From 71c399b381fda5a5bb865583517ab68346205b90 Mon Sep 17 00:00:00 2001
From: swetank18 <tankrst2005@gmail.com>
Date: Fri, 23 Jan 2026 05:54:54 +0530
Subject: [PATCH] Fix edge case in analytical integral of exponential TF1

---
 hist/hist/src/AnalyticalIntegrals.cxx | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/hist/hist/src/AnalyticalIntegrals.cxx b/hist/hist/src/AnalyticalIntegrals.cxx
index ca7c402625c6b..3fe57034b888c 100644
--- a/hist/hist/src/AnalyticalIntegrals.cxx
+++ b/hist/hist/src/AnalyticalIntegrals.cxx
@@ -40,10 +40,20 @@ Double_t AnalyticalIntegral(TF1 *f, Double_t a, Double_t b)
       return TMath::QuietNaN();
    }
 
-   if   (num == 200)//expo: exp(p0+p1*x)
-   {
-      result = ( exp(p[0]+p[1]*xmax) - exp(p[0]+p[1]*xmin))/p[1];
+   else if (num == 200) { // expo: exp(p0 + p1*x)
+   const double p0 = p[0];
+   const double p1 = p[1];
+
+   if (p1 == 0) {
+      // Limit p1 -> 0: integral of constant exp(p0)
+      result = std::exp(p0) * (xmax - xmin);
+   } else {
+      const double ea = p0 + p1 * xmin;
+      const double eb = p0 + p1 * xmax;
+      result = (std::exp(eb) - std::exp(ea)) / p1;
    }
+}
+
    else if (num == 100)//gaus: [0]*exp(-0.5*((x-[1])/[2])^2))
    {
       double amp   = p[0];