Hi Vitaliy --

Thanks for your interest!

As for how long it took me: Somewhere between a couple of hundred cups of coffee and way too long.

More qualitatively: I purchased the Mathematica 8 Home Edition about three years ago to play with as a retirement hobby. I upgraded to MMA 9 and MMA/WL 10.1 as they became available. During that 3-year period, I worked on and off on DeepCube at an average of very roughly 10 hours a week (with huge variations from week to week and month to month). So, if you want a number, (I hate to admit that) maybe 1500 hours would be a reasonable but very rough estimate. But I had a lot to learn and had a lot of fun.

As far as the most difficult part: Well, Mathematica and the Wolfram Language were new to me. So, there was a learning curve that was further compounded by a big "unlearning" curve to undo old habits from decades of writing code in procedural languages. I should add that the Help system and availability of examples on the Wolfram sites were a huge aid in learning the Wolfram Language.

Another thing that probably accounted for the long development time is that I am not very good at knowing when to quit. There's always the temptation to add more features and to make things more elegant and tidy. And I have yielded to that temptation perhaps more than I should have. But that's part of the fun one can have when one is retired.

This is by far the most complete program (from a UI standpoint) I've ever seen in the Wolfram Language. Obviously there's a lot of content, I doubt I understand even the broad flow, but I hope to follow whats going on soon.

Is the deep cube a known or studied puzzle?

A computer scientist I know is always keen to stress that I can't build serious interfaces in Mathematica, and thus will always be a tinkerer before a programmer. Here I finally have ammunition for a retort!