Cron.h

#include <chrono>
#include <string>
#include <sstream>
#include <vector>
#include <iterator>

namespace Bosma {
    using Clock = std::chrono::system_clock;

    inline void add(std::tm &tm, Clock::duration time) {
      auto tp = Clock::from_time_t(std::mktime(&tm));
      auto tp_adjusted = tp + time;
      auto tm_adjusted = Clock::to_time_t(tp_adjusted);
      tm = *std::localtime(&tm_adjusted);
    }

    class BadCronExpression : public std::exception {
    public:
        explicit BadCronExpression(std::string msg) : msg_(std::move(msg)) {}

        const char *what() const noexcept override { return (msg_.c_str()); }

    private:
        std::string msg_;
    };

    inline void
    verify_and_set(const std::string &token, const std::string &expression, int &field, const int lower_bound,
                   const int upper_bound, const bool adjust = false) {
      if (token == "*")
        field = -1;
      else {
        try {
          field = std::stoi(token);
        } catch (const std::invalid_argument &) {
          throw BadCronExpression("malformed cron string (`" + token + "` not an integer or *): " + expression);
        } catch (const std::out_of_range &) {
          throw BadCronExpression("malformed cron string (`" + token + "` not convertable to int): " + expression);
        }
        if (field < lower_bound || field > upper_bound) {
          std::ostringstream oss;
          oss << "malformed cron string ('" << token << "' must be <= " << upper_bound << " and >= " << lower_bound
              << "): " << expression;
          throw BadCronExpression(oss.str());
        }
        if (adjust)
          field--;
      }
    }

    class Cron {
    public:
        explicit Cron(const std::string &expression) {
          std::istringstream iss(expression);
          std::vector<std::string> tokens{std::istream_iterator<std::string>{iss},
                                          std::istream_iterator<std::string>{}};

          if (tokens.size() != 5) throw BadCronExpression("malformed cron string (must be 5 fields): " + expression);

          verify_and_set(tokens[0], expression, minute, 0, 59);
          verify_and_set(tokens[1], expression, hour, 0, 23);
          verify_and_set(tokens[2], expression, day, 1, 31);
          verify_and_set(tokens[3], expression, month, 1, 12, true);
          verify_and_set(tokens[4], expression, day_of_week, 0, 6);
        }

        // http://stackoverflow.com/a/322058/1284550
        Clock::time_point cron_to_next(const Clock::time_point from = Clock::now()) const {
          // get current time as a tm object
          auto now = Clock::to_time_t(from);
          std::tm next(*std::localtime(&now));
          // it will always at least run the next minute
          next.tm_sec = 0;
          add(next, std::chrono::minutes(1));
          while (true) {
            if (month != -1 && next.tm_mon != month) {
              // add a month
              // if this will bring us over a year, increment the year instead and reset the month
              if (next.tm_mon + 1 > 11) {
                next.tm_mon = 0;
                next.tm_year++;
              } else
                next.tm_mon++;

              next.tm_mday = 1;
              next.tm_hour = 0;
              next.tm_min = 0;
              continue;
            }
            if (day != -1 && next.tm_mday != day) {
              add(next, std::chrono::hours(24));
              next.tm_hour = 0;
              next.tm_min = 0;
              continue;
            }
            if (day_of_week != -1 && next.tm_wday != day_of_week) {
              add(next, std::chrono::hours(24));
              next.tm_hour = 0;
              next.tm_min = 0;
              continue;
            }
            if (hour != -1 && next.tm_hour != hour) {
              add(next, std::chrono::hours(1));
              next.tm_min = 0;
              continue;
            }
            if (minute != -1 && next.tm_min != minute) {
              add(next, std::chrono::minutes(1));
              continue;
            }
            break;
          }

          // telling mktime to figure out dst
          next.tm_isdst = -1;
          return Clock::from_time_t(std::mktime(&next));
        }

        int minute, hour, day, month, day_of_week;
    };
}