[RF] Fix RooHistPdf sub-range integrals with interpolation

roofit/roofitcore/src/RooHistPdf.cxx

@@ -300,6 +300,7 @@ Int_t RooHistPdf::getAnalyticalIntegral(RooArgSet& allVars,
 
   Int_t code = 0;
   Int_t frcode = 0;
+  bool directSubRange = false;
   for (unsigned int n=0; n < pdfObsList.size() && n < histObsList.size(); ++n) {
     const auto pa = pdfObsList[n];
     const auto ha = histObsList[n];
@@ -309,6 +310,9 @@ Int_t RooHistPdf::getAnalyticalIntegral(RooArgSet& allVars,
       analVars.add(*pa);
       if (fullRange(*pa, *ha, rangeName)) {
         frcode |= 2 << n;
+      } else if (pa->isFundamental()) {
+        // Sub-range integral over the histogram observable itself (no transform).
+        directSubRange = true;
       }
     }
   }
@@ -319,10 +323,12 @@ Int_t RooHistPdf::getAnalyticalIntegral(RooArgSet& allVars,
     code |= 1;
   }
 
-  // Disable partial analytical integrals if interpolation is used, and we
-  // integrate over sub-ranges, but leave them enabled when we integrate over
-  // the full range of one or several variables
-  if (intOrder > 1 && !(code & 1)) {
+  // the full range. For interpolated histograms (intOrder > 0), fall back to
+  // numerical integration over a direct sub-range of the histogram observable.
+  // A derived (non-fundamental) observable, such as a linear transform used to
+  // renormalize the components of a RooMomentMorphFuncND, keeps the analytical
+  // path it relies on.
+  if (intOrder > 0 && directSubRange) {
     analVars.removeAll();
     return 0;
   }

roofit/roofitcore/test/testRooHistPdf.cxx

@@ -12,6 +12,9 @@
 
 #include <gtest/gtest.h>
 
+#include <cmath>
+#include <memory>
+
 // Verify that RooFit correctly uses analytic integration when having a
 // RooLinearVar as the observable of a RooHistPdf.
 TEST(RooHistPdf, AnalyticIntWithRooLinearVar)
@@ -36,6 +39,113 @@ TEST(RooHistPdf, AnalyticIntWithRooLinearVar)
    EXPECT_EQ(integ.anaIntVars().size(), 1);
 }
 
+namespace {
+
+// Narrow Gaussian peak (sigma smaller than the bin width) on a steeply falling
+// exponential, so the sub-range area genuinely depends on the interpolation
+// order (a shape well sampled by the binning would let the bug below slip
+// through).
+void fillPeakedHist(RooDataHist &dataHist, RooRealVar &xHist)
+{
+   double const mean = 100.;
+   double const sigma = 8.;
+   for (int i = 0; i < xHist.numBins(); ++i) {
+      xHist.setBin(i);
+      double const c = xHist.getVal();
+      double const d = (c - mean) / sigma;
+      dataHist.set(i, 5000. * std::exp(-0.5 * d * d) + 3000. * std::exp(-0.05 * c) + 10., 0.0);
+   }
+}
+
+// Integral of the raw RooHistPdf curve over the full range of "x", sampling
+// pdf.getVal() directly. Independent of the integration machinery under test and
+// correct for any interpolation order.
+double trapezoidalReference(RooHistPdf &pdf, RooRealVar &x)
+{
+   int const n = 200000;
+   double const lo = x.getMin();
+   double const hi = x.getMax();
+   double const h = (hi - lo) / n;
+   double sum = 0.0;
+   for (int i = 0; i <= n; ++i) {
+      x.setVal(lo + i * h);
+      double const w = (i == 0 || i == n) ? 0.5 : 1.0;
+      sum += w * pdf.getVal();
+   }
+   return sum * h;
+}
+
+} // namespace
+
+// The analytical integral (RooDataHist::sum()) integrates the piecewise-constant
+// histogram, which only matches the interpolated curve over the full range. Over
+// a sub-range with intOrder > 0 RooHistPdf used to take that shortcut anyway,
+// returning the intOrder == 0 area regardless of the interpolation order.
+//
+// This checks the integral *value* (against an independent reference) rather
+// than which strategy is used, so it stays valid both for the current numerical
+// fallback and for a possible future analytical sub-range integral.
+TEST(RooHistPdf, SubRangeIntegralWithInterpolation)
+{
+   // Histogram observable, wider than the pdf observable "x" below.
+   RooRealVar xHist{"xHist", "xHist", -30., 270.};
+   xHist.setBins(20);
+
+   RooDataHist dataHist{"dataHist", "dataHist", xHist};
+   fillPeakedHist(dataHist, xHist);
+
+   // The pdf observable "x" spans only a sub-range of the histogram.
+   RooRealVar x{"x", "x", 0., 200.};
+
+   double refByOrder[2] = {0., 0.};
+
+   for (int intOrder : {0, 1}) {
+      RooHistPdf pdf{"pdf", "pdf", x, xHist, dataHist, intOrder};
+      double const ref = trapezoidalReference(pdf, x);
+      refByOrder[intOrder] = ref;
+
+      std::unique_ptr<RooAbsReal> integ{pdf.createIntegral(x)};
+      EXPECT_NEAR(integ->getVal(), ref, 1e-3 * ref) << "Wrong sub-range integral for intOrder=" << intOrder;
+   }
+
+   // Guard against the histogram shape becoming too smooth to tell the
+   // interpolation orders apart, which would make this test insensitive.
+   EXPECT_GT(std::abs(refByOrder[1] - refByOrder[0]), 0.02 * refByOrder[1])
+      << "Test histogram is too smooth to distinguish interpolation orders";
+}
+
+// Analytic integration is used in the cases where RooDataHist::sum() is exact:
+// non-interpolated histograms (any range) and full-range integrals (any order).
+TEST(RooHistPdf, AnalyticIntForStepFunctionAndFullRange)
+{
+   RooRealVar xHist{"xHist", "xHist", -30., 270.};
+   xHist.setBins(20);
+
+   RooDataHist dataHist{"dataHist", "dataHist", xHist};
+   fillPeakedHist(dataHist, xHist);
+
+   // Sub-range observable (narrower than the histogram).
+   RooRealVar xSub{"xSub", "xSub", 0., 200.};
+   // Full-range observable (matches the histogram range).
+   RooRealVar xFull{"xFull", "xFull", -30., 270.};
+
+   // Sub-range, no interpolation: analytical (step function is exact).
+   {
+      RooHistPdf pdf{"pdf", "pdf", xSub, xHist, dataHist, 0};
+      RooRealIntegral integ{"integ", "integ", pdf, xSub};
+      integ.getVal();
+      EXPECT_EQ(integ.anaIntVars().size(), 1) << "intOrder=0 sub-range should be analytical";
+   }
+
+   // Full-range, with interpolation: analytical (area is conserved over the full range).
+   {
+      RooHistPdf pdf{"pdf", "pdf", xFull, xHist, dataHist, 1};
+      RooRealIntegral integ{"integ", "integ", pdf, xFull};
+      integ.getVal();
+      EXPECT_EQ(integ.anaIntVars().size(), 1) << "intOrder=1 full-range should be analytical";
+   }
+}
+
 // Regression test for https://github.com/root-project/root/issues/21159, which
 // uncovered that the values were not clipped to be positive when evaluating a
 // RooHistPdf with the new vectorizing evaluation backend.