feat: Add option for initial step size

Core/include/Acts/Propagator/AtlasStepper.hpp

@@ -160,7 +160,9 @@ class AtlasStepper {
     state.pathAccumulated = 0;
     state.nSteps = 0;
     state.nStepTrials = 0;
-    state.stepSize = ConstrainedStep(state.options.maxStepSize);
+    state.stepSize = ConstrainedStep();
+    state.stepSize.setAccuracy(state.options.initialStepSize);
+    state.stepSize.setUser(state.options.maxStepSize);
     state.previousStepSize = 0;
     state.statistics = StepperStatistics();
 

Core/include/Acts/Propagator/EigenStepper.ipp

@@ -46,7 +46,9 @@ void Acts::EigenStepper<E>::initialize(State& state,
   state.pathAccumulated = 0;
   state.nSteps = 0;
   state.nStepTrials = 0;
-  state.stepSize = ConstrainedStep(state.options.maxStepSize);
+  state.stepSize = ConstrainedStep();
+  state.stepSize.setAccuracy(state.options.initialStepSize);
+  state.stepSize.setUser(state.options.maxStepSize);
   state.previousStepSize = 0;
   state.statistics = StepperStatistics();
 

Core/include/Acts/Propagator/StepperOptions.hpp

@@ -11,6 +11,7 @@
 #include "Acts/Definitions/Units.hpp"
 
 #include <functional>
+#include <limits>
 
 namespace Acts {
 
@@ -35,8 +36,11 @@ struct StepperPlainOptions {
   /// Cut-off value for the step size
   double stepSizeCutOff = 0.;
 
+  /// Initial step size
+  double initialStepSize = 10 * Acts::UnitConstants::m;
+
   /// Absolute maximum step size
-  double maxStepSize = 10 * Acts::UnitConstants::m;
+  double maxStepSize = std::numeric_limits<double>::max();
 
   /// Maximum number of Runge-Kutta steps for the stepper step call
   unsigned int maxRungeKuttaStepTrials = 10000;

Core/src/Propagator/StraightLineStepper.cpp

@@ -39,7 +39,9 @@ void StraightLineStepper::initialize(State& state,
   state.pathAccumulated = 0;
   state.nSteps = 0;
   state.nStepTrials = 0;
-  state.stepSize = ConstrainedStep(state.options.maxStepSize);
+  state.stepSize = ConstrainedStep();
+  state.stepSize.setAccuracy(state.options.initialStepSize);
+  state.stepSize.setUser(state.options.maxStepSize);
   state.previousStepSize = 0;
   state.statistics = StepperStatistics();
 

Core/src/Propagator/SympyStepper.cpp

@@ -46,7 +46,9 @@ void SympyStepper::initialize(State& state, const BoundVector& boundParams,
   state.pathAccumulated = 0;
   state.nSteps = 0;
   state.nStepTrials = 0;
-  state.stepSize = ConstrainedStep(state.options.maxStepSize);
+  state.stepSize = ConstrainedStep();
+  state.stepSize.setAccuracy(state.options.initialStepSize);
+  state.stepSize.setUser(state.options.maxStepSize);
   state.previousStepSize = 0;
   state.statistics = StepperStatistics();
 

Tests/UnitTests/Core/Propagator/EigenStepperTests.cpp

@@ -217,6 +217,7 @@ BOOST_AUTO_TEST_CASE(eigen_stepper_test) {
 
   EigenStepper<>::Options esOptions(tgContext, mfContext);
   esOptions.maxStepSize = stepSize;
+  esOptions.initialStepSize = 10_m;
 
   // Build the stepper and the state
   EigenStepper<> es(bField);

Tests/UnitTests/Core/Propagator/StraightLineStepperTests.cpp

@@ -31,6 +31,7 @@
 #include <optional>
 #include <string>
 
+using namespace Acts::UnitLiterals;
 using Acts::VectorHelpers::makeVector4;
 
 namespace Acts::Test {
@@ -114,6 +115,7 @@ BOOST_AUTO_TEST_CASE(straight_line_stepper_test) {
 
   StraightLineStepper::Options options(tgContext, mfContext);
   options.maxStepSize = stepSize;
+  options.initialStepSize = 10_m;
 
   // Build the stepper and the state
   StraightLineStepper sls;

Tests/UnitTests/Core/Propagator/SympyStepperTests.cpp

@@ -192,6 +192,7 @@ BOOST_AUTO_TEST_CASE(sympy_stepper_test) {
 
   SympyStepper::Options esOptions(tgContext, mfContext);
   esOptions.maxStepSize = stepSize;
+  esOptions.initialStepSize = 10_m;
 
   // Build the stepper and the state
   SympyStepper es(bField);