Example to access HCal segmentation from DD4hep in a ROOT script

.rootlogon.C

+{
+  gROOT->ProcessLine(".include(\"/usr/local/include/DD4hep\")")
+}

README.md

+# HCal segmentation example Example 
+
+To run the example, please follow these steps:
+
+1. Download the latest `eic-shell` container in your location of preference:
+```bash
+curl https://eicweb.phy.anl.gov/containers/eic_container/-/raw/master/install.sh | bash
+```
+
+2. Start eic-shell from this location:
+```bash
+eic-shell
+```
+
+3. Load the canyonlands v2.1 detector:
+```bash
+source /opt/detector/athena-canyonlands-v2.1/setup.sh
+```
+
+4. Go to your directory where you unpacked this example, and execute the root macro in
+   compiled mode (it will NOT work in normal interpreted mode):
+```bash
+root -q -b hcal_segmentation.cxx+
+```
+
+5. That's all!

hcal_segmentation.cxx

+
+R__LOAD_LIBRARY(libDDRec)
+R__LOAD_LIBRARY(libfmt)
+
+// Includes necessary for compilation
+#include <DD4hep/Detector.h>
+#include <DDRec/CellIDPositionConverter.h>
+#include <DDSegmentation/BitFieldCoder.h>
+
+#include <iostream>
+#include <vector>
+
+#include <ROOT/RDataFrame.hxx>
+
+#include <dd4pod/CalorimeterHitData.h>
+#include <fmt/format.h>
+
+int hcal_segmentation() {
+  std::cout << "hello world" << std::endl;
+
+  // Get a handle to the global detector
+  dd4hep::Detector* det = &(dd4hep::Detector::getInstance());
+
+  // Load the detector from the compact file. Use the correct compact file for
+  // your simulation here. Also make sure your environment variables are setup
+  // correctly Example:
+  // - source /opt/detector/athena-canyonlands-v2.1/setup.sh
+  // - use "/opt/detector/athena-canyonlands-v2.1/share/athena/athena.xml"
+  det->fromCompact(
+      "/opt/detector/athena-canyonlands-v2.1/share/athena/athena.xml");
+  det->volumeManager();
+  det->apply("DD4hepVolumeManager", 0, 0);
+
+  // Create our cellID converter. This is useful to get cell coordinates. Not
+  // used below.
+  // auto cellIDConverter = std::make_shared<const
+  // dd4hep::rec::CellIDPositionConverter>(*det);
+
+  // Get our readout ID spec for HcalBarrelHits
+  auto idSpecBarrel = det->readout("HcalBarrelHits").idSpec();
+  auto idSpecEndcapN = det->readout("HcalEndcapNHits").idSpec();
+  auto idSpecEndcapP = det->readout("HcalEndcapPHits").idSpec();
+
+  // Get our cell ID decoder
+  auto idDecBarrel = idSpecBarrel.decoder();
+  auto idDecEndcapN = idSpecEndcapN.decoder();
+  auto idDecEndcapP = idSpecEndcapP.decoder();
+
+  // Get a bit mask for module, layer, x and y
+  // module: Which stave are we on?
+  // layer: layer within the module (stave)
+  // (x,): pad coordinates within the layer
+  //
+  const int moduleIdxBarrel = idDecBarrel->index("module");
+  const int layerIdxBarrel = idDecBarrel->index("layer");
+  const int xIdxBarrel = idDecBarrel->index("x");
+  const int yIdxBarrel = idDecBarrel->index("y");
+  // same for negative and positive endcaps (they don't use module)
+  const int layerIdxEndcapN = idDecEndcapN->index("layer");
+  const int xIdxEndcapN = idDecEndcapN->index("x");
+  const int yIdxEndcapN = idDecEndcapN->index("y");
+  const int layerIdxEndcapP = idDecEndcapP->index("layer");
+  const int xIdxEndcapP = idDecEndcapP->index("x");
+  const int yIdxEndcapP = idDecEndcapP->index("y");
+
+  // Simple lambda function that prints out the hit segmentation for the barrel
+  auto HcalBarrelPrinter =
+      [&](int event, const std::vector<dd4pod::CalorimeterHitData>& h) {
+        for (size_t i = 0; i < h.size(); ++i) {
+          // Only do hits above .1 MeV
+          if (h[i].energyDeposit < 1e-4) {
+            continue;
+          }
+          const int module = idDecBarrel->get(h[i].cellID, moduleIdxBarrel);
+          const int layer = idDecBarrel->get(h[i].cellID, layerIdxBarrel);
+          const int x = idDecBarrel->get(h[i].cellID, xIdxBarrel);
+          const int y = idDecBarrel->get(h[i].cellID, yIdxBarrel);
+          fmt::print(
+              "HcalBarrel event: {:3d} hit: {:3d} module: {:2d} layer: {:2d} x: {:4d} "
+              "y {:4d} energy: {:1.3e}\n",
+              event, i, module, layer, x, y, h[i].energyDeposit);
+        }
+      };
+
+  // Open the ROOT file from full simulation, change this by the name of
+  // your ROOT file
+  ROOT::RDataFrame d{"events", "dis.root"};
+
+  // For this demonstration we only use the first 50 events, not something
+  // you would want to do in general. For production, remove this line and
+  // change all occurences of d50 with d
+  auto d50 = d.Range(50);
+
+  // Print out hit segmentation data for the barren HCal
+  int event = 0;
+  d50.Define("event", [&]{return event++;}, {})
+     .Foreach(HcalBarrelPrinter, {"event", "HcalBarrelHits"});
+
+  // Let's do the same for the negative endcap
+  auto HcalEndcapNPrinter =
+      [&](int event, const std::vector<dd4pod::CalorimeterHitData>& h) {
+        for (size_t i = 0; i < h.size(); ++i) {
+          // Only do hits above .1 MeV
+          if (h[i].energyDeposit < 1e-4) {
+            continue;
+          }
+          const int layer = idDecEndcapN->get(h[i].cellID, layerIdxEndcapN);
+          const int x = idDecEndcapN->get(h[i].cellID, xIdxEndcapN);
+          const int y = idDecEndcapN->get(h[i].cellID, yIdxEndcapN);
+          fmt::print(
+              "HcalEndcapN event: {:3d} hit: {:3d} layer: {:2d} x: {:4d} "
+              "y {:4d} energy: {:1.3e}\n",
+              event, i, layer, x, y, h[i].energyDeposit);
+        }
+      };
+
+  event = 0;
+  d50.Define("event", [&]{return event++;}, {})
+     .Foreach(HcalEndcapNPrinter, {"event", "HcalEndcapNHits"});
+
+  return 0;
+}