From 2e4ae8ca0a5c520c9072bf0a6bbc9563e349d82c Mon Sep 17 00:00:00 2001
From: Varun Agrawal <varagrawal@gmail.com>
Date: Tue, 28 Jan 2025 22:08:17 -0500
Subject: [PATCH] add explanatory comments

---
 gtsam/discrete/DiscreteFactorGraph.cpp | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/gtsam/discrete/DiscreteFactorGraph.cpp b/gtsam/discrete/DiscreteFactorGraph.cpp
index 321ec7147c..f2bae4b9bf 100644
--- a/gtsam/discrete/DiscreteFactorGraph.cpp
+++ b/gtsam/discrete/DiscreteFactorGraph.cpp
@@ -215,6 +215,7 @@ namespace gtsam {
   EliminateDiscrete(const DiscreteFactorGraph& factors,
                     const Ordering& frontalKeys) {
     gttic(product);
+    // `product` is scaled later to prevent underflow.
     DiscreteFactor::shared_ptr product = factors.product();
     gttoc(product);
 
@@ -222,8 +223,11 @@ namespace gtsam {
     gttic(sum);
     DiscreteFactor::shared_ptr sum = product->sum(frontalKeys);
     gttoc(sum);
-    
+
     // Normalize/scale to prevent underflow.
+    // We divide both `product` and `sum` by `max(sum)`
+    // since it is faster to compute and when the conditional
+    // is formed by `product/sum`, the scaling term cancels out.
     gttic(scale);
     DiscreteFactor::shared_ptr denominator = sum->max(sum->size());
     product = product->operator/(denominator);