Here are explanations of various people:

http://www.ezcodesample.com/abs/abs_article.html

http://cbloomrants.blogspot.com/2014/02/02-18-14-understandi...

http://fastcompression.blogspot.fr/2013/12/finite-state-entr... (Yann Collet, he has many posts)

https://fgiesen.wordpress.com/2014/02/02/rans-notes/

interactive simulator: http://demonstrations.wolfram.com/DataCompressionUsingAsymme...