Method evaluatePostfixStack

src/Eval.cpp:203–386

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void Eval::evaluatePostfixStack(const std::vector<std::pair<std::string, Lexer::Type>>& tokens,
                                Variant& result) const {
  if (tokens.size() == 0) throw std::string("No expression to evaluate.");

  // This is stack used by the postfix evaluator.
  std::vector<Variant> values;
  values.reserve(tokens.size());

  for (const auto& token : tokens) {
    // Unary operators.
    if (token.second == Lexer::Type::op && token.first == "!") {
      if (values.size() < 1) throw std::string("The expression could not be evaluated.");

      Variant right = values.back();
      values.pop_back();
      Variant result = !right;
      values.push_back(result);
      if (_debug)
        Context::getContext().debug(format("Eval {1} ↓'{2}' → ↑'{3}'", token.first,
                                           (std::string)right, (std::string)result));
    } else if (token.second == Lexer::Type::op && token.first == "_neg_") {
      if (values.size() < 1) throw std::string("The expression could not be evaluated.");

      Variant right = values.back();
      values.pop_back();

      Variant result(0);
      result -= right;
      values.push_back(result);

      if (_debug)
        Context::getContext().debug(format("Eval {1} ↓'{2}' → ↑'{3}'", token.first,
                                           (std::string)right, (std::string)result));
    } else if (token.second == Lexer::Type::op && token.first == "_pos_") {
      // The _pos_ operator is a NOP.
      if (_debug)
        Context::getContext().debug(format("[{1}] eval op {2} NOP", values.size(), token.first));
    }

    // Binary operators.
    else if (token.second == Lexer::Type::op) {
      if (values.size() < 2) throw std::string("The expression could not be evaluated.");

      Variant right = values.back();
      values.pop_back();

      Variant left = values.back();
      values.pop_back();

      auto contextTask = Context::getContext().currentTask;

      // Ordering these by anticipation frequency of use is a good idea.
      Variant result;
      if (token.first == "and")
        result = left && right;
      else if (token.first == "or")
        result = left || right;
      else if (token.first == "&&")