The code here is following the examples from Peter Shirley's books

1. "Ray Tracing in One Weekend"
2. "Ray Tracing: The Next Week"
3. "Ray Tracing the Rest of Your Life"

I tagged the different version according to book Chapter where they have been introduced, for example v1.6 is the code as of Chapter 6 (chapter 6) on Antialiasing in "Ray Tracing in One Weekend" (book 1). The tags for the other books are v2.x and v3.x.