---

---

Table of Contents

• Introduction
  • Design Philosophy and Workflow
  • Funding
• Usage
  • Python installation
    • pypi
    • source
    • source for developers
    • patching shared object files
  • Automatic Build and Installation
    • Script Build and Installation Instructions
    • Currently, known good platforms
  • Manual Build Instructions
    • Prerequisites
    • Install Scripts
    • Dependency Installation on Common Platforms
    • Building the C++ Library
    • Installing the Library
    • Minimum compiler versions
• Code Architecture and Logical Flow
• Engines
  • GraphEngine
  • GraphEngine Configuration Options
  • Available Partition Functions
  • AutoDiff
• Reaclib in GridFire
• Engine Views
  • A Note about composability
• Numerical Solver Strategies
  • NetworkSolverStrategy<EngineT>
  • NetIn and NetOut
  • DirectNetworkSolver (Implicit Rosenbrock Method)
  • Algorithmic Workflow in DirectNetworkSolver
  • Future Solver Implementations
• Python Extensibility
• Usage Examples
  • C++
    • GraphEngine Initialization
    • Adaptive Network View
    • Composition Initialization
    • Common Workflow Example
    • Callback Example
  • Python
    • Common Workflow Example
    • Python callbacks
• Related Projects

Version and Notes

This repository is currently tracking GridFire version v0.7.0-alpha. Note that this is a development version which has known scientific and build issues:

1. Over long runs (say 10Gyr) repartitioning stages can introduce discontinuities into abundances
2. We do not currently produce He-4 at a rate consistent with literature values. This is a known issue and is being addressed.
3. When using Weak Rate Library (WRL) weak reactions the network becomes pathologically stiff. Reaclib includes a limited set of reactions which can be used to close the CNO cycle. Network construction then defaults to using all reaclib reactions while we address pathological stiffness with WRL rates.
4. WRL reactions do track energy loss due to neutrinos and neutrino flux; however, these are not currently reported to the user. They will be in the final v0.7.0 release.
5. There is a current bug in meson-python which results in multiple duplicate LC_RPATH entries in any shared object files compiled for python linking. Recent versions of the macos dynamic loader (XCode command line tools versions >= 16) refuse to load shared object files with duplicat rpath entries. Because of this running pip install. in the root will result in a broken gridfire python install. Instead we have bundled a helper script pip_install_mac_patch.sh which should be used to install python bindings on macos for the time being.

Introduction

GridFire is a C++ library designed to perform general nuclear network evolution. It is part of the larger SERiF project within the 4D-STAR collaboration. GridFire is primarily focused on modeling the most relevant burning stages for stellar evolution modeling. Currently, there is limited support for inverse reactions. Therefore, GridFire has a limited set of tools to evolve a fusing plasma in NSE; however, this is not the primary focus of the library and has therefore not had significant development. For those interested in modeling super nova, neutron star mergers, or other high-energy astrophysical phenomena, we strongly recommend using SkyNet.

Design Philosophy and Workflow

GridFire is architected to balance physical fidelity, computational efficiency, and extensibility when simulating complex nuclear reaction networks. Users begin by defining a composition, which is used to construct a full GraphEngine representation of the reaction network. A GraphNetwork uses JINA Reaclib reaction rates (Cyburt et al., ApJS 189 (2010) 240.) along with a dynamically constructed network topology. To manage the inherent stiffness and multiscale nature of these networks, GridFire employs a layered view strategy: partitioning algorithms isolate fast and slow processes, adaptive culling removes negligible reactions at runtime, and implicit solvers stably integrate the remaining stiff system. This modular pipeline allows researchers to tailor accuracy versus performance trade-offs, reuse common engine components, and extend screening or partitioning models without modifying core integration routines.

Funding

GridFire is a part of the 4D-STAR collaboration.

4D-STAR is funded by European Research Council (ERC) under the Horizon Europe programme (Synergy Grant agreement No. 101071505: 4D-STAR) Work for this project is funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council.

Usage

Python installation

By far the easiest way to install is with pip. This will install either pre-compiled wheels or, if your system has not had a wheel compiled for it, it will try to build locally (this may take a long time). The python bindings are just that and should maintain nearly the same speed as the C++ code. End users are strongly encouraged to use the python module rather than the C++ code.

pypi

Installing from pip is as simple as. Note that this will install gridfire v0.5.0, currently the latest version on pip. Once v0.7.0 is released this will be pushed to pip.

pip install gridfire

These wheels have been compiled on many systems

Version	Platform	Architecture	CPython Versions	PyPy Versions
0.5.0	macOS	arm64	3.8, 3.9, 3.10, 3.11, 3.12, 3.13 (std & t), 3.14 (std & t)	3.10, 3.11
0.5.0	Linux	aarch64	3.8, 3.9, 3.10, 3.11, 3.12, 3.13 (std & t), 3.14 (std & t)	3.10, 3.11
0.5.0	Linux	x86_64	3.8, 3.9, 3.10, 3.11, 3.12, 3.13 (std & t), 3.14 (std & t)	3.10, 3.11

Note

: Currently macOS x86_64 does not have a precompiled wheel. Due to that platform being phased out it is likely that there will never be precompiled wheels or releases for it.

Note: macOS wheels were targeted to macOS 12 Monterey and should work on any version more recent than that (at least as of August 2025).

Note: Linux wheels were compiled using manylinux_2_28 and are expected to work on Debian 10+, Ubuntu 18.10+, Fedora 29+, or CentOS/RHEL 8+

Note: If your system does not have a prebuilt wheel the source distribution will download from pypi and try to build. This may simply not work if you do not have the correct system dependencies installed. If it fails the best bet is to try to build boost >= 1.83.0 from source and install (https://www.boost.org/) as that is the most common broken dependency.

source

The user may also build the python bindings directly from source

git clone https://github.com/4D-STAR/GridFire
cd GridFire
pip install .

Note: that if you do not have all system dependencies installed this will fail, the steps in further sections address these in more detail.

Note: If you are using macos you should use the included pip_install_mac_patch.sh script instead of pip install . as this will automatically patch the build shared object libraries such that they can be loaded by the macos dynamic loader.

source for developers

If you are a developer and would like an editable and incremental python install meson-python makes this very easy

git clone https://github.com/4D-STAR/GridFire
cd GridFire
pip install -e . --no-build-isolation -vv

This will generate incremental builds whenever source code changes, and you run a python script automatically (note that since meson setup must run for each of these it does still take a few seconds to recompile regardless of how small a source code change you have made). It is strongly recommended that developers use this approach and end users do not.

Patching Shared Object Files

If you need to patch shared object files generated by meson-python directly you should first located the shared object file these will be in the site-packages and site-packages/fourdst directories for your python enviroment.

Look for files named

• site-packages/gridfire.cpython-3*-darwin.so
• site-packages/fourdst/_phys.cpython-3*-darwin.so

then, for each of these files, run

otool -l <Path/to/file> | grep RPATH -A2

count the number of occurences of duplicate RPATH entries (these should look like @loaderpath/.gridfire.mesonpy.libs or @loaderpath/../.fourdst.mesonpy.libs). Then use install_name_tool to remove all but one of these from each shared object file.

If for example there are 4 occurrences of the path @loader_path/../.fourdst.mesonpy.libs in _phys.cpython-3*-darwin.so then you should run the following command 3 times

install_name_tool -delete_rpath @loader_path/../.fourdst.mesonpy.libs site-packages/fourdst/_phys.cpython-314-darwin.so

the same for the other shared object file (make sure to count the duplicate rpath entries for each separately as there may be a different number of duplicates in each shared object file).

Automatic Build and Installation

Script Build and Installation Instructions

The easiest way to build GridFire is using the install.sh or install-tui.sh scripts in the root directory. To use these scripts, simply run:

./install.sh
# or
./install-tui.sh

The regular installation script will select a standard "ideal" set of build options for you. If you want more control over the build options, you can use the install-tui.sh script, which will provide a text-based user interface to select the build options you want.

Generally, both are intended to be easy to use and will prompt you automatically to install any missing dependencies.

Currently, known good platforms

The installation script has been tested and found to work on clean installations of the following platforms:

• MacOS 15.3.2 (Apple Silicon + brew installed)
• Fedora 42.0 (aarch64)
• Ubuntu 25.04 (aarch64)
• Ubuntu 22.04 (X86_64)

Note: On Ubuntu 22.04 the user needs to install boost libraries manually as the versions in the Ubuntu repositories are too old. The installer automatically detects this and will instruct the user in how to do this.

Manual Build Instructions

Prerequisites

These only need to be manually installed if the user is not making use of the install.sh or install-tui.sh

Required

• C++ compiler supporting C++23 standard
• Meson build system (>= 1.5.0)
• Python 3.8 or newer
• CMake 3.20 or newer
• ninja 1.10.0 or newer
• Python packages: meson-python>=0.15.0
• Boost libraries (>= 1.83.0) installed system-wide (or at least findable by meson with pkg-config)

Optional

• dialog (used by the install.sh script, not needed if using pip or meson directly)
• pip (used by the install.sh script or by calling pip directly, not needed if using meson directly)

Note: Boost is the only external library dependency used by GridFire directly.

Note: Windows is not supported at this time and there are no plans to support it in the future. Windows users are encouraged to use WSL2 or a Linux VM.

Note: If install-tui.sh is not able to find a usable version of boost it will provide directions to fetch, compile, and install a usable version.

Install Scripts

GridFire ships with an installer (install.sh) which is intended to make the process of installation both easier and more repeatable.

Ease of Installation

Both scripts are intended to automate installation more or less completely. This includes dependency checking. In the event that a dependency cannot be found they try to install (after explicitly asking for user permission). If that does not work they will provide a clear message as to what went wrong.

Reproducibility

The TUI mode provides easy modification of meson build system and compiler settings which can then be saved to a config file. This config file can then be loaded by either tui mode or cli mode (with the --config) flag meaning that build configurations can be made and reused. Note that this is not a deterministically reproducible build system as it does not interact with any system dependencies or settings, only meson and compiler settings.

Examples

TUI config and saving

TUI config loading and meson setup

CLI config loading, setup, and build

Note: install-tui.sh is simply a script which calls install.sh with the --tui flag. You can get the exact same results by running install.sh --tui.

Note: Call install.sh with the --help or --h flag to see command line options

Note: clang tends to compile GridFire much faster than gcc thus why I select it in the above asciinema recording.

Dependency Installation on Common Platforms

• Ubuntu/Debian:

sudo apt-get update
sudo apt-get install -y build-essential meson python3 python3-pip libboost-all-dev

Note: Depending on the ubuntu version you have the libboost-all-dev libraries may be too old. If this is the case refer to the boost documentation for how to download and install a version >=1.83.0

Note: On recent versions of ubuntu python has switched to being externally managed by the system. We strongly recommend that if you install manually all python packages are installed inside some kind of virtual environment (e.g. pyenv, conda, python-venv, etc...). When using the installer script this is handled automatically using python-venv.

• Fedora/CentOS/RHEL:

sudo dnf install -y gcc-c++ meson python3 python3-pip boost-devel

• macOS (Homebrew):

brew update
brew install boost meson python

Building the C++ Library

meson setup build
meson compile -C build

Clang vs. GCC

As noted above clang tends to compile GridFire much faster than gcc. If your system has both clang and gcc installed you may force meson to use clang via environmental variables

CC=clang CXX=clang++ meson setup build_clang
meson compile -C build_clang

Installing the Library

meson install -C build

Minimum compiler versions

GridFire uses C++23 features and therefore only compilers and standard library implementations which support C++23 are supported. Generally we have found that gcc >= 13.0.0 or clang >= 16.0.0 work well.

Code Architecture and Logical Flow

GridFire is organized into a series of composable modules, each responsible for a specific aspect of nuclear reaction network modeling. The core components include:

• Engine Module: Core interfaces and implementations (e.g., GraphEngine) that evaluate reaction network rate equations and energy generation. Also implemented Views submodule.
• Engine::Views Module: Composable engine optimization and modification (e.g. MultiscalePartitioningEngineView) which can be used to make a problem more tractable or applicable.
• Screening Module: Implements nuclear reaction screening corrections (e.g. WeakScreening (Salpeter, 1954), BareScreening) affecting reaction rates.
• Reaction Module: Parses and manages Reaclib reaction rate data, providing temperature- and density-dependent rate evaluations.
• Partition Module: Implements partition functions (e.g., GroundStatePartitionFunction, RauscherThielemannPartitionFunction (Rauscher & Thielemann, 2000) to weight reaction rates based on nuclear properties.
• Solver Module: Defines numerical integration strategies (e.g., CVODESolverStrategy) for solving the stiff ODE systems arising from reaction networks.
• io Module: Defines shared interface for parsing network data from files
• trigger Module: Defines interface for complex trigger logic so that repartitioning can be followed.
• Python Interface: Exposes almost all C++ functionality to Python, allowing users to define compositions, configure engines, and run simulations directly from Python scripts.

Generally a user will start by selecting a base engine (currently we only offer GraphEngine), which constructs the full reaction network graph from a given composition. The user can then apply various engine views to adapt the network topology, such as partitioning fast and slow reactions, adaptively culling low-flow pathways, or priming the network with specific species. Finally, a numerical solver is selected to integrate the network over time, producing updated abundances and diagnostics.

Engines

GridFire is, at its core, based on a series of Engines. These are constructs which know how to report information on series of ODEs which need to be solved to evolve abundances. The important thing to understand about Engines is that they contain all the detailed physics GridFire uses. For example a Solver takes an Engine but does not compute physics itself. Rather, it asks the Engine for stuff like the jacobian matrix, stoichiometry, nuclear energy generation rate, and change in abundance with time.

Refer to the API documentation for the exact interface which an Engine must implement to be compatible with GridFire solvers.

Currently, we only implement GraphEngine which is intended to be a very general and adaptable Engine.

GraphEngine

In GridFire the GraphEngine will generally be the most fundamental building block of a nuclear network. A GraphEngine represents a directional hypergraph connecting some set of atomic species through reactions listed in the JINA Reaclib database.

GraphEngines are constructed from a seed composition of species from which they recursively expand their topology outward, following known reaction pathways and adding new species to the tracked list as they expand.

GraphEngine Configuration Options

GraphEngine exposes runtime configuration methods to tailor network construction and rate evaluations:

• Constructor Parameters:

  • composition: The initial seed composition to start network construction from.
  • BuildDepthType (Full, Shallow, SecondOrder, etc...): controls number of recursions used to construct the network topology. Can either be a member of the NetworkBuildDepth enum or an integer.
  • partition::PartitionFunction: Partition function used when evaluating detailed balance for inverse rates.
  • NetworkConstructionFlags: A bitwise flag telling the network how to construct itself. That is, what reaction types should be used in construction. For example one might use NetworkConstructionFlags::STRONG | NetworkConstructionFlags::BETA_PLUS to use all strong reactions and β+ decay. By Default this is set to use reaclib strong and reaclib weak (no WRL included by default due to current pathological stiffness issues).

• setPrecomputation(bool precompute):

  • Enable/disable caching of reaction rates and stoichiometric data at initialization.
  • Effect: Reduces per-step overhead; increases memory and setup time.

• setScreeningModel(ScreeningType type):

  • Choose plasma screening (models: BARE, WEAK).
  • Effect: Alters rate enhancement under dense/low-T conditions, impacting stiffness.

• setUseReverseReactions(bool useReverse):

  • Toggle inclusion of reverse (detailed balance) reactions.
  • Effect: Improves equilibrium fidelity; increases network size and stiffness.

Available Partition Functions

Function Name	Identifier / Enum	Description
GroundStatePartitionFunction	"GroundState"	Weights using nuclear ground-state spin factors.
RauscherThielemannPartitionFunction	"RauscherThielemann"	Interpolates normalized g-factors per Rauscher & Thielemann.
CompositePartitionFunction	"Composite"	Combines multiple partition functions for situations where different partitions functions are used for different domains

AutoDiff

One of the primary tasks any engine must accomplish is to report the jacobian matrix of the system to the solver. GraphEngine uses CppAD, a C++ auto differentiation library, to generate analytic jacobian matrices very efficiently.

Reaclib in GridFire

All reactions in JINA Reaclib which only include reactants iron and lighter were downloaded on June 17th, 2025 where the most recent documented change on the JINA Reaclib site was on June 24th, 2021.

All of these reactions have been compiled into a header file which is then statically compiled into the gridfire binaries (specifically into lib_reaction_reaclib.cpp.o). This does increase the binary size by a few MB; however, the benefit is faster load times and more importantly no need for end users to manage resource files.

If a developer wants to add new reaclib reactions we include a script at utils/reaclib/format.py which can ingest a reaclib data file and produce the needed header file. More details on this process are included in utils/reaclib/readme.md

Engine Views

The GridFire engine supports multiple engine view strategies to adapt or restrict network topology. Generally when extending GridFire the approach is likely to be one of adding new EngineViews.

View Name	Purpose	Algorithm / Reference	When to Use
AdaptiveEngineView	Dynamically culls low-flow species and reactions during runtime	Iterative flux thresholding to remove reactions below a flow threshold	Large networks to reduce computational cost
DefinedEngineView	Restricts the network to a user-specified subset of species and reactions	Static network masking based on user-provided species/reaction lists	Targeted pathway studies or code-to-code comparisons
FileDefinedEngineView	Load a defined engine view from a file using some parser	Same as DefinedEngineView but loads from a file	Same as DefinedEngineView
MultiscalePartitioningEngineView	Partitions the network into fast and slow subsets based on reaction timescales	Network partitioning following Hix & Thielemann Silicon Burning I & II (DOI:10.1086/177016,10.1086/306692)	Stiff, multi-scale networks requiring tailored integration
NetworkPrimingEngineView	Primes the network with an initial species or set of species for ignition studies	Single-species ignition and network priming	Investigations of ignition triggers or initial seed sensitivities

These engine views implement the common Engine interface and may be composed in any order to build complex network pipelines. New view types can be added by deriving from the EngineView base class, and linked into the composition chain without modifying core engine code.

A Note about composability

There are certain functions for which it is expected that a call to an engine view will propagate the result down the chain of engine views, eventually reaching the base engine (e.g. DynamicEngine::update). We do not strongly enforce this as it is not hard to contrive a situation where that is not the mose useful behavior; however, we do strongly encourage developers to think carefully about passing along calls to base engine methods when implementing new views.

Numerical Solver Strategies

GridFire defines a flexible solver architecture through the networkfire::solver::NetworkSolverStrategy interface, enabling multiple ODE integration algorithms to be used interchangeably with any engine that implements the Engine or DynamicEngine contract.

NetworkSolverStrategy<EngineT>:

All GridFire solvers implement the abstract strategy templated by NetworkSolverStrategy which enforces only that there is some evaluate method with the following signature

NetOut evaluate(const NetIn& netIn);

Which is intended to integrate some network over some time and returns updated abundances, temperature, density, and diagnostics.

NetIn and NetOut

GridFire solvers use a unified input and output type for their public interface (though as developers will quickly learn, internally these are immediately broken down into simpler data structures). All solvers expect a NetIn struct for the input type to the evaluate method and return a NetOut struct.

NetIn

A NetIn struct contains

• The composition to start the timestep at. (NetIn::composition)
• The temperature in Kelvin (NetIn::temperature)
• The density in g/cm^3 (NetIn::density)
• The max time to evolve the network to in seconds (NetIn::tMax)
• The initial timestep to use in seconds (NetIn::dt0)
• The initial energy in the system in ergs (NetIn::energy)

Note: It is often useful to set NetIn::dt0 to something very small and let an iterative time stepper push the timestep up. Often for main sequence burning I use ~1e-12 for dt0

Note: The composition must be a fourdst::composition::Composition object. This is made available through the foursdt library and the fourdst/composition/Composition.h header. fourdst is installed automatically with GridFire

Note: In Python composition comes from fourdst.composition.Composition and similarly is installed automatically when building GridFire python bindings.

NetOut

A NetOut struct contains

• The final composition after evolving to tMax (NetOut::composition)
• The number of steps the solver took to evolve to tmax (NetOut::num_steps)
• The final energy generated by the network while evolving to tMax (NetOut::energy)

Note: Currently GraphEngine only considers energy due to nuclear mass defect and not neutrino loss.

CVODESolverStrategy

We use the CVODE module from SUNDIALS as our primary numerical solver. Specifically we use the BDF linear multistep method from that which includes advanced adaptive timestepping.

Further, we use a trigger system to periodically repartition the network as the state of the network changes. This keeps the stiffness of the network tractable. The algorithm we use for that is

1. Trigger every 1000th time that the simulation time exceeds the simulationTimeInterval
2. OR if any off-diagonal Jacobian entry exceeds the offDiagonalThreshold
3. OR every 10th time that the timestep growth exceeds the timestepGrowthThreshold (relative or absolute)
4. OR if the number of convergence failures grows more than 100% from one step to the next or exceeds 5 at any given step.

Moreover, callback functions can be registered in either python or C++ which will take a const CVODESolverStrategy::TimestepContext& struct as argument. This allows for more complex logging logic. Note that callbacks do not let you reach inside the solver and adjust the state of the network. They are only intended for investigation not extension of physics. If you wish to extend the physics this must be implemented at the engine or engine view level.

Python Extensibility

Through the Python bindings, users can subclass engine view classes directly in Python, override methods like evaluate or generateStoichiometryMatrix, and pass instances back into C++ solvers. This enables rapid prototyping of custom view strategies without touching C++ sources.

Usage Examples

C++

GraphEngine Initialization

#include "gridfire/engine/engine_graph.h"
#include "fourdst/composition/composition.h"

int main(){
    // Define a composition and initialize the engine
    fourdst::composition::Composition comp;
    gridfire::GraphEngine engine(comp);
}

Adaptive Network View

#include "gridfire/engine/views/engine_adaptive.h"
#include "gridfire/engine/engine_graph.h"

int main(){
    fourdst::composition::Composition comp;
    gridfire::GraphEngine baseEngine(comp);
    // Dynamically adapt network topology based on reaction flows
    gridfire::AdaptiveEngineView adaptiveView(baseEngine);
}

Composition Initialization

#include "fourdst/composition/composition.h"
#include <vector>
#include <string>

#include <iostream>

int main() {
    fourdst::composition::Composition comp;

    std::vector<std::string> symbols = {"H-1", "He-4", "C-12"};
    std::vector<double> massFractions = {0.7, 0.29, 0.01};

    comp.registerSymbols(symbols);
    comp.setMassFraction(symbols, massFractions);

    comp.finalize(true);

    std::cout << comp << std::endl;
}

Common Workflow Example

A representative workflow often composes multiple engine views to balance accuracy, stability, and performance when integrating stiff nuclear networks:

#include "gridfire/engine/engine.h" // Unified header for real usage
#include "gridfire/solver/solver.h" // Unified header for solvers
#include "fourdst/composition/composition.h"

int main(){
    // 1. Define initial composition
    fourdst::composition::Composition comp;

    std::vector<std::string> symbols = {"H-1", "He-4", "C-12"};
    std::vector<double> massFractions = {0.7, 0.29, 0.01};

    comp.registerSymbols(symbols);
    comp.setMassFraction(symbols, massFractions);

    comp.finalize(true);

    // 2. Create base network engine (full reaction graph)
    gridfire::GraphEngine baseEngine(comp, NetworkBuildDepth::SecondOrder)

    // 3. Partition network into fast/slow subsets (reduces stiffness)
    gridfire::MultiscalePartitioningEngineView msView(baseEngine);

    // 4. Adaptively cull negligible flux pathways (reduces dimension & stiffness)
    gridfire::AdaptiveEngineView adaptView(msView);

    // 5. Construct implicit solver (handles remaining stiffness)
    gridfire::CVODESolverStrategey solver(adaptView);

    // 6. Prepare input conditions
    NetIn input{
        comp,     // composition
        1.5e7,      // temperature [K]
        1.5e2,      // density [g/cm^3]
        1e-12,     // initial timestep [s]
        3e17      // integration end time [s]
    };

    // 7. Execute integration
    NetOut output = solver.evaluate(input);
    std::cout << "Final results are: " << output << std::endl;
}

Callback Example

Custom callback functions can be registered with any solver. Because it might make sense for each solver to provide different context to the callback function, you should use the struct gridfire::solver::<SolverName>::TimestepContext as the argument type for the callback function. This struct contains all the information provided by that solver to the callback function.

#include "gridfire/engine/engine.h" // Unified header for real usage
#include "gridfire/solver/solver.h" // Unified header for solvers
#include "fourdst/composition/composition.h"
#include "fourdst/atomic/species.h"

#include <iostream>

void callback(const gridfire::solver::CVODESolverStrategy::TimestepContext& context) {
    int H1Index = context.engine.getSpeciesIndex(fourdst::atomic::H_1);
    int He4Index = context.engine.getSpeciesIndex(fourdst::atomic::He_4);

    std::cout << context.t << "," << context.state(H1Index) << "," << context.state(He4Index) << "\n";
}

int main(){
    // 1. Define initial composition
    fourdst::composition::Composition comp;

    std::vector<std::string> symbols = {"H-1", "He-4", "C-12"};
    std::vector<double> massFractions = {0.7, 0.29, 0.01};

    comp.registerSymbols(symbols);
    comp.setMassFraction(symbols, massFractions);

    comp.finalize(true);

    // 2. Create base network engine (full reaction graph)
    gridfire::GraphEngine baseEngine(comp, NetworkBuildDepth::SecondOrder)

    // 3. Partition network into fast/slow subsets (reduces stiffness)
    gridfire::MultiscalePartitioningEngineView msView(baseEngine);

    // 4. Adaptively cull negligible flux pathways (reduces dimension & stiffness)
    gridfire::AdaptiveEngineView adaptView(msView);

    // 5. Construct implicit solver (handles remaining stiffness)
    gridfire::CVODESolverStrategy solver(adaptView);
    solver.set_callback(callback);

    // 6. Prepare input conditions
    NetIn input{
        comp,     // composition
        1.5e7,      // temperature [K]
        1.5e2,      // density [g/cm^3]
        1e-12,     // initial timestep [s]
        3e17      // integration end time [s]
    };

    // 7. Execute integration
    NetOut output = solver.evaluate(input);
    std::cout << "Final results are: " << output << std::endl;
}

Note: If you want to see exactly why each repartitioning stage was triggered in a human readable manner add the flag True to solver.evaluate (solver.evaluate(input, true)).

Note: A fully detailed list of all available information in the TimestepContext struct is available in the API documentation.

Note: The order of species in the boost state vector (ctx.state) is not guaranteed to be any particular order run over run. Therefore, in order to reliably extract values from it, you must use the getSpeciesIndex method of the engine to get the index of the species you are interested in (these will always be in the same order).

If you wish to know what is provided by a solver context without investigating the code you can simply do

void callback(const gridfire::solver::SolverContextBase& context) {
    for (const auto& [parameterName, description] : context.describe()) {
        std::cout << parameterName << ": " << description << "\n";
    }
    std::cout << std::flush();
    exit(0);
}

If you set this as the callback (to any solver strategy) it will print out the available parameters and what they are and then close the code. This is useful when writing new callbacks.

Callback Context

Since each solver may provide different context to the callback function, and it may be frustrating to refer to the documentation every time, we also enforce that all solvers must implement a descripe_callback_context method which returns a vector of tuples<string, string> where the first element is the name of the field and the second is its datatype. It is on the developer to ensure that this information is accurate.

...
std::cout << solver.describe_callback_context() << std::endl;

Python

The python bindings intentionally look very similar to the C++ code. Generally all examples can be adapted to python by replacing includes of paths with imports of modules such that

#include "gridfire/engine/GraphEngine.h" becomes import gridfire.engine.GraphEngine

All GridFire C++ types have been bound and can be passed around as one would expect.

Common Workflow Example

This example implements the same logic as the above C++ example

from gridfire.engine import GraphEngine, MultiscalePartitioningEngineView, AdaptiveEngineView
from gridfire.solver import CVODESolverStrategey
from gridfire.type import NetIn

from fourdst.composition import Composition

symbols : list[str] = ["H-1", "He-3", "He-4", "C-12", "N-14", "O-16", "Ne-20", "Mg-24"]
X : list[float]     = [0.708, 2.94e-5, 0.276, 0.003, 0.0011, 9.62e-3, 1.62e-3, 5.16e-4]


comp = Composition()
comp.registerSymbol(symbols)
comp.setMassFraction(symbols, X)
comp.finalize(True)

print(f"Initial H-1 mass fraction {comp.getMassFraction("H-1")}")

netIn = NetIn()
netIn.composition = comp
netIn.temperature = 1.5e7
netIn.density = 1.6e2
netIn.tMax = 1e-9
netIn.dt0 = 1e-12

baseEngine = GraphEngine(netIn.composition, 2)
baseEngine.setUseReverseReactions(False)

qseEngine = MultiscalePartitioningEngineView(baseEngine)

adaptiveEngine = AdaptiveEngineView(qseEngine)

solver = CVODESolverStrategey(adaptiveEngine)

results = solver.evaluate(netIn)

print(f"Final H-1 mass fraction {results.composition.getMassFraction("H-1")}")

Python callbacks

Just like in C++, python users can register callbacks to be called at the end of each successful timestep. Note that these may slow down code significantly as the interpreter needs to jump up into the slower python code therefore these should likely only be used for debugging purposes.

The syntax for registration is very similar to C++

from gridfire.engine import GraphEngine, MultiscalePartitioningEngineView, AdaptiveEngineView
from gridfire.solver import DirectNetworkSolver
from gridfire.type import NetIn

from fourdst.composition import Composition
from fourdst.atomic import species

symbols : list[str] = ["H-1", "He-3", "He-4", "C-12", "N-14", "O-16", "Ne-20", "Mg-24"]
X : list[float]     = [0.708, 2.94e-5, 0.276, 0.003, 0.0011, 9.62e-3, 1.62e-3, 5.16e-4]


comp = Composition()
comp.registerSymbol(symbols)
comp.setMassFraction(symbols, X)
comp.finalize(True)

print(f"Initial H-1 mass fraction {comp.getMassFraction("H-1")}")

netIn = NetIn()
netIn.composition = comp
netIn.temperature = 1.5e7
netIn.density = 1.6e2
netIn.tMax = 1e-9
netIn.dt0 = 1e-12

baseEngine = GraphEngine(netIn.composition, 2)
baseEngine.setUseReverseReactions(False)

qseEngine = MultiscalePartitioningEngineView(baseEngine)

adaptiveEngine = AdaptiveEngineView(qseEngine)

solver = DirectNetworkSolver(adaptiveEngine)


data: List[Tuple[float, Dict[str, Tuple[float, float]]]] = []
def callback(context):
    engine = context.engine
    abundances: Dict[str, Tuple[float, float]] = {}
    for species in engine.getNetworkSpecies():
        sid = engine.getSpeciesIndex(species)
        abundances[species.name()] = (species.mass(), context.state[sid])
    data.append((context.t,abundances))

solver.set_callback(callback)
results = solver.evaluate(netIn)

print(f"Final H-1 mass fraction {results.composition.getMassFraction("H-1")}")

Related Projects

GridFire integrates with and builds upon several key 4D-STAR libraries:

• fourdst: hub module managing versioning of libcomposition, libconfig, liblogging, and libconstants
• libcomposition (docs): Composition management toolkit.
• libconfig: Configuration file parsing utilities.
• liblogging: Flexible logging framework.
• libconstants: Physical constants