From ff6649cb8971dcd7047771ddb864b5ef3139c26b Mon Sep 17 00:00:00 2001
From: Jonas Rembser <jonas.rembser@cern.ch>
Date: Tue, 24 Sep 2024 14:24:25 +0200
Subject: [PATCH] [RF] Support scalar categorical inputs in RooFit::Evaluator

Closes #16419.

Should be backported also to 6.32.
---
 roofit/roofitcore/src/RooFit/Evaluator.cxx | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/roofit/roofitcore/src/RooFit/Evaluator.cxx b/roofit/roofitcore/src/RooFit/Evaluator.cxx
index 7c199f7747efb..67f637a99b5a1 100644
--- a/roofit/roofitcore/src/RooFit/Evaluator.cxx
+++ b/roofit/roofitcore/src/RooFit/Evaluator.cxx
@@ -332,7 +332,7 @@ void Evaluator::updateOutputSizes()
 Evaluator::~Evaluator()
 {
    for (auto &info : _nodes) {
-      if(!info.isVariable) {
+      if (!info.isVariable) {
          info.absArg->resetDataToken();
       }
    }
@@ -342,8 +342,6 @@ void Evaluator::computeCPUNode(const RooAbsArg *node, NodeInfo &info)
 {
    using namespace Detail;
 
-   auto nodeAbsReal = static_cast<RooAbsReal const *>(node);
-
    const std::size_t nOut = info.outputSize;
 
    double *buffer = nullptr;
@@ -372,7 +370,17 @@ void Evaluator::computeCPUNode(const RooAbsArg *node, NodeInfo &info)
    if (nOut > 1) {
       _evalContextCPU.enableVectorBuffers(true);
    }
-   nodeAbsReal->doEval(_evalContextCPU);
+   if (info.isCategory) {
+      auto nodeAbsCategory = static_cast<RooAbsCategory const *>(node);
+      if (nOut == 1) {
+         buffer[0] = nodeAbsCategory->getCurrentIndex();
+      } else {
+         throw std::runtime_error("RooFit::Evaluator - non-scalar category values are not supported!");
+      }
+   } else {
+      auto nodeAbsReal = static_cast<RooAbsReal const *>(node);
+      nodeAbsReal->doEval(_evalContextCPU);
+   }
    _evalContextCPU.resetVectorBuffers();
    _evalContextCPU.enableVectorBuffers(false);
    if (info.copyAfterEvaluation) {