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Introduction

libcomposition is a modern, C++23 library, for the creation, manipulation, and analysis of astrophysical chemical compositions. It provides a robust and type‑safe interface for assembling a set of isotopes together with their molar abundances and for deriving commonly used bulk properties (mass fractions, number fractions, canonical X/Y/Z, mean particle mass, and electron abundance). libcomposition is designed to be tighly integrated into SERiF and related projects such as GridFire.

Key Features

• Type–Safe Species Representation: Strongly typed isotopes (fourdst::atomic::Species) generated from evaluated nuclear data (AME2020 / NUBASE2020).
• Molar Abundance Core: Stores absolute molar abundances and derives all secondary quantities (mass / number fractions, mean particle mass, electron abundance) on demand, with internal caching.
• Canonical Composition Support: Direct computation of canonical (X: Hydrogen, Y: Helium, Z: Metals) mass fractions via getCanonicalComposition().
• Convenience Construction: Helper utilities for constructing compositions from a vector or set of mass fractions (buildCompositionFromMassFractions).
• Deterministic Ordering: Species are always stored and iterated lightest→heaviest (ordering defined by atomic mass) enabling uniform vector interfaces.
• Clear Exception Hierarchy: Explicit error signaling for invalid symbols, unregistered species, and inconsistent input data.
• Meson + pkg-config Integration: Simple build, install, and consumption in external projects.

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Installation

libcomposition can be installed either from source or as part of the fourdst project.

libcomposition uses the Meson build system. A C++23 compatible compiler is required.

Build Steps

Setup the build directory:

The first step is to use meson to set up an out of source build. Note that this means that you can have multiple builds configured and cleanly separated!

meson setup builddir

Compile the library:

meson by default uses ninja to compile so it should be very fast; however, gcc is very slow when compiling the species database so that might take some time (clang tends to be very fast for this).

meson compile -C builddir

Install the library:

This will also install a pkg-config file!

sudo meson install -C builddir

Build Options

You can enable the generation of a pkg-config file during the setup step, which simplifies linking the library in other projects. By default this is true; it can be useful to disable this when using some build system orchestrator (such as meson-python).

# Enable pkg-config file generation
meson setup builddir -Dpkg-config=true

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Usage

Below are focused examples illustrating the current API. All examples assume headers are available via pkg-config or your include path.

1. Constructing a Composition from Symbols

#include <iostream>
#include "fourdst/composition/composition.h"

int main() {
    using namespace fourdst::composition;

    // Register symbols upon construction (no molar abundances yet -> default 0.0)
    Composition comp({"H-1", "He-4", "C-12"});

    // Set molar abundances (absolute counts; they need not sum to 1.0)
    comp.setMolarAbundance("H-1", 10.0);
    comp.setMolarAbundance("He-4", 3.0);
    comp.setMolarAbundance("C-12", 0.25);

    // Query derived properties
    double x_h1 = comp.getMassFraction("H-1");
    double y_he4 = comp.getNumberFraction("He-4");
    auto canon = comp.getCanonicalComposition(); // X, Y, Z mass fractions

    std::cout << "H-1 mass fraction: " << x_h1 << "\n";
    std::cout << "He-4 number fraction: " << y_he4 << "\n";
    std::cout << canon << "\n"; // <CanonicalComposition: X=..., Y=..., Z=...>
}

2. Constructing from Strongly Typed Species

#include <iostream>
#include "fourdst/composition/composition.h"
#include "fourdst/atomic/species.h"

int main() {
    using namespace fourdst::composition;
    using namespace fourdst::atomic;

    // Build directly from species constants
    Composition comp(std::vector<Species>{H_1, He_4, O_16});

    comp.setMolarAbundance(H_1, 5.0);
    comp.setMolarAbundance(He_4, 2.5);
    comp.setMolarAbundance(O_16, 0.1);

    std::cout << "Mean particle mass: " << comp.getMeanParticleMass() << " g/mol\n";
    std::cout << "Electron abundance (Ye): " << comp.getElectronAbundance() << "\n";
}

3. Building from Mass Fractions (Helper Utility)

#include <iostream>
#include "fourdst/composition/utils.h"

int main() {
    using namespace fourdst::composition;

    std::vector<std::string> symbols = {"H-1", "He-4", "C-12"};
    std::vector<double> mf       = {0.70, 0.28, 0.02}; // Must sum to ~1 within tolerance

    Composition comp = buildCompositionFromMassFractions(symbols, mf);

    auto canon = comp.getCanonicalComposition();
    std::cout << canon << "\n";
}

4. Iterating and Sorted Vector Interfaces

#include <iostream>
#include "fourdst/composition/composition.h"

int main() {
    using namespace fourdst::composition;

    Composition comp({"H-1", "C-12", "He-4"}); // Internally sorted by mass (H < He < C)
    comp.setMolarAbundance({"H-1", "He-4", "C-12"}, {10.0, 3.0, 0.25});

    // Ordered iteration (lightest -> heaviest)
    for (const auto &[sp, y] : comp) {
        std::cout << sp << ": molar = " << y << "\n";
    }

    // Vector access (index corresponds to ordering by atomic mass)
    auto molarVec = comp.getMolarAbundanceVector();
    auto massVec  = comp.getMassFractionVector();

    size_t idx_he4 = comp.getSpeciesIndex("He-4");
    std::cout << "He-4 index: " << idx_he4 << ", molar abundance at index: " << molarVec[idx_he4] << "\n";
}

5. Accessing Specific Derived Quantities

// Assume 'comp' is already populated.

double mf_c12   = comp.getMassFraction("C-12");
double nf_c12   = comp.getNumberFraction("C-12");
double mol_c12  = comp.getMolarAbundance("C-12");
double meanA    = comp.getMeanParticleMass();
double Ye       = comp.getElectronAbundance();
auto   canon    = comp.getCanonicalComposition();

6. Exception Handling Examples

#include <iostream>
#include "fourdst/composition/composition.h"
#include "fourdst/composition/exceptions/exceptions_composition.h"

int main() {
    using namespace fourdst::composition;
    using namespace fourdst::composition::exceptions;

    Composition comp;

    try {
        // Unknown symbol (not in species database)
        comp.registerSymbol("Xx-999");
    } catch (const UnknownSymbolError &e) {
        std::cerr << "Caught UnknownSymbolError: " << e.what() << "\n";
    }

    comp.registerSymbol("H-1");
    try {
        // Unregistered symbol used in a setter
        comp.setMolarAbundance("He-4", 1.0); // He-4 not registered yet
    } catch (const UnregisteredSymbolError &e) {
        std::cerr << "Caught UnregisteredSymbolError: " << e.what() << "\n";
    }

    comp.registerSymbol("He-4");
    try {
        comp.setMolarAbundance("H-1", -3.0);
    } catch (const InvalidCompositionError &e) { 
        std::cerr << "Caught InvalidCompositionError: " << e.what() << "\n";
    }

    // Mass fraction construction validation
    try {
        Composition bad = buildCompositionFromMassFractions({"H-1", "He-4"}, {0.6, 0.5}); // sums to 1.1
    } catch (const InvalidCompositionError &e) {
        std::cerr << "Caught InvalidCompositionError: " << e.what() << "\n";
    }
}

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@section exceptions_sec Possible Exception States

The library surfaces errors through a focused hierarchy in fourdst::composition::exceptions:

Exception Type	When It Occurs
UnknownSymbolError	A string symbol does not correspond to any known isotope in the compiled species database.
UnregisteredSymbolError	A valid species/symbol is used before being registered with a Composition instance.
InvalidCompositionError	Construction from mass fractions fails validation (sum deviates from unity beyond tolerance) or canonical (X+Y+Z) check fails.
CompositionError	Base class; may be thrown for generic composition-level issues (e.g. negative abundances via the documented InvalidAbundanceError contract).

Recommended patterns:

• Validate externally provided symbol lists before calling bulk registration.
• Use species‑based overloads (strongly typed) where possible for slightly lower overhead (no symbol resolution).
• Wrap construction from mass fractions in a try/catch to surface normalization issues early.

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Linking and Integration

Linking with pkg-config

If you installed libcomposition with the pkg-config option enabled, you can get the necessary compiler and linker flags easily:

# Get compiler flags (include paths)
pkg-config --cflags fourdst_composition

# Get linker flags (library paths and names)
pkg-config --libs fourdst_composition

Example compilation command:

g++ my_app.cpp $(pkg-config --cflags --libs fourdst_composition) -o my_app