如何向后读取文件以有效地查找子字符串

我有一个巨大的这种结构的日志文件：

“时间戳”：{“标识符”：值}

"1463403600":{"AA":74.42},
"1463403601":{"AA":29.55},
"1463403603":{"AA":24.78},
"1463403604":{"AA":8.46},
"1463403605":{"AA":44.84},
"1463403607":{"AA":87.05},
"1463403608":{"AA":54.81},
"1463403609":{"AA":93.1},
"1463403611":{"AA":77.64},
"1463403612":{"AA":33.39},
"1463403613":{"AA":69.2},

我想在给定的时间戳之后提取内容，例如：

std::ifstream * myfunc( uint32_t timestamp) 

例子：

myfunc(1463403611);
/* returns
"1463403611":{"AA":77.64},
"1463403612":{"AA":33.39},
"1463403613":{"AA":69.2},
*/

日志文件很长 - 太长而无法将其保留在内存中。该代码将在资源有限的嵌入式设备（80Mhz，~10kB 可用内存）上运行，因此我正在寻找一些有效解决方案的想法。

日志文件可能有 500k+ 条目，并且 99% 的时间时间戳将位于最后 100 行，因此从文件的开头开始并检查每一行是否有正确的时间戳将非常低效。

所以我想我正在寻找一种解决方案来逐行向后读取文件。 我真的没有一个解决方案如何在不将大块加载到内存的情况下有效地做到这一点。

我尝试从 EOF 开始读取 200 字节的块，但遇到了这样的问题：在许多情况下，该块将时间戳减半。我试图检测到这一点，并在需要时重新选择一些字节，但我感​​觉，必须有一个聪明的解决方案。

嗯，我发现这很有趣，所以我为它做了一个概念验证二分查找 https://en.wikipedia.org/wiki/Binary_search_algorithm idea.

这没有经过充分的测试，可能还有一些问题，但到目前为止似乎有效，并且演示了分而治之的思想。您检查文件的中间，然后根据是否太高或太低，将数据分成两部分并搜索相关的一半。您递归地执行此操作，直到足够接近为止。

#include <ctime>
#include <cmath>
#include <cstdlib>
#include <string>
#include <fstream>
#include <iostream>

// Don't use this, its just to show how many reads
// are being done to find the record.
int global_counter;

std::streampos find_stamp(std::istream& is, long stamp, std::streampos pos, std::streampos end)
{
    ++global_counter;

    if(pos == 0) // can't divide zero
        return 0;

    std::string s;
    long found_stamp;

    // extract nearest timestamp after pos
    is.seekg(pos);
    if(!(std::getline(std::getline(is, s, ','), s, '"') >> found_stamp))
        return end;

    // if its too big check first half of this region
    if(found_stamp > stamp)
        return find_stamp(is, stamp, pos / 2, pos);

    // if its not within 10 timestamp seconds check end half of this region
    if(stamp - found_stamp > 10)
        return find_stamp(is, stamp, (pos + end) / 2, end);

    // read record by record (prolly more efficient than skipping)
    pos = is.tellg();
    while(std::getline(std::getline(is, s, ','), s, '"') >> found_stamp)
    {
        if(found_stamp > stamp)
            return pos;
        pos = is.tellg();
    }
    return end;
}

void print_after(const std::string& filename, long stamp)
{
    // open at end of file (to get length)
    std::ifstream ifs(filename, std::ios::ate);

    std::streampos end = ifs.tellg();
    auto pos = end / 2; // start checking in middle

    // find position before required record
    // (may be in the middle of a record)
    if((pos = find_stamp(ifs, stamp, pos, end)) != end)
    {
        ifs.seekg(pos);

        std::string line;
        std::getline(ifs, line, ','); // skip to next whole record

        // print out all following recors
        while(std::getline(ifs, line, ','))
            std::cout << line;
    }
}

inline
std::string leading_zeros(int n, int zeros = 2)
{
    std::string s;
    for(int z = std::pow(10, zeros - 1); z; z /= 10)
        s += (n < z ? "0":"");
    return s + std::to_string(n);
}

int main()
{
    std::srand(std::time(0));

    // generate some test data
    std::ofstream ofs("test.txt");

    for(int i = 0; i < 1000; ++i)
    {
        ofs << '"' << leading_zeros(i, 10) << '"';
        ofs << ":{\"AA\":" << (std::rand() % 100);
        ofs << '.' << (std::rand() % 100) << "},\n";
    }

    ofs.close();

    global_counter = 0;
    print_after("test.txt", 993);

    std::cout << "find checked " << global_counter << " places in the file\n";
}

Output:

"0000000994":{"AA":80.6}
"0000000995":{"AA":11.90}
"0000000996":{"AA":16.43}
"0000000997":{"AA":53.11}
"0000000998":{"AA":68.43}
"0000000999":{"AA":79.77}
find checked 6 places in the file