Those are the bsp.exe and vis.exe source code, modified in order to really understand the PVS building system
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common
light
maps
qbsp
qc
qcc
vis
xcode
README
qbsp.sln

README

This is the source code of qbsp and vis: Quake preprocessing tools.

They have been modified in order to trace the portal building system
and the PVS building systems.

Quake 1 maps have been released as GPL by John Romero on October 12, 2006 (In honour of Quake's 10th year).
 
You can also use Q3Radiant: It features a built-in viewprt Portal Viewer that work on Quake1 files.