  We present the Flowgen tool, which generates flowcharts from annotated C++
source code. The tool generates a set of interconnected high-level UML activity
diagrams, one for each function or method in the C++ sources. It provides a
simple and visual overview of complex implementations of numerical algorithms.
Flowgen is complementary to the widely-used Doxygen documentation tool. The
ultimate aim is to render complex C++ computer codes accessible, and to enhance
collaboration between programmers and algorithm or science specialists. We
describe the tool and a proof-of-concept application to the VINCIA plug-in for
simulating collisions at CERN's Large Hadron Collider.
