easi is a library for the Easy Initialization of models in three (or less or more) dimensional domains. The purpose of easi is to evaluate functions \(f:\mathbb{R}^m\rightarrow \mathbb{R}^n\), which are described in a YAML configuration file. In grid-based simulation software, such as SeisSol, easi may be used to define models. In SeisSol, the function f maps every point in space (say x,y,z) to a vector of parameters (e.g. ρ, μ, and λ), which define a rheological model. Here, over 5000 lines of model-specific Fortran code could be replaced with YAML files.

An easi model consists of only two components: Map and Filter. These components may be wired as a tree, e.g. as in the following figure:


The procedure is as follows: A point x,y,z enters the tree at the root. The first Map takes a 3-dimensional vector as input and returns a n-dimensional vector. The following Filters decide if they accept this n-dimensional vector or reject it (e.g. accept if it lies in a hypercube and reject otherwise). Then, the branch which accepts a vector is taken (a Map accepts everything by default). The final parameter vector is, in this example, always 3-dimensional as it contains the density and the Lamé parameters.