  We develop a package using the computer algebra system GAP for computing the
decomposition of a representation $\rho$ of a finite group $G$ over
$\mathbb{C}$ into irreducibles, as well as the corresponding decomposition of
the centraliser ring of $\rho(G)$. Currently, the only open-source programs for
decomposing representations are for non-zero characteristic fields. While
methods for characteristic zero are known, there are no open-source computer
programs that implement these methods, nor are details on how to achieve good
performance of such an implementation published. We aim to record such details
and demonstrate an application of our program in reducing the size of
semidefinite programs.
