#include <cassert>
#include <ostream>
#include <sstream>
#include <string>
#include <vector>

namespace {

struct range_t {
  int from;
  int to;

  range_t(int from, int to) : from(from), to(to) {}
  void extend() { to++; }

  int size() const { return to - from + 1; }
  bool prefixes(int x) const { return x == to + 1; }
};

std::ostream &operator<<(std::ostream &stream, const range_t &range) {
  if (range.size() >= 3) {
    stream << range.from << "-" << range.to;
  } else {
    stream << range.from;

    for (int i = range.from + 1; i <= range.to; i++) {
      stream << "," << i;
    }
  }
  return stream;
}

} // namespace

std::string range_extraction(std::vector<int> args) {
  if (args.empty()) {
    return "";
  }

  std::vector<range_t> ranges;

  for (int x : args) {
    if (ranges.size() && ranges.back().prefixes(x)) {
      ranges.back().extend();
    } else {
      ranges.emplace_back(x, x);
    }
  }

  std::stringstream out;

  auto it = ranges.cbegin();
  out << (*it);

  for (++it; it != ranges.cend(); ++it) {
    out << "," << (*it);
  }

  return out.str();
}

int main() {
  assert(range_extraction({-6, -3, -2, -1, 0,  1,  3,  4,  5,  7,
                           8,  9,  10, 11, 14, 15, 17, 18, 19, 20}) ==
         "-6,-3-1,3-5,7-11,14,15,17-20");
  assert(range_extraction({-3, -2, -1, 2, 10, 15, 16, 18, 19, 20}) ==
         "-3--1,2,10,15,16,18-20");
}