Sort the branches alphabetically when writing Frames

src/ROOTFrameWriter.cc

@@ -26,7 +26,7 @@ void ROOTFrameWriter::writeFrame(const podio::Frame& frame, const std::string& c
   // been initialized
   if (catInfo.tree == nullptr) {
     catInfo.idTable = frame.getCollectionIDTableForWrite();
-    catInfo.collsToWrite = collsToWrite;
+    catInfo.collsToWrite = root_utils::sortAlphabeticaly(collsToWrite);
     catInfo.tree = new TTree(category.c_str(), (category + " data tree").c_str());
     catInfo.tree->SetDirectory(m_file.get());
   }

src/rootUtils.h

@@ -11,6 +11,8 @@
 #include "TClass.h"
 #include "TTree.h"
 
+#include <algorithm>
+#include <cctype>
 #include <iostream>
 #include <string>
 #include <string_view>
@@ -182,6 +184,25 @@ inline auto reconstructCollectionInfo(TTree* eventTree, podio::CollectionIDTable
   return collInfo;
 }
 
+/**
+ * Sort the input vector of strings alphabetically, case insensitive.
+ */
+inline std::vector<std::string> sortAlphabeticaly(std::vector<std::string> strings) {
+  // Obviously there is no tolower(std::string) in c++, so this is slightly more
+  // involved and we make use of the fact that lexicographical_compare works on
+  // ranges and the fact that we can feed it a dedicated comparison function,
+  // where we convert the strings to lower case char-by-char. The alternative is
+  // to make string copies inside the first lambda, transform them to lowercase
+  // and then use operator< of std::string, which would be effectively
+  // hand-writing what is happening below.
+  std::sort(strings.begin(), strings.end(), [](const auto& lhs, const auto& rhs) {
+    return std::lexicographical_compare(
+        lhs.begin(), lhs.end(), rhs.begin(), rhs.end(),
+        [](const auto& cl, const auto& cr) { return std::tolower(cl) < std::tolower(cr); });
+  });
+  return strings;
+}
+
 } // namespace podio::root_utils
 
 #endif