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Developer Details

This page is for contributors who want to modify algorithms, add engines, or extend the project.

# create/sync venv with dev + exts
uv sync --python 3.13 --dev --extra exts

# install editable local packages into the active venv
uv pip install -e ./py_ballisticcalc.exts
uv pip install -e .

# activate & test
source .venv/bin/activate   # Linux/macOS
# .\.venv\Scripts\activate  # Windows
python -m pytest tests --engine="rk4_engine"

Notes:

  • The repo includes a sitecustomize.py that disables user site-packages and warns if you are not using the local .venv, to prevent stale/external packages from shadowing your build.
  • If you prefer pip, using python -m pip install -e ./py_ballisticcalc.exts (then python -m pip install -e .) works fine when the venv is activated.

CI and uv.lock

Development dependencies and reproducible developer/CI installs are pinned in uv.lock.

  • This lockfile is for maintainers and CI reproducibility; it is not used by library consumers who install via pip/pyproject.
  • If you use uv for environment management, run uv sync --dev (optionally with --extra exts to install the Cython subproject) to produce the locked environment used by CI.

Code locations & responsibilities

  • py_ballisticcalc/ — core Python package.
    • engines/ — Python engine implementations and TrajectoryDataFilter.
    • trajectory_data.py — BaseTrajData, TrajectoryData, HitResult, TrajFlag, interpolation helpers.
    • conditions.py, munition.py — shot and environment objects.
    • drag_model.py, drag_tables.py — drag lookup and interpolation.
  • py_ballisticcalc.exts/ — Cython subproject.
    • py_ballisticcalc_exts/base_engine.pyx — Cython wrapper that orchestrates C-layer stepping and defers event logic to Python.
    • py_ballisticcalc_exts/ rk4_engine.pyx, euler_engine.pyx — Cython engine implementations.
    • py_ballisticcalc_exts/cy_bindings.pyx/.pxd — helper functions and bridging helpers for C structs.

How engines are wired

Public call flow (simplified): 1. Calculator.fire() calls engine.integrate(). 2. BaseIntegrationEngine.integrate() converts units, calls engine _integrate(), which feeds TrajectoryDataFilter. 3. _integrate() returns a HitResult consisting of TrajectoryData rows and post-processing functions.

Testing & parity

  • The project runs many parity tests that assert identical results between Python and Cython engines. When adding features, run the whole test suite using the --engine="engine_name" argument.
  • Focus tests on:
    • Event parity (ZERO_UP/ZERO_DOWN/MACH/APEX) and interpolation accuracy.
    • Search functions (find_zero_angle, find_max_range, find_apex).
    • Dense output correctness (HitResult.base_data) and shape.

Cython notes & common pitfalls

  • Cython is used only for performance-critical numeric loops. Keep higher-level semantics in Python to avoid code duplication and subtle parity issues.
  • Common Cython pitfalls observed in this codebase:
    • Indentation and cdef scoping errors — ensure cdef declarations live at the top of a C function or appropriate scope.
    • Avoid using Python booleans when declaring typed C variables (use bint and 0/1 assignment in the C context).
    • Keep initialisation of C structs and memory allocation clear; release resources in _free_trajectory.

Build / test commands

# optional: install editable C extensions and main package
py -m pip install -e ./py_ballisticcalc.exts
py -m pip install -e .

# run a single test file
py -m pytest tests/test_exts_basic.py

# run full tests
py -m pytest

Where to ask questions

  • Open an issue on the repository with a minimal reproduction and a note about the engine(s) involved.