ALICE3: ACTS tracking geometry provider

Adds the Acts::TrackingGeometry that an ACTS-based reconstruction needs, built
from the TGeo geometry O2 already has live in memory. ACTS stays an optional
dependency: without it these packages produce no targets.

O2::ACTSInterface (Detectors/ACTS/interface), no DPL dependency:
  - TrackingGeometryManager, a lazily-built process-wide provider following the
    o2::base::Propagator::Instance() / GeometryTGeo::Instance() pattern. It
    fetches nothing itself; callers make gGeoManager available first, normally
    by declaring GRPGeomRequest::Aligned.
  - ITrackingGeometryBuilder, the detector-specific seam, plus the element store
    the geometry does not own but depends on.
  - SurfaceIndexMap, the O2 sensor <-> ACTS surface lookup.
  - MagneticFieldAdapter, an Acts::MagneticFieldProvider over the O2 field.

O2::ACTSWorkflow (Detectors/ACTS/workflow): ActsGeometryService, the same
provider reachable through the DPL ServiceRegistry with ServiceKind::DeviceGlobal.

O2::ALICE3ACTS (Detectors/Upgrades/ALICE3/ACTS): the Gen3 Blueprint builder,
ported from actsO2 ActsAlgorithms/Geometry/src/ALICE3Gen3Geometry.cpp. The
construction logic is unchanged so the geometry identifiers stay compatible with
the standalone chain's material maps and digitisation/seeding configs; verified
by decorating an O2-built geometry with actsO2's own material map. Differences
from the original: it reads a TGeoManager handed in by the caller instead of
importing a ROOT file, and its process-lifetime static stores became member
state so several builders can coexist in one process.

Sensors are identified by the TGeo node the surface was built from, not by
position. A transform is not an identity: the vertex-detector petals are tube
segments, so the three layers of a petal share both origin and rotation and
differ only in radius, which the O2 matrix cache does not carry. Position-only
matching is kept as a fallback and reports such cases as ambiguous instead of
assigning them arbitrarily.

macros/run_test.sh generates an ALICE 3 geometry with o2-sim and checks every
TRK chip against its ACTS surface; config/gen3_geometry_config.json is the
matching geometry description.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: pbutti

Detectors/ACTS/CMakeLists.txt

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+# Copyright 2019-2026 CERN and copyright holders of ALICE O2.
+# See https://alice-o2.web.cern.ch/copyright for details of the copyright holders.
+# All rights not expressly granted are reserved.
+#
+# This software is distributed under the terms of the GNU General Public
+# License v3 (GPL Version 3), copied verbatim in the file "COPYING".
+#
+# In applying this license CERN does not waive the privileges and immunities
+# granted to it by virtue of its status as an Intergovernmental Organization
+# or submit itself to any jurisdiction.
+
+# The whole package is optional: ACTS is an optional O2 dependency
+# (see dependencies/O2Dependencies.cmake) and standard builds do not ship it.
+if(NOT Acts_FOUND)
+  message(STATUS "ACTS not found, skipping the O2 ACTS interface")
+  return()
+endif()
+
+add_subdirectory(interface)
+add_subdirectory(workflow)

Detectors/ACTS/README.md

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+# ACTS tracking geometry in O2
+
+Provides the shared, immutable `Acts::TrackingGeometry` that ACTS-based
+reconstruction needs, built from the TGeo geometry O2 already has live in memory.
+
+ACTS is an **optional** O2 dependency (`dependencies/O2Dependencies.cmake`). Without
+it this package produces no targets and `O2_WITH_ACTS` is not defined, so anything
+using it must be guarded.
+
+## Libraries
+
+| Target | Contents |
+| --- | --- |
+| `O2::ACTSInterface` | `TrackingGeometryManager` (the provider), `ITrackingGeometryBuilder` (the detector-specific seam), `SurfaceIndexMap` (sensor ↔ surface lookup), `MagneticFieldAdapter` (`Acts::MagneticFieldProvider` over the O2 field). No DPL dependency. |
+| `O2::ACTSWorkflow` | `ActsGeometryService`, the same provider reachable through the DPL `ServiceRegistry`. |
+| `O2::ALICE3ACTS` | `Gen3BlueprintBuilder`, the ALICE 3 builder (under `Detectors/Upgrades/ALICE3/ACTS`). |
+
+## Using it from a DPL task
+
+The manager fetches nothing itself: it needs `gGeoManager` to be live, which is what
+`GRPGeomRequest::Aligned` arranges. Note that a task that only used the material LUT
+before will have been passing `GRPGeomRequest::None` and has to be switched.
+
+```cpp
+// spec factory
+auto ggRequest = std::make_shared<o2::base::GRPGeomRequest>(
+    false, false, false, true /*GRPMagField*/, false,
+    o2::base::GRPGeomRequest::Aligned, inputs, true);
+
+void init(InitContext& ic)
+{
+  o2::base::GRPGeomHelper::instance().setRequest(mCCDBReq);
+
+  o2::alice3::Gen3BlueprintBuilder::Config cfg;
+  cfg.geometryConfigFile = ic.options().get<std::string>("acts-geometry-config");
+  o2::acts::TrackingGeometryManager::instance().setBuilder(
+      std::make_unique<o2::alice3::Gen3BlueprintBuilder>(cfg));
+}
+
+void run(ProcessingContext& pc)
+{
+  o2::base::GRPGeomHelper::instance().checkUpdates(pc);      // gGeoManager now live
+  const auto& tg = o2::acts::TrackingGeometryManager::instance().get();  // built once
+}
+```
+
+To reach it through the registry instead, put `o2::acts::defaultServicesWithActsGeometry()`
+into the `DataProcessorSpec`'s `requiredServices` and use
+`pc.services().get<o2::acts::ActsGeometryService>()`. Both paths hand out the same object.
+
+## Mapping O2 clusters onto ACTS surfaces
+
+`makeSurfaceIndex()` (or `TrackingGeometryManager::getIndex(cache, tol, pathProvider)`,
+which caches per detector) maps every sensor of a `DetMatrixCache`-derived geometry helper
+onto a sensitive ACTS surface.
+
+**Pass a `SensorPathProvider` whenever the detector has one.** With it, sensors are
+identified by the TGeo node the surface was built from — an identity, so it cannot
+mis-assign:
+
+```cpp
+auto* trkGeo = o2::trk::GeometryTGeo::Instance();
+trkGeo->fillMatrixCache(o2::math_utils::bit2Mask(o2::math_utils::TransformType::L2G));
+const auto& index = mgr.getIndex(*trkGeo, 1e-3,
+    [trkGeo](int chipID) { return std::string(trkGeo->getMatrixPath(chipID).Data()); });
+```
+
+Without it the fallback matches on the sensor's local-to-global transform, which is exact
+where a transform identifies a sensor but not always: the ALICE 3 vertex-detector petals
+are tube segments, so the three layers of a petal share both origin and rotation and
+differ only in radius, which the matrix cache does not carry. Those are reported as
+ambiguous rather than assigned arbitrarily, and `makeSurfaceIndex()` throws. One surface
+per sensor is enforced in both modes.
+
+## The Gen3 geometry description
+
+`Detectors/Upgrades/ALICE3/ACTS/config/gen3_geometry_config.json` carries everything
+geometry-specific: sensor name globs, region boundaries, per-subsystem clustering
+tolerances, passive material structures and the pinned volume IDs. All lengths in mm.
+
+It is tuned for the layout `run_test.sh` generates — verified against an o2-sim geometry of
+that layout: the sensor globs cover every sensitive volume family (and correctly exclude
+`FT3Sensor_Inactive_*`), `*EOSCard*` matches the 504 end-of-stave volumes, the passive
+cylinder radii and half-lengths match the real support volumes, and the resulting
+volume/layer/surface table is identical to the one actsO2's reference geometry produces.
+
+One known imprecision, inherited and present for both geometries: `TRK_MID_CarbonSupport`
+is declared with `halfZ = 1420` while the real `TRK_MID_CARBONSUPPORT` volume has
+`dz = 1410 mm`. It only affects the z-extent of that passive layer's material receiver.
+
+## Material
+
+Gen3 construction takes no `IMaterialDecorator`, so an ACTS JSON material map is applied
+afterwards via `setMaterialMapFile()`. Material maps are keyed on
+`Acts::GeometryIdentifier`, so a structural change to a builder invalidates existing
+maps; the manager raises rather than silently producing a material-free geometry.
+
+## Checking a geometry
+
+`Detectors/Upgrades/ALICE3/ACTS/macros/run_test.sh` does the whole thing: it runs
+`o2-sim-serial-run5` with the ALICE 3 layout the ACTS chain is developed against, then
+builds the tracking geometry from the result and checks every TRK chip against its ACTS
+surface. Run it from a scratch directory — `o2-sim` writes into `$PWD`:
+
+```bash
+mkdir -p /tmp/acts && cd /tmp/acts
+bash <O2-source>/Detectors/Upgrades/ALICE3/ACTS/macros/run_test.sh
+```
+
+The geometry description it needs is shipped in this package
+(`Detectors/Upgrades/ALICE3/ACTS/config/gen3_geometry_config.json`, installed to
+`$O2_ROOT/share/Detectors/Upgrades/ALICE3/ACTS/config/`) and matches the layout the script
+generates. Set `GEN3_CONFIG` to point at a different geometry's config.
+
+Knobs: `nEvents`, `generator`, `modules`, `GEN3_CONFIG`, `SKIP_ALIGNMENT`. The last one
+defaults to 1 because `o2-sim` finishes by fetching alignment from CCDB, which needs a
+valid alien token and aborts without one — after the geometry file has already been
+written. The ACTS geometry is built from the ideal geometry, so skipping it costs nothing.
+
+The macro `CheckActsTrackingGeometry.C` can also be run on its own against an existing
+`o2sim_geometry.root`; it prints the volume/layer/surface table, which is the authority for
+the `(volume, layer)` keys that digitisation and seeding configurations use. It needs ACTS,
+Eigen and `$O2_ROOT/include` on `ROOT_INCLUDE_PATH` — `run_test.sh` sets that up.

Detectors/ACTS/interface/CMakeLists.txt

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+# Copyright 2019-2026 CERN and copyright holders of ALICE O2.
+# See https://alice-o2.web.cern.ch/copyright for details of the copyright holders.
+# All rights not expressly granted are reserved.
+#
+# This software is distributed under the terms of the GNU General Public
+# License v3 (GPL Version 3), copied verbatim in the file "COPYING".
+#
+# In applying this license CERN does not waive the privileges and immunities
+# granted to it by virtue of its status as an Intergovernmental Organization
+# or submit itself to any jurisdiction.
+
+o2_add_library(ACTSInterface
+               TARGETVARNAME targetName
+               SOURCES src/TrackingGeometryManager.cxx
+                       src/SurfaceIndexMap.cxx
+                       src/MagneticFieldAdapter.cxx
+               PUBLIC_LINK_LIBRARIES O2::DetectorsBase
+                                     O2::DetectorsCommonDataFormats
+                                     O2::Field
+                                     O2::Framework
+                                     ROOT::Geom
+                                     Acts::Core
+                                     Acts::PluginRoot
+                                     Acts::PluginJson)
+
+target_compile_definitions(${targetName} PUBLIC O2_WITH_ACTS)

Detectors/ACTS/interface/include/ACTSInterface/ITrackingGeometryBuilder.h

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+// Copyright 2019-2026 CERN and copyright holders of ALICE O2.
+// See https://alice-o2.web.cern.ch/copyright for details of the copyright holders.
+// All rights not expressly granted are reserved.
+//
+// This software is distributed under the terms of the GNU General Public
+// License v3 (GPL Version 3), copied verbatim in the file "COPYING".
+//
+// In applying this license CERN does not waive the privileges and immunities
+// granted to it by virtue of its status as an Intergovernmental Organization
+// or submit itself to any jurisdiction.
+
+///
+/// \file ITrackingGeometryBuilder.h
+/// \brief Detector-agnostic interface for building an Acts::TrackingGeometry from TGeo
+/// \author Paolo Butti
+///
+
+#ifndef ALICEO2_ACTS_ITRACKINGGEOMETRYBUILDER_H
+#define ALICEO2_ACTS_ITRACKINGGEOMETRYBUILDER_H
+
+#include <memory>
+#include <vector>
+
+#include "Acts/Geometry/GeometryContext.hpp"
+#include "Acts/Geometry/TrackingGeometry.hpp"
+#include "Acts/Surfaces/SurfacePlacementBase.hpp"
+
+#include "ACTSInterface/SurfaceIndexMap.h"
+
+class TGeoManager;
+
+namespace o2::acts
+{
+
+/// Everything a builder produces, kept together because the parts have a
+/// lifetime dependency on each other.
+struct TrackingGeometryOutput {
+  /// Non-const so the manager can still decorate it with material before
+  /// handing it out; it is published as shared_ptr<const> afterwards.
+  std::shared_ptr<Acts::TrackingGeometry> geometry;
+
+  /// Detector elements backing the sensitive surfaces.
+  ///
+  /// Acts::TrackingGeometry does NOT own these: Acts::Surface keeps a raw,
+  /// non-owning back-pointer to its placement (Surface::surfacePlacement()).
+  /// Destroying this store while \a geometry is alive is a dangling-pointer bug,
+  /// so it must be kept for at least as long as the geometry.
+  std::vector<std::shared_ptr<const Acts::SurfacePlacementBase>> elementStore;
+
+  /// Sensor <-> surface lookup. May be empty if the builder cannot provide one.
+  SurfaceIndexMap index;
+};
+
+/// Builds an Acts::TrackingGeometry from a TGeo tree.
+///
+/// Implementations are detector specific and live with their detector (see
+/// o2::alice3::Gen3BlueprintBuilder). They are injected into
+/// TrackingGeometryManager at runtime, so this library never depends on any
+/// concrete detector.
+class ITrackingGeometryBuilder
+{
+ public:
+  virtual ~ITrackingGeometryBuilder() = default;
+
+  /// Build from an already-loaded TGeo tree.
+  ///
+  /// \param tgeo the geometry manager to read; implementations must not import
+  ///             or otherwise replace it, since it is normally O2's live
+  ///             gGeoManager shared with the rest of the workflow
+  /// \param gctx nominal context used while placing surfaces
+  virtual TrackingGeometryOutput build(TGeoManager& tgeo, const Acts::GeometryContext& gctx) = 0;
+
+  /// Short name, used in log messages.
+  virtual const char* getName() const = 0;
+};
+
+} // namespace o2::acts
+
+#endif // ALICEO2_ACTS_ITRACKINGGEOMETRYBUILDER_H

Detectors/ACTS/interface/include/ACTSInterface/MagneticFieldAdapter.h

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+// Copyright 2019-2026 CERN and copyright holders of ALICE O2.
+// See https://alice-o2.web.cern.ch/copyright for details of the copyright holders.
+// All rights not expressly granted are reserved.
+//
+// This software is distributed under the terms of the GNU General Public
+// License v3 (GPL Version 3), copied verbatim in the file "COPYING".
+//
+// In applying this license CERN does not waive the privileges and immunities
+// granted to it by virtue of its status as an Intergovernmental Organization
+// or submit itself to any jurisdiction.
+
+///
+/// \file MagneticFieldAdapter.h
+/// \brief Acts::MagneticFieldProvider backed by the O2 field
+/// \author Paolo Butti
+///
+
+#ifndef ALICEO2_ACTS_MAGNETICFIELDADAPTER_H
+#define ALICEO2_ACTS_MAGNETICFIELDADAPTER_H
+
+#include "Acts/MagneticField/MagneticFieldProvider.hpp"
+
+namespace o2::field
+{
+class MagneticField;
+}
+
+namespace o2::acts
+{
+
+/// Exposes the O2 magnetic field to ACTS.
+///
+/// Holds no field of its own: it reads the process-global field that
+/// o2::base::Propagator::initFieldFromGRP() installs into
+/// TGeoGlobalMagField::Instance() from the GRPMagField CCDB object. The
+/// underlying object stays valid across field rescalings -- GRPGeomHelper
+/// rescales the existing MagneticField in place rather than replacing it -- so
+/// an adapter built once keeps returning up-to-date values.
+///
+/// Units: O2 works in kGauss and cm, ACTS in its native tesla/mm system, and the
+/// conversion is applied here.
+class MagneticFieldAdapter : public Acts::MagneticFieldProvider
+{
+ public:
+  /// Use the field currently installed in TGeoGlobalMagField.
+  /// \throw std::runtime_error if no field has been initialised
+  MagneticFieldAdapter();
+
+  /// Use an explicitly provided field.
+  explicit MagneticFieldAdapter(o2::field::MagneticField* field);
+
+  Cache makeCache(const Acts::MagneticFieldContext& mctx) const final;
+
+  Acts::Result<Acts::Vector3> getField(const Acts::Vector3& position, Cache& cache) const final;
+
+  const o2::field::MagneticField* getO2Field() const { return mField; }
+
+ private:
+  struct CacheImpl {
+  };
+
+  o2::field::MagneticField* mField = nullptr;
+};
+
+} // namespace o2::acts
+
+#endif // ALICEO2_ACTS_MAGNETICFIELDADAPTER_H