  Quasi-Separatrix Layers (QSLs) are a useful proxy for the locations where
current sheets can develop in the solar corona, and give valuable information
about the connectivity in complicated magnetic field configurations. However,
calculating QSL maps even for 2-dimensional slices through 3-dimensional models
of coronal magnetic fields is a non-trivial task as it usually involves tracing
out millions of magnetic field lines with immense precision. Thus, extending
QSL calculations to three dimensions has rarely been done until now. In order
to address this challenge, we present QSL Squasher -- a public, open-source
code, which is optimized for calculating QSL maps in both two and three
dimensions on GPUs. The code achieves large processing speeds for three
reasons, each of which results in an order-of-magnitude speed-up. 1) The code
is parallelized using OpenCL. 2) The precision requirements for the QSL
calculation are drastically reduced by using perturbation theory. 3) A new
boundary detection criterion between quasi-connectivity domains is used, which
quickly identifies possible QSL locations which need to be finely sampled by
the code. That boundary detection criterion relies on finding the locations of
abrupt field-line length changes, which we do by introducing a new Field-line
Length Edge (FLEDGE) map. We find FLEDGE maps useful on their own as a
quick-and-dirty substitute for QSL maps. QSL Squasher allows constructing
high-resolution 3D FLEDGE maps in a matter of minutes, which is two orders of
magnitude faster than calculating the corresponding 3D QSL maps. We include a
sample of calculations done using QSL Squasher to demonstrate its capabilities
as a QSL calculator, as well as to compare QSL and FLEDGE maps.
