If you are referring to this post (not a previous one months where i just started crypto and missed the concept of difficulty), it is basic actuarial stuff and not intended to be a script. Perhaps using a variable named "a" is improper since "a" usually refers to various applications of an annuity of $1, and here I used it as the daily pay out which traditionally probably would have been named P. http://faculty.atu.edu/mfinan/actuarieshall/mainf.pdf
Much actuarial is just applications of the geometric series [and probability]. Having a programming background too, prior to studying math I probably would choose a for loop instead of the geometric series.