#include <cstring>
#include <fstream>
#include <iostream>
#include <string>

int main(int argc, char *argv[]) {
  if (argc != 4) {
    std::cerr << "Usage: " << argv[0] << " <filename> <s1> <s2>" << std::endl;
    return 2;
  }

  std::string in_filename = argv[1];
  std::string out_filename = in_filename + ".replace";
  std::string s1 = argv[2];
  std::string s2 = argv[3];

  std::ifstream infile(in_filename.c_str(), std::ios::in);

  if (!infile.is_open()) {
    std::cerr << "Failed to open file " << in_filename << ": "
              << strerror(errno) << std::endl;
    return 1;
  }

  std::ofstream outfile(out_filename.c_str());

  if (!outfile.is_open()) {
    std::cerr << "Failed to open file " << out_filename << ": "
              << strerror(errno) << std::endl;
    return 1;
  }

  std::string line;
  while (!infile.eof() && !infile.fail()) {
    if (infile.fail()) {
      std::cerr << "Failed to read from file " << in_filename << ": "
                << strerror(errno) << std::endl;
      return 1;
    }
    std::getline(infile, line);
    size_t pos = 0;
    while (pos < line.length()) {
      size_t offset = line.find(s1, pos);
      outfile << line.substr(pos, offset - pos);
      if (offset == std::string::npos) {
        break;
      }
      outfile << s2;
      pos = offset + s1.length();
    }
    if (!infile.eof() && !infile.fail())
      outfile << std::endl;
  }

  return 0;
}