使用 C++23 协程实现第一个 co_yield 同步风格调用接口--Qt计算排列组合

上一篇介绍了 co_await 的例子。与 co_await 类似,在C++23的协程特性里, co_yield 用于从协程执行过程中暂停,并返回值。这个功能乍一听起来很奇怪,网上的例子大多是用一个计数器来演示多次中断协程函数,返回顺序的计数值。这看起来毫无意义。

其实这个功能主要想演示的就是协程 co_yield 具备打断一个函数的执行,并多次返回值的能力。这种能力允许实现一种隐式状态机,每次使用时,返回下一个状态。这对于极为复杂的状态计算来说,是很有用的。它(协程)避免了显式的设置状态记忆句柄,大大简化了实现难度。同时,由于可以任意打断执行,便于在中间获取、展示一些数据状态、甚至单步调试,对构造一些教学程序意义重大。典型的是观察堆排序的中间态,不需要大幅度修改排序算法插入很多的printf,而是在函数外部做。

我们以产生任意P(N,M)、C(N,M)这样的排列、组合数序列为例子,看看传统算法和协程的区别。

1. 回溯法迭代排列组合

传统的回溯法,求取一个排列的算法如下:

cpp 复制代码
void pnm_calc(const int n, const int m)
{
	std::vector<int> vec_buf,vec_bz;
	int swim = 0;
	bool finished = false;
	for (int i=0;i<m;++i)    vec_buf.push_back(0);
	for (int i=0;i<n;++i)    vec_bz.push_back(0);
	do
	{
		int ch = 0;
		if (swim<m)
		{
			while (vec_bz[ch])
				++ch;
			vec_buf[swim] = ch;
			vec_bz[ch] = 1;
			++swim;
		}
		if (swim==m)
		{
			//打印
			for (int i=0;i<m;++i)
				printf("%d,",vec_buf[i]);
			printf("\n");
			bool hit = false;
			do
			{
				if (swim<m && swim >=0) vec_bz[vec_buf[swim]] = 0;
				--swim;
				if (swim>=0)
				{
					int nextv = vec_buf[swim];
					do
					{
						++nextv;
						if (nextv >=n)
							break;
						if (vec_bz[nextv]==0)
							hit = true;
					} while (hit == false);
					if (hit==true)
					{
						vec_bz[vec_buf[swim]] = 0;
						vec_buf[swim] = nextv;
						vec_bz[nextv] = 1;
						++swim;
					}
				}
				else
					finished = true;
			} while (finished == false && hit == false);
		}
	}while(finished == false);
};
int main(int argc, char *argv[])
{
	pnm_calc(4,3);

	return 0;
}

输出:

txt 复制代码
0,1,2,
0,1,3,
0,2,1,
0,2,3,
...
3,1,2,
3,2,0,
3,2,1,

2 传统状态机封装

上述打印显示结果演示的是回溯法本身。若为了更好地使用组合数,需要对算法进行封装,以便于批量的获取、运用组合数的各组结果。比如考虑到总数可能很大,需要分批次返回结果等功能,显著增加了工作量。

cpp 复制代码
#include <vector>
#include <cstdio>
struct tag_NM_State
{
	std::vector<unsigned short> vec_buf;
	std::vector<unsigned short> vec_bz;
	int swim;
	bool finished;
};
/*!
	\brief pnm 快速算法,使用带有记忆效应的 tag_NM_State 记录穷尽进度很好的避免了重新计算的耗时
	\fn pnm
	\param n				N,集合数
	\param m				M, 子集
	\param vec_output		存储结果的集合,本集合会自动增长
	\param state			状态存储
	\param limit			本次最多样本数
	\return int			本次给出的样本数
	*/
int pnm(int n, int m, std::vector<std::vector <unsigned short> > & vec_output,tag_NM_State * state, int limit/* = 0*/)
{
	std::vector<unsigned short> & vec_buf = state->vec_buf,
		& vec_bz = state->vec_bz;
	int &swim = state->swim;
	bool &finished = state->finished;
	const bool firstRun = vec_output.size()?false:true;
	if (vec_bz.size()==0)
	{
		for (int i=0;i<m;++i)    vec_buf.push_back(0);
		for (int i=0;i<n;++i)    vec_bz.push_back(0);
		swim = 0;
		finished = false;
	}
	if (finished==true)
		return 0;
	int group = 0;
	do
	{
		int ch = 0;
		if (swim<m)
		{
			while (vec_bz[ch])
				++ch;
			vec_buf[swim] = ch;
			vec_bz[ch] = 1;
			++swim;
		}
		if (swim==m)
		{
			if (!firstRun)
				memcpy(vec_output[group].data(),vec_buf.data(),m*sizeof(unsigned short));
			else
				vec_output.push_back(vec_buf);
			++group;
			bool hit = false;
			do
			{
				if (swim<m && swim >=0) vec_bz[vec_buf[swim]] = 0;
				--swim;
				if (swim>=0)
				{
					int nextv = vec_buf[swim];
					do
					{
						++nextv;
						if (nextv >=n)
							break;
						if (vec_bz[nextv]==0)
							hit = true;
					} while (hit == false);
					if (hit==true)
					{
						vec_bz[vec_buf[swim]] = 0;
						vec_buf[swim] = nextv;
						vec_bz[nextv] = 1;
						++swim;
					}
				}
				else
					finished = true;
			} while (finished == false && hit == false);
			if (group>=limit && limit>0)
				break;
		}
	}while(finished == false);
	return group;
}

int main(int argc, char *argv[])
{
	QCoreApplication a(argc, argv);
	using std::vector;
	tag_NM_State state;
	const int n = 4, m = 3, group = 10;
	vector<vector<unsigned short> > result;
	int ret = pnm(n,m,result,&state,group);
	while (ret>0)
	{
		printf("\ngroup contains %d results:\n",ret);
		for (int i=0;i<ret;++i)
		{
			printf("\n\t");
			for (int j=0;j<m;++j)
				printf("%d ",result[i][j]);
		}
		ret = pnm(n,m,result,&state,group);
	}
	printf("\nFinished\n");
	return 0;
}

分批输出:

txt 复制代码
group contains 10 results:

        0 1 2
        0 1 3
        0 2 1
        0 2 3
        0 3 1
        0 3 2
        1 0 2
        1 0 3
        1 2 0
        1 2 3
group contains 10 results:

        1 3 0
        1 3 2
        2 0 1
        2 0 3
        2 1 0
        2 1 3
        2 3 0
        2 3 1
        3 0 1
        3 0 2
group contains 4 results:

        3 1 0
        3 1 2
        3 2 0
        3 2 1
Finished

详细算法参考 https://goldenhawking.blog.csdn.net/article/details/80037669

3. 协程封装

使用C++23 协程后,使用变得非常简洁:

cpp 复制代码
int main(int argc, char *argv[])
{
	const int n = 4 , m = 3;
	nmCalc pnm = pnm_calc(n,m);
	while (pnm.next())
	{
		const int * res = pnm.currResult();
		printf("\n\t");
		for (int j=0;j<m;++j)
			printf("%d ",res[j]);
	}
}

每次调用 pnm.next() 就返回下一组结果且无需记忆状态。

但这也是有代价的!为了达到上述的效果,协程封装如下:

cpp 复制代码
#ifndef NMCALC_H
#define NMCALC_H
#include<coroutine>
#include<vector>
class nmCalc
{
public:
	struct promise_type {
		//记录本次排列组合的结果
		const int * m_currResult;
		auto get_return_object() { return nmCalc{ handle::from_promise(*this) }; }
		auto initial_suspend() { return std::suspend_always{}; }
		auto final_suspend() noexcept { return std::suspend_always{}; }
		void unhandled_exception() { return ;}
		void return_void(){}
		auto yield_value(const int *  result ) {this->m_currResult=result; return std::suspend_always{}; }
	};
	using handle = std::coroutine_handle<promise_type>;
private:
	handle hCoroutine;
	nmCalc(handle handle) :hCoroutine(handle) {}
public:
	nmCalc(nmCalc&& other)noexcept :hCoroutine(other.hCoroutine) { other.hCoroutine = nullptr; }
	~nmCalc() { if (hCoroutine) hCoroutine.destroy(); }
	//请求下一组结果,调用后 co_yield继续。
	bool next() const { return hCoroutine && (hCoroutine.resume(), !hCoroutine.done()); }
	const int *  currResult() const { return hCoroutine.promise().m_currResult; }
};

nmCalc pnm_calc(const int n, const int m)
{
	std::vector<int> vec_buf,vec_bz;
	int swim = 0;
	bool finished = false;
	for (int i=0;i<m;++i)    vec_buf.push_back(0);
	for (int i=0;i<n;++i)    vec_bz.push_back(0);
	do
	{
		int ch = 0;
		if (swim<m)
		{
			while (vec_bz[ch])
				++ch;
			vec_buf[swim] = ch;
			vec_bz[ch] = 1;
			++swim;
		}
		if (swim==m)
		{
			//返回一组结果!!!!!
			co_yield vec_buf.data();
			bool hit = false;
			do
			{
				if (swim<m && swim >=0) vec_bz[vec_buf[swim]] = 0;
				--swim;
				if (swim>=0)
				{
					int nextv = vec_buf[swim];
					do
					{
						++nextv;
						if (nextv >=n)
							break;
						if (vec_bz[nextv]==0)
							hit = true;
					} while (hit == false);
					if (hit==true)
					{
						vec_bz[vec_buf[swim]] = 0;
						vec_buf[swim] = nextv;
						vec_bz[nextv] = 1;
						++swim;
					}
				}
				else
					finished = true;
			} while (finished == false && hit == false);
		}
	}while(finished == false);
};

4. 体会与思考

这种封装方式,显著提高了算法流程的紧凑程度。无需考虑如何巧妙的保留状态,而是直接借助协程随时打断并返回。

这在算法极其复杂的情况下,尤其有效。同时,对于单步演示,比如按一下按钮出一次,也很方便,主要代码参考:

https://gitcode.net/coloreaglestdio/qtcpp_demo/-/tree/master/qt_coro_test

运行效果:

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