Implement a `fix_convolution` method #368

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Merged

Radonirinaunimi merged 21 commits into master from fix-convolution

Jan 25, 2026

CHANGELOG.md

-Original file line number
+Diff line change
@@ Expand Up @@
     ## [Unreleased]
+    ### Added
+    - added a `fix_convolution` method to integrate out one of the convolution
+      dimensions in a grid by convolving it with a non-perturbative function
     ## [1.3.0] - 06/12/2025
     ### Added
@@ Expand Down @@

examples/cpp/Makefile

-Original file line number
+Diff line change
@@ Expand Up / @@ -22,7 +22,8 @@ PROGRAMS = \ @@
     	display-orders \
     	display-orders-deprecated \
     	merge-grids \
-    	modify-grid
+    	modify-grid \
+    	fix-convolution
     all: $(PROGRAMS)
@@ Expand All / @@ -47,6 +48,9 @@ convolve-grid-deprecated: convolve-grid-deprecated.cpp @@
     convolve-grid: convolve-grid.cpp
     	$(CXX) $(CXXFLAGS) $< $(LHAPDF_DEPS) $(PINEAPPL_DEPS) -o $@
+    fix-convolution: fix-convolution.cpp
+    	$(CXX) $(CXXFLAGS) $< $(LHAPDF_DEPS) $(PINEAPPL_DEPS) -o $@
     deprecated: deprecated.cpp
     	$(CXX) $(CXXFLAGS) $< $(LHAPDF_DEPS) $(PINEAPPL_DEPS) -o $@
@@ Expand Down @@

examples/cpp/fix-convolution.cpp

-Original file line number
+Diff line change
@@ -0,0 +1,101 @@
+    #include <cassert>
+    #include <cmath>
+    #include <cstddef>
+    #include <iomanip>
+    #include <ios>
+    #include <iostream>
+    #include <memory>
+    #include <string>
+    #include <vector>
+    #include <LHAPDF/LHAPDF.h>
+    #include <pineappl_capi.h>
+    void print_results(const std::vector<double>& r1, const std::vector<double>& r2) {
+        std::cout << std::scientific << std::setprecision(6);
+        std::cout << "Bin | Original Grid   | Fixed Grid      | Rel. Diff." << std::endl;
+        std::cout << "--- | --------------- | --------------- | -----------------" << std::endl;
+        for (size_t i = 0; i < r1.size(); ++i) {
+            double rel_diff = 0.0;
+            if (r1[i] != 0.0) {
+                rel_diff = std::abs((r1[i] - r2[i]) / r1[i]);
+            }
+            std::cout << std::setw(3) << i << " | "
+                        << std::setw(15) << r1[i] << " | "
+                        << std::setw(15) << r2[i] << " | "
+                        << std::setw(17) << rel_diff << std::endl;
+            assert(rel_diff < 1e-9);
+        }
+    }
+    int main() {
+        // Grid with 3 convolutions (2x pol PDF, 1x unpol FF)
+        const std::string filename = "../../test-data/SIHP-PP-POLARIZED-STAR-NLO.pineappl.lz4";
+        const std::string pol_pdf_set = "NNPDFpol11_100";
+        const std::string ff_set = "MAPFF10NLOPIsum";
+        LHAPDF::setVerbosity(0);
+        auto pol_pdf = std::unique_ptr<LHAPDF::PDF>(LHAPDF::mkPDF(pol_pdf_set, 0));
+        auto ff = std::unique_ptr<LHAPDF::PDF>(LHAPDF::mkPDF(ff_set, 0));
+        pineappl_grid* grid = pineappl_grid_read(filename.c_str());
+        assert(grid != nullptr);
+        auto xfx = [](int32_t id, double x, double q2, void* pdf) {
+            return static_cast <LHAPDF::PDF*> (pdf)->xfxQ2(id, x, q2);
+        };
+        auto alphas = [](double q2, void* pdf) {
+            return static_cast <LHAPDF::PDF*> (pdf)->alphasQ2(q2);
+        };
+        // Convolve original grid
+        size_t bins = pineappl_grid_bin_count(grid);
+        std::vector<double> results_orig(bins);
+        std::vector<LHAPDF::PDF*> pdfs_orig_vec = { pol_pdf.get(), pol_pdf.get(), ff.get() };
+        void** pdfs_orig = reinterpret_cast<void**>(pdfs_orig_vec.data());
+        std::vector<double> mu_scales = { 1.0, 1.0, 1.0 };
+        pineappl_grid_convolve(
+            grid,
+            xfx,
+            alphas,
+            pdfs_orig,
+            pol_pdf.get(),
+            nullptr,
+            nullptr,
+            nullptr,
+,
+            mu_scales.data(),
+            results_orig.data()
+        );
+        // Fix the third convolution (fragmentation function)
+        pineappl_grid* grid_fixed = pineappl_grid_fix_convolution(grid, 2, xfx, ff.get(), 1.0);
+        assert(grid_fixed != nullptr);
+        // Convolve the new grid
+        std::vector<double> results_fixed(bins);
+        std::vector<LHAPDF::PDF*> pdfs_fixed_vec = { pol_pdf.get(), pol_pdf.get() };
+        void** pdfs_fixed = reinterpret_cast<void**>(pdfs_fixed_vec.data());
+        pineappl_grid_convolve(
+            grid_fixed,
+            xfx,
+            alphas,
+            pdfs_fixed,
+            pol_pdf.get(),
+            nullptr,
+            nullptr,
+            nullptr,
+,
+            mu_scales.data(),
+            results_fixed.data()
+        );
+        print_results(results_orig, results_fixed);
+        std::cout << "\nSuccess: results from original and fixed grid match." << std::endl;
+        pineappl_grid_delete(grid);
+        pineappl_grid_delete(grid_fixed);
+        return 0;
+    }

examples/cpp/fix-convolution.output

-Original file line number
+Diff line change
@@ -0,0 +1,10 @@
+    Bin | Original Grid   | Fixed Grid      | Rel. Diff.
+    --- | --------------- | --------------- | -----------------
+|    2.260512e+03 |    2.260512e+03 |      1.207021e-15
+|    1.036130e+03 |    1.036130e+03 |      1.097226e-15
+|    4.894751e+02 |    4.894751e+02 |      1.045182e-15
+|    2.402394e+02 |    2.402394e+02 |      2.011198e-15
+|    1.246446e+02 |    1.246446e+02 |      7.980768e-16
+|    5.268035e+01 |    5.268035e+01 |      8.092688e-16
+    Success: results from original and fixed grid match.

examples/object-oriented-cpp/PineAPPL.hpp

-Original file line number
+Diff line change
@@ Expand Up / @@ -13,6 +13,7 @@ @@
     #include <cstdio>
     #include <string>
     #include <vector>
+    #include <memory>
     /** @brief Object oriented interface to PineAPPL.*/
     namespace PineAPPL {
@@ Expand Down Expand Up / @@ -123,8 +124,7 @@ struct Grid { @@
       /** @brief Underlying raw object. */
       pineappl_grid *raw;
-      /** @brief Constructor (protected to avoid direct instantiation). */
-     protected:
+      /** @brief Constructor. */
       Grid(pineappl_grid *grid) : raw(grid) {}
       /** @brief Deleted copy/move semantics. */
@@ Expand Down Expand Up / @@ -320,6 +320,31 @@ struct Grid { @@
                                raw_mu_scales.data(), results.data());
         return results;
       }
+      /**
+       * @brief Fix one of the convolutions in the Grid and return a new Grid with
+       * lower dimension.
+       *
+       * @param conv_idx index of the convolution to fix
+       * @param pdf the PDF set to use for the convolution
+       * @param xi the scale factor (xif or xia)
+       * @return a new grid with one less convolution
+       */
+      std::unique_ptr<Grid> fix_convolution(std::size_t conv_idx, LHAPDF::PDF *pdf,
+                                            double xi) const {
+        auto xfx = [](std::int32_t id, double x, double q2, void *pdf_ptr) {
+          return static_cast<LHAPDF::PDF *>(pdf_ptr)->xfxQ2(id, x, q2);
+        };
+        pineappl_grid *new_raw_grid =
+            pineappl_grid_fix_convolution(this->raw, conv_idx, xfx, pdf, xi);
+        if (new_raw_grid == nullptr) {
+          return nullptr;
+        }
+        return std::unique_ptr<Grid>(new Grid(new_raw_grid));
+      }
     };
     }  // namespace PineAPPL
@@ Expand Down @@

pineappl/src/convolutions.rs

-Original file line number
+Diff line change
@@ Expand Up / @@ -319,6 +319,12 @@ impl ConvType { @@
         pub const fn is_pdf(&self) -> bool {
             matches!(self, Self::UnpolPDF | Self::PolPDF)
         }
+        /// TODO
+        #[must_use]
+        pub const fn is_ff(&self) -> bool {
+            matches!(self, Self::UnpolFF | Self::PolFF)
+        }
     }
     /// Data type that indentifies different types of convolutions.
@@ Expand Down @@

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