From da6af32ca835d205349d757624f17e7a0b42c71e Mon Sep 17 00:00:00 2001
From: Christopher Schwan <handgranaten-herbert@posteo.de>
Date: Sat, 4 Nov 2023 12:04:45 +0100
Subject: [PATCH] Add new examples showing the extraction of channel
 information

---
 examples/cpp/Makefile             | 10 +++--
 examples/cpp/display-channels.cpp | 62 +++++++++++++++++++++++++++++++
 2 files changed, 69 insertions(+), 3 deletions(-)
 create mode 100644 examples/cpp/display-channels.cpp

diff --git a/examples/cpp/Makefile b/examples/cpp/Makefile
index a30d4073..22e7758f 100644
--- a/examples/cpp/Makefile
+++ b/examples/cpp/Makefile
@@ -3,16 +3,20 @@ CXXFLAGS = -std=c++11 -O3 -Wall -Wextra
 PINEAPPL_DEPS != pkg-config --cflags --libs pineappl_capi
 LHAPDF_DEPS != pkg-config --cflags --libs lhapdf
 
-all: convolute-grid fill-custom-grid fill-grid
+all: convolute-grid display-channels fill-custom-grid fill-grid
 
-test-examples: convolute-grid fill-custom-grid fill-grid
+test-examples: convolute-grid display-channels fill-custom-grid fill-grid
 	./fill-grid
 	./fill-custom-grid
 	./convolute-grid
+	./display-channels
 
 convolute-grid: convolute-grid.cpp
 	$(CXX) $(CXXFLAGS) $< $(LHAPDF_DEPS) $(PINEAPPL_DEPS) -o $@
 
+display-channels: display-channels.cpp
+	$(CXX) $(CXXFLAGS) $< $(PINEAPPL_DEPS) -o $@
+
 fill-custom-grid: fill-grid.cpp
 	$(CXX) $(CXXFLAGS) $< -DUSE_CUSTOM_GRID_PARAMETERS $(PINEAPPL_DEPS) -o $@
 
@@ -22,4 +26,4 @@ fill-grid: fill-grid.cpp
 PHONY: clean
 
 clean:
-	rm -f convolute-grid fill-custom-grid fill-grid
+	rm -f convolute-grid display-channels fill-custom-grid fill-grid
diff --git a/examples/cpp/display-channels.cpp b/examples/cpp/display-channels.cpp
new file mode 100644
index 00000000..e3670aa8
--- /dev/null
+++ b/examples/cpp/display-channels.cpp
@@ -0,0 +1,62 @@
+#include <pineappl_capi.h>
+
+#include <cstddef>
+#include <iomanip>
+#include <iostream>
+#include <string>
+#include <vector>
+
+int main(int argc, char* argv[]) {
+    std::string filename = "drell-yan-rap-ll.pineappl.lz4";
+
+    switch (argc) {
+    case 2:
+        filename = argv[1];
+    case 1:
+        break;
+
+    default:
+        std::cout << "Usage: " << argv[0] << " [grid] [pdf]\n";
+    }
+
+    // read the grid from a file
+    auto* grid = pineappl_grid_read(filename.c_str());
+
+    // extract all channels
+    auto* channels = pineappl_grid_lumi(grid);
+
+    // how many channels are there?
+    auto channel_count = pineappl_lumi_count(channels);
+
+    for (std::size_t channel = 0; channel != channel_count; ++channel) {
+        // print channel index
+        std::cout << std::setw(4) << channel << ' ';
+
+        // how many partonic combinations does this channel have?
+        auto combinations = pineappl_lumi_combinations(channels, channel);
+
+        std::vector<double> factors(combinations);
+        std::vector<int> pids(2 * combinations);
+
+        pineappl_lumi_entry(channels, channel, pids.data(), factors.data());
+
+        for (std::size_t combination = 0; combination != combinations; ++combination) {
+            auto factor = factors.at(combination);
+            auto pida = pids.at(2 * combination + 0);
+            auto pidb = pids.at(2 * combination + 1);
+
+            if (combination != 0) {
+                std::cout << " + ";
+            }
+
+            std::cout << factor << " x (" << std::setw(3) << pida << ',' << std::setw(4) << pidb
+                << ")";
+        }
+
+        std::cout << '\n';
+    }
+
+    // release memory
+    pineappl_lumi_delete(channels);
+    pineappl_grid_delete(grid);
+}