Lattice Hydrodynamics

Building the project

Navigate to a new build directory and run: cmake path/to/source && make.

Design Notes

Preamble

First and foremost, this code is written in the style of a modern C++ library, and is heavily influenced by the style of xtensor, which it has as a dependency. C++ library implementation is a highly niche skill and consists of many facets that are have no analogues in other programming languages, or in different use-cases.

The principles of good design in implementing a C++ library are:

Be opaque to the user: The user should be able to use the library without ever having to understand its internals. The interface forwarded to the user should be dictated by requiring only the inputs that are logically necessary for a theoretical specification of the functionality.
Be customizable: Offer the user a wide range of behaviors, and generalize everything that may be done at no performance cost (e.g. for a numeric library, the user should be able to change the floating point precision).
Avoid bloat: The user should pay, in computation time and executable size, only for the features which they use. Essentially, one strive for design in which the library performs as though it was written with the same generality with which it is used.
Be extensible: Different axes of functionality should be orthogonal. Each unit of functionality should use the others only by way of prescribed interfaces. Hence, if the library is to be extended as to account for different behavior along one such axis, this would require changing the code in a minimal number of locations.
Be unmodifiable: If a section of code means what it says, and one thinks of a different meaning, then one can make the code mean a different thing by modifying it. This should be avoided!

Q: But where is the university?

A: The meaning of the code, and thus the reason why it works, should be in the gestalt of the design. This somewhat a logical necessity because C++ requires a lot of duck typing - if it walks like a duck and quacks like a duck, then it is a duck - i.e. there isn't a mechanism in C++17 to specify behavior expectations on template parameters (the somewhat pretentiously named concepts of C++20 provide a partial solution to this problem). An all-encompassing design makes it so that, in respecting all implicit or explicit interfaces, there is very little leeway in how things get implemented. In light of the previous point, this can be thought of as saying that the library consists of two parts: everything that has been written, and everything that has not been written. In particular, the latter has quite a lot of ontology to it.

In C++, unlike most mainstream languages, these principles are non-contradictory. The mechanism enabling this is templates, which should be thought of as machines that build code. The problem of deducing template types at compile time is Turing-complete. This means that any arbitrary computation may performed at compile time as to decide how to combine various sections of code. Most of what could be seen as the unusual aspects of this project, arise from specifying logic and meaning to the compiler, all of which ceases to exist once the project is built. The mechanisms for doing so are quite unlike the rest of the language features.

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