feat(combinator): displayExpression with prefix notation

src/combinator.cpp

@@ -26,6 +26,7 @@
 #include <fmt/format.h>
 
 #include <numeric>
+#include <unordered_set>
 
 using Log = metricq::logger::nitro::Log;
 
@@ -49,6 +50,47 @@ Combinator::~Combinator()
 {
 }
 
+std::string displayExpression(const nlohmann::json& expression) {
+    static const std::unordered_set<std::string> validOperators = { "+", "-", "*", "/" };
+
+    // Check if the expression is a number and return it as a string
+    if (expression.is_number()) {
+        double value = expression.get<double>();
+        return (value == static_cast<int>(value)) ? std::to_string(static_cast<int>(value)) : std::to_string(value);
+    }
+
+    if (expression.is_object() && expression.contains("operation")) {
+        std::string operation = expression.value("operation", "");
+
+        // Validate if the operation is supported
+        if (validOperators.find(operation) == validOperators.end()) {
+            return operation;
+        }
+
+        // Helper lambda to process operands
+        auto processOperand = [&](const std::string& key) -> std::string {
+            if (expression.contains(key)) {
+                const auto& operand = expression[key];
+                if (operand.is_string()) return operand.get<std::string>();
+                if (operand.is_number()) return std::to_string(operand.get<int>());
+                return displayExpression(operand);
+            }
+            return "";
+        };
+
+        std::string leftStr = processOperand("left");
+        std::string rightStr = processOperand("right");
+        return operation + (leftStr.empty() ? "" : " " + leftStr) + (rightStr.empty() ? "" : " " + rightStr);
+    }
+
+    // If the expression is a string, return it directly
+    if (expression.is_string()) {
+        return expression.get<std::string>();
+    }
+
+    return "Unknown expression format";
+}
+
 void Combinator::on_transformer_config(const metricq::json& config)
 {
     input_metrics.clear();
@@ -93,6 +135,8 @@ void Combinator::on_transformer_config(const metricq::json& config)
         // Register the combined metric as a new source metric
         auto& metric = (*this)[combined_name];
 
+        metric.metadata["displayExpression"] = displayExpression(combined_expression);
+
         if (combined_config.count("chunk_size"))
         {
             auto chunk_size = combined_config["chunk_size"].get<int>();