排序分为内排序和外排序，内排序是在内存中的排序。外排序指在磁盘中文件的排序，因为在磁盘中，不能进行下标访问，归并排序经常用于磁盘中文件的排序

假如有10亿个整形数据在磁盘中，要对它排序，内存中只有1G空间，10亿需要4G。可以将这个文件划分成好几块1G大小的排序，再对这些1G文件两两归并，得到4G的排序后文件

方便演示外排序，手动生成100的数据量，命名为data.txt文件每10个数据读到内存中一排序，可以用快速排序，排完序写到新文件，sub+编号.txt，不断循环对这些小文件归并汇总并生成新文件

#define _CRT_SECURE_NO_WARNINGS 1
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void Swap(int* a, int* b)
{
	int temp = *a;
	*a = *b;
	*b = temp;
}

//三数取中
int GetIndex(int ary[], int left, int right)
{
	//int mid = (left + right) / 2;
	int mid = left + rand() % (right - left);

	if (ary[mid] > ary[left])
	{
		if (ary[mid] < ary[right])
		{
			return mid;
		}
		else if (ary[left] > ary[right])
		{
			return left;
		}
		else
		{
			return right;
		}
	}
	//ary[mid] < ary[left]
	else
	{
		if (ary[mid] > ary[right])
		{
			return mid;
		}
		else if (ary[left] > ary[right])
		{
			return right;
		}
		else
		{
			return left;
		}
	}
}

int QSplit(int ary[], int left, int right)
{
	//三数取中
	int mid = GetIndex(ary, left, right);
	Swap(&ary[left], &ary[mid]);

	int keyi = left;
	while (left < right)
	{
		//右边找小
		while (left < right && ary[right] >= ary[keyi])
		{
			right--;
		}
		//左边找大
		while (left < right && ary[left] <= ary[keyi])
		{
			left++;
		}

		Swap(&ary[left], &ary[right]);
	}

	Swap(&ary[left], &ary[keyi]);

	return left;
}

//升序排列 区间[left,right]
void Qsort(int ary[], int left, int right)
{
	//区间错误,返回
	if (left >= right)
	{
		return;
	}


	int keyi = QSplit(ary, left, right);
	//递归左右区间,keyi处除外
	Qsort(ary, left, keyi - 1);
	Qsort(ary, keyi + 1, right);

}

void MergeFile(const char* file1, const char* file2, const char* mfile)
{
	FILE* fout1;
	fout1 = fopen(file1, "r");

	FILE* fout2;
	fout2 = fopen(file2, "r");

	FILE* fin;
	fin = fopen(mfile, "w");

	int num1 = 0;
	int num2 = 0;
	int ret1 = fscanf(fout1, "%d", &num1);
	int ret2 = fscanf(fout2, "%d", &num2);

	while (ret1 != EOF && ret2 != EOF)
	{
		if (num1 <= num2)
		{
			fprintf(fin, "%d\n", num1);
			ret1 = fscanf(fout1, "%d", &num1);
		}
		else
		{
			fprintf(fin, "%d\n", num2);
			ret2 = fscanf(fout2, "%d", &num2);
		}
	}

	
	while (ret1 != EOF)
	{
		fprintf(fin, "%d\n", num1);
		ret1 = fscanf(fout1, "%d", &num1);
	}

	while (ret2 != EOF)
	{
		fprintf(fin, "%d\n", num2);
		ret2 = fscanf(fout2, "%d", &num2);

	}
	fclose(fout1);
	fclose(fout2);
	fclose(fin);
}

int main()
{
	FILE* fdata;
	fdata = fopen("data.txt", "r");

	//分割一段段数据,内存排序写到小文件
	int ary[10];
	int n = 10; //每10个排序
	int num = 0; //临时读取内容
	char subfile[20]; //小文件名字
	int filei = 1;  //小文件编号
	int i = 0;

	while (fscanf(fdata, "%d", &num) != EOF)
	{
		if (i < n)
		{
			ary[i++] = num;
		}
		//排序
		if (i == 10)
		{
			Qsort(ary, 0, n - 1);
			sprintf(subfile, "sub%d.txt", filei++);
			FILE* fsub;
			fsub = fopen(subfile, "w");

			for (int i = 0; i < n; i++)
			{
				fprintf(fsub, "%d\n", ary[i]);
			}
			fclose(fsub);

			i = 0;
		}
		
	}

	//互相归并到文件,实现整体有序
	char file1[100] = "sub1.txt";
	char file2[100] = "sub2.txt";
	char mfile[100] = "merge.txt";

	for (int i = 2; i <= filei - 1; i++)
	{
		//读取file1和file2,归并出mfile
		MergeFile(file1, file2, mfile);

		remove(file1);
		remove(file2);

		strcpy(file1, mfile);
		sprintf(file2, "sub%d.txt", i + 1);
		sprintf(mfile, "merge%d.txt",i + 1);
	}
	printf("hello world\r\n");
	return 0;
}