Why is the mark ring called a 'ring' and not a 'stack'?

Question

From the description of the mark ring on the Emacs website (link) it seems to me that it is basically just a stack with a maximum size set to be 16. So I am curious why this data-structure is called a "ring" in Emacs parlance and not a mark "stack" which intuitively seems a more reasonable name.

As a side question, what is the mark ring internally implemented as?

Answer 1

The Elisp manual, node Rings, tells you about rings in Emacs.

It starts off with a description:

A ring is a fixed-size data structure that supports insertion, deletion, rotation, and modulo-indexed reference and traversal. An efficient ring data structure is implemented by the ring package. It provides the functions listed in this section.

Note that several rings in Emacs, like the kill ring and the mark ring, are actually implemented as simple lists, not using the ring package...

As you can see from the basic ring functions described there, a ring is not just a list or just a stack. It has its own set of properties.

But the mark-ring is a list (of former marks). It is used together with the (current) mark. You can use it like a stack, but you can also directly access any of its elements.