Development and verification

Setting up

git clone --depth 1 https://github.com/Espe13/ceridwen.git
cd ceridwen
pip install -e .
export CERIDWEN_TEST_SSP=$(python -c "from ceridwen.ssps import fetch_grid; print(fetch_grid('mist_bpass_v2'))")

The suite's main SSP grid is the published BPASS grid. A grid-dependent test skips when it finds no grid, so read the -ra summary at the end of a run: a skip is not a pass. Do not run scripts from the folder that contains the clone: Python then finds the clone folder ceridwen before the installed package.

The checks

Each layer answers a different question.

layer	command	what it asserts	on CI
Environment	python -m ceridwen.check	Python version, dependencies, filter curves and attenuation laws, float64, nested sampling, $SPS_HOME and its data files	yes, in the wheel job
Tests	pytest -m "not fsps and not gpu" -q -ra	units, conventions, broadening, likelihoods, samplers, post-processing, the misuse guards	every push and pull request
Regression baselines	pytest tests/regression/test_regression.py -q	18 categories of forward-model and likelihood output (plus 4 optional ones, skipped where their grid is absent) against stored arrays at atol=1e-10, rtol=1e-7, and a maximum relative residual of 1e-6; writes comparison figures to tests/regression/figures/	no, needs $SPS_HOME
Golden spectra	pytest tests/csp/test_lookback_flip_invariant.py -q	6 SFH and metallicity configurations: SSP weights at rtol=1e-12; spectra, line fluxes and photometry at rtol=1e-6 (they pass through float32 contractions)	no, needs $SPS_HOME
Misuse report	python tests/regression/misuse_report.py	each known user error ends in an exception or a warning; a SILENT row is a bug	its assertions run in the tests
Static API check	python scripts/check_api_usage.py	every call in tests/, examples/ and the Python blocks of README.md, GOTCHAS.md and docs/ uses only keyword arguments that exist and no removed name	no
Byte identity	python scripts/bit_identity_check.py --save old.npz on one commit, --compare old.npz on the next	a refactor changes nothing: every output array equal under tobytes(); --ns adds a nested-sampling run at the same RNG key	no

CI has no FSPS data, so the nebular tests, the regression baselines and the golden spectra run only on a machine with $SPS_HOME set. Byte identity is a CPU statement: on a GPU, the scatter-add that paints emission lines can differ in the last bit from run to run.

How this code was built

CERIDWEN is written and maintained by one person, with AI coding assistants used for drafting, refactoring and review. The safeguard is not trust in the tool but the checks above, and a few rules that apply to every change, whoever or whatever proposes it:

• Conventions are written down where a tool reads them first. AGENTS.md holds the conventions that are easy to get wrong (solar-relative logzsol, lookback-time ordering, units and frames), the hard requirements and the module map; GOTCHAS.md is the misuse guide.
• A change has to say what it is. A refactor or optimisation must be byte-identical on CPU. An intended change to the physics must come with its predicted size, and a new feature must add a golden configuration of its own. Existing baselines are not re-captured to make a test pass.
• Wrong input fails loudly. Every guard sits in construction or setup code, or runs once at trace time, so the compiled sampling path is unchanged.
• Independent references where they exist. The spectral broadening is tested against direct quadrature and, when sedpy is installed, against its smoothing routines (tests/test_broadening.py).