This milestone will be complete when baseline questions about code structure have been hammered out and implemented through PRs. For example:

• What classes are needed (core, pebble, material, simulation, input, output, plot, physics?)
• How does one interact with didymus (input files, interactive function calls, both?)
• What kinds of major data and functions will be associated with which objects?

This milestone will be complete when didymus can take a starting map of pebble positions and progress their movement through the core at constant scalar velocity, producing a new map of pebble positions at some future time t. The velocity will be a single number invariant in time and space.

This milestone will be complete when didymus can take a starting map of pebble positions and progress their movement through the core at temporally constant, but axially variable velocity, producing a new map of pebble positions at some future time t. The velocity will be invariant in time but varying over z (axially) in space.

This milestone will be complete when didymus can take a starting map of pebble positions and progress their movement through the core with a velocity profile that is constant in time but varies radially in space. Didymus should produce a new map of pebble positions at some future time t. The velocity will be a vector invariant in time but varying over r in space.

This milestone will be complete when functional unit tests and integration tests have been written to cover major functionality of:

• Pebble class
• MCReader class
• Pebble packing
• Pebble movement
• Core class

This milestone is not meant to be comprehensive, as a test framework will live alongside the code with continuous improvement. However, a baseline set of tests that demonstrate what this code is meant to do will drive development. This can be closed when some basic tests exist to establish those fundamental features.