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#ifndef BPT_MEMORYRIVER_HPP
#define BPT_MEMORYRIVER_HPP
#include <fstream>
using std::fstream;
using std::ifstream;
using std::ofstream;
using std::string;
template <class T, int info_len = 2>
class MemoryRiver {
private:
struct Metadata {
bool hasNext : 1;
int next : 31;
};
fstream file;
string file_name;
static constexpr size_t kSzData = std::max(sizeof(T), sizeof(int));
static constexpr size_t kOffsetData = sizeof(int) * (info_len + 1);
static size_t offsetof_ (int id) {
return kOffsetData + kSzData * id;
}
public:
MemoryRiver () = default;
MemoryRiver (const string &file_name) : file_name(file_name) {}
void initialise (string FN = "") {
if (FN != "") file_name = FN;
file.open(file_name, std::ios::out);
int tmp = 0;
for (int i = 0; i <= info_len; ++i) file.write(reinterpret_cast<char *>(&tmp), sizeof(int));
file.close();
}
//读出第n个int的值赋给tmp，1_base
void get_info (int &tmp, int n) {
if (n > info_len) return;
file.open(file_name, std::ios::in);
file.seekg(n * sizeof(tmp));
file.read(reinterpret_cast<char *>(&tmp), sizeof(tmp));
file.close();
}
//将tmp写入第n个int的位置，1_base
void write_info(int tmp, int n) {
if (n > info_len) return;
file.open(file_name);
file.seekp(n * sizeof(tmp));
file.write(reinterpret_cast<char *>(&tmp), sizeof(tmp));
file.close();
}
//在文件合适位置写入类对象t，并返回写入的位置索引index
//位置索引意味着当输入正确的位置索引index，在以下三个函数中都能顺利的找到目标对象进行操作
//位置索引index可以取为对象写入的起始位置
int write(T &t) {
file.open(file_name);
Metadata meta;
file.seekg(0);
file.read(reinterpret_cast<char *>(&meta), sizeof(int));
int id = meta.next;
if (meta.hasNext) {
file.seekg(offsetof_(id));
Metadata nextMeta;
file.read(reinterpret_cast<char *>(&nextMeta), sizeof(int));
file.seekp(0);
file.write(reinterpret_cast<char *>(&nextMeta), sizeof(int));
} else {
Metadata nextMeta = meta;
++nextMeta.next;
file.seekp(0);
file.write(reinterpret_cast<char *>(&nextMeta), sizeof(int));
}
file.seekp(offsetof_(id));
file.write(reinterpret_cast<char *>(&t), sizeof(t));
file.close();
return id;
}
//用t的值更新位置索引index对应的对象，保证调用的index都是由write函数产生
void update(T &t, const int id) {
file.open(file_name);
file.seekp(offsetof_(id));
file.write(reinterpret_cast<char *>(&t), sizeof(t));
file.close();
}
//读出位置索引index对应的T对象的值并赋值给t，保证调用的index都是由write函数产生
void read(T &t, const int id) {
file.open(file_name, std::ios::in);
file.seekg(offsetof_(id));
file.read(reinterpret_cast<char *>(&t), sizeof(t));
file.close();
}
//删除位置索引index对应的对象(不涉及空间回收时，可忽略此函数)，保证调用的index都是由write函数产生
void Delete(int id) {
Metadata meta;
file.open(file_name);
file.seekg(0);
file.read(reinterpret_cast<char *>(&meta), sizeof(int));
file.seekp(0);
Metadata newMeta { .hasNext = true, .next = id };
file.write(reinterpret_cast<char *>(&newMeta), sizeof(int));
file.seekp(offsetof_(id));
file.write(reinterpret_cast<char *>(&meta), sizeof(int));
file.close();
}
};
#endif // BPT_MEMORYRIVER_HPP
#ifndef ONLINE_JUDGE
#include <iostream>
#include <cstring>
using std::cin, std::cout, std::endl;
#define print(x) cout << x << endl
struct Payload {
int a;
char c[10];
bool b;
};
int main () {
MemoryRiver<Payload> instance;
instance.initialise("fn");
instance.write_info(233, 1);
instance.write_info(666, 2);
Payload p { .a = 19260817, .b = true };
strcpy(p.c, "hello");
print(instance.write(p));
++p.a;
print(instance.write(p));
p.b = false;
print(instance.write(p));
Payload r;
instance.read(r, 0);
print(r.a);
print(r.b);
print(r.c);
instance.Delete(1);
instance.Delete(2);
print(instance.write(p));
print(instance.write(p));
print(instance.write(p));
print(instance.write(p));
print(instance.write(p));
return 0;
}
#endif