Taking n lines from a file which contains m lines, repeating the file (lazily) if necessary

Question

I'm looking for a way to use a text file to generate as much text as I need by simply repeating the as few times as needed.

In a purely functional language, like Haskell, the solution seems trivial: here I've posted the code for the review, and it is pretty short, even though I'm pretty sure it can be improved.

But in C++, I have little clue of where to start from, except that I'm pretty sure Boost Hana offers a lot of the tools I need to design the solution.

Here's an example input file,

line 1
line 2
line 3

and, if I ask for 7 lines, here's what I'd like to put in a variable (for instance in a single std::string with embedded '\n's),

line 1
line 2
line 3
line 1
line 2
line 3
line 1

I guess the function can have a declaration like this:

std::string function(std::string s, int n);

Answer 1

Assuming file is some input stream, and you want to repeat the lines in file for n lines, you can use the range-v3 library like this:

namespace rv = ranges::views;

auto lines = ranges::getlines(file) 
           | ranges::to<std::vector<std::string>>;
  
auto result = lines 
            | rv::cycle
            | rv::take(n) 
            | rv::join('\n') 
            | ranges::to<std::string>;

Here's a demo.

Answer 2

To do that in C++, you would need to loop similar to the following:

std::string readLines(std::string filename, int nlines)
{
    std::ifstream in(filename);
    std::string line, result;

    while (nlines > 0)
    {
        while (std::getline(in, line))
        {
            result += line;
            result += '\n';
            if (--nlines == 0)
                return result;
        }

        if (!in) // an error occurred...
            break; // or throw...

        // must be eof, rewind and start over...

        in.clear(); // <-- seekg() clears eofbit in C++11 and later...

        if (!in.seekg(0)) // an error occurred...
            break; // or throw...
    }

    return result;
}