text: // See www.openfst.org for extensive documentation on this weighted
  // finite-state transducer library.
  //
  // Performs the dynamic replacement of arcs in one FST with another FST,
  // allowing for the definition of FSTs analogous to RTNs.
  
  #include <cstring>
  #include <string>
  #include <utility>
  #include <vector>
  
  #include <fst/flags.h>
  #include <fst/types.h>
  #include <fst/script/getters.h>
  #include <fst/script/replace.h>
  
  DECLARE_string(call_arc_labeling);
  DECLARE_string(return_arc_labeling);
  DECLARE_int64(return_label);
  DECLARE_bool(epsilon_on_replace);
  
  namespace fst {
  namespace script {
  namespace {
  
  void Cleanup(std::vector<std::pair<int64, const FstClass *>> *pairs) {
    for (const auto &pair : *pairs) delete pair.second;
    pairs->clear();
  }
  
  }  // namespace
  }  // namespace script
  }  // namespace fst
  
  int fstreplace_main(int argc, char **argv) {
    namespace s = fst::script;
    using fst::script::FstClass;
    using fst::script::VectorFstClass;
    using fst::ReplaceLabelType;
  
    std::string usage =
        "Recursively replaces FST arcs with other FST(s).\n\n"
        "  Usage: ";
    usage += argv[0];
    usage += " root.fst rootlabel [rule1.fst label1 ...] [out.fst]\n";
  
    std::set_new_handler(FailedNewHandler);
    SET_FLAGS(usage.c_str(), &argc, &argv, true);
    if (argc < 4) {
      ShowUsage();
      return 1;
    }
  
    const std::string in_name = argv[1];
    const std::string out_name = argc % 2 == 0 ? argv[argc - 1] : "";
  
    auto *ifst = FstClass::Read(in_name);
    if (!ifst) return 1;
  
    std::vector<std::pair<int64, const FstClass *>> pairs;
    // Note that if the root label is beyond the range of the underlying FST's
    // labels, truncation will occur.
    const auto root = atoll(argv[2]);
    pairs.emplace_back(root, ifst);
  
    for (auto i = 3; i < argc - 1; i += 2) {
      ifst = FstClass::Read(argv[i]);
      if (!ifst) {
        s::Cleanup(&pairs);
        return 1;
      }
      // Note that if the root label is beyond the range of the underlying FST's
      // labels, truncation will occur.
      const auto label = atoll(argv[i + 1]);
      pairs.emplace_back(label, ifst);
    }
  
    ReplaceLabelType call_label_type;
    if (!s::GetReplaceLabelType(FLAGS_call_arc_labeling, FLAGS_epsilon_on_replace,
                                &call_label_type)) {
      LOG(ERROR) << argv[0] << ": Unknown or unsupported call arc replace "
                 << "label type: " << FLAGS_call_arc_labeling;
    }
    ReplaceLabelType return_label_type;
    if (!s::GetReplaceLabelType(FLAGS_return_arc_labeling,
                                FLAGS_epsilon_on_replace, &return_label_type)) {
      LOG(ERROR) << argv[0] << ": Unknown or unsupported return arc replace "
                 << "label type: " << FLAGS_return_arc_labeling;
    }
  
    s::ReplaceOptions opts(root, call_label_type, return_label_type,
                           FLAGS_return_label);
  
    VectorFstClass ofst(ifst->ArcType());
    s::Replace(pairs, &ofst, opts);
    s::Cleanup(&pairs);
  
    return !ofst.Write(out_name);
  }
  