ji.c is the "J incunabulum", a small prototype for an array language interpreter:

One summer weekend in 1989, Arthur Whitney visited Ken Iverson at Kiln Farm and produced — on one page and in one afternoon—an interpreter fragment on the AT&T 3B1 computer. I studied this interpreter for about a week for its organization and programming style; and on Sunday, August 27, 1989, at about four o'clock in the afternoon, wrote the first line of code that became the implementation described in this document. -- Roger Hui, An Implementation of J

The code was rather cryptic to begin with and no longer compiles on modern C compilers.

mj.c is a partial rewrite that I created in modern C to try and understand the code. I did my best to emulate the style of the original code, but with more comments, and a little more whitespace.

Although the code is terse and may seem unreadable at first, it can be understood with a little effort. It may help to understand that the code was written for and by people who were very familiar with array-centric languages (APL) and who came from a strong mathematical background. Indeed, one of the main reasons for adopting a style like this is that it meakes it easier to reason about and manipulate the code algebraically.

For an interesting interview with Arthur Whitney, see : http://queue.acm.org/detail.cfm?id=1531242

For more on the J language, see http://jsoftware.com/