Apple has donated the the MacPaint source code to the Computer History Museum. Bill Atkinson was responsible for the code, including QuickDraw, which formed a large portion of the MacOS.
A reporter asked Steve Jobs, “How many man-years did it take to write Quick Draw?” Steve asked Bill, who said, “Well, I worked on it on and off for four years.” Steve then told the reporter, “Twenty-four man-years”. Obviously Steve figured, with ample justification, that one Atkinson year was the equivalent of six ordinary programmer years.
The main source is written in Pascal, and is quite beautiful to read — you can tell that he took pride in it. The rest of the code is written in assembler language for the 68000 processor.