進(jìn)位
template<class?A>?struct?BI_Carry;
template<class?A,?class?B>?struct?BI_Add?:?BI_Carry<typename?BI_AddNotCarry::type>?{};

template<class?Mod,?class?Div,?class?ShouldCalc?=?typename?BI_IsZero
::type>?struct?BI_Carry_Impl;
template<class?Mod,?class?Div>?struct?BI_Carry_Impl?:?Mod?{};
template<class?Mod,?class?Div>?struct?BI_Carry_Impl
????????:?BI_Addtypename?BI_SHL
::type?>?{};
template?struct?BI_Carry>
????????:?BI_Remove_Zeros<typename?BI_Carry_Impl,?BigInteger<(I?/?Base)...>>::type>?{};

//?乘以X并自動進(jìn)位
template<class?A,?int?X>?struct?BI_MulX;
templateint?X>?struct?BI_MulX?,?X>
????????:?BI_Carry>?{};

//?計算階乘
template<int?X>?struct?BI_Fact?:?BI_MulX<typename?BI_Fact-1>::type,?X>?{};
template<>?struct?BI_Fact<0>?:?BigInteger<1>?{};

template
std::ostream?&operator<<(std::ostream?&out,?BigInteger)?{
????return?((out?<std::setfill('0')?<std::setw(lgBase)),?...);
}

int?main()
{
????std::cout?<typename?BI_Fact<20>::type()?<std::endl;
}

原回答：

https://www.zhihu.com/question/365763395/answer/971009059

不得不服，這位老哥真是人才，把 C++ 玩出這么多花樣，不愧是語言學(xué)家。當(dāng)然，現(xiàn)實工作中不會有人這樣寫代碼的，所以大家圖個樂就好，不用較真。

你們在第幾個版本倒下了？

我反正看到面向?qū)ο髮＜野姹揪鸵呀?jīng)忍不住了···