  Magnetic reconnection preferentially takes place at the intersection of two
separatrices or two quasi-separatrix layers, which can be quantified by the
squashing factor Q, whose calculation is computationally expensive due to the
need to trace as many field lines as possible. We developed a method (FastQSL)
optimized for obtaining Q and the twist number in a 3D data cube. FastQSL
utilizes the hardware acceleration of the graphic process unit (GPU) and adopts
a step-size adaptive scheme for the most computationally intensive part:
tracing magnetic field lines. As a result, it achieves a computational
efficiency of 4.53 million Q values per second. FastQSL is open source, and
user-friendly for data import, export, and visualization.
