Although I have not seen the source code, I was mind blown when first heard of and saw the demo of the Sketchpad, a program created by Ivan Sutherland in 1963.

http://scihi.org/ivan-sutherland-computer-graphics/

Ph.D. thesis: - https://dspace.mit.edu/handle/1721.1/14979 (1963 original), - https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-574.{html,pd... (2003 electronic edition)