List Commands Library for 50g

[DavidM]

(05-22-2018 05:35 PM)John Keith Wrote:  To make a reasonably fast RPL program requires a lot of stack gymnastics that make the program hard to write and impossible to read.

Are there some particular types of stack-based operations that would be helpful for this?

One of the concepts I've toyed with from time to time is creating some "generic" RPL commands to do stack-targeted arithmetic, similar to traditional RPN commands such as ST+Z, ST*Y, etc. In the RPL context, perhaps those commands might look more like ST+3, ST*3, etc. (3=stack level 3). Or possibly the stack level indicator could be an indirect reference. I'm not sure if this would be helpful, but it's something that I've thought about in the past that might apply to this kind of concept.

(05-22-2018 05:35 PM)John Keith Wrote:  Basically it amounts to the well-known problem with RPL that building lists by appending one element at a time is much slower than doing the work on the stack and collecting everything into a list at the end.

Yes, if the data is to be stored in a list, there's no way to avoid a basic truth here: RPL objects are essentially COW (Copy On Write) data structures. There's no way to simply add one element to a list -- the entire list is copied to a new memory location with the change, and a pointer to that new list is pushed onto the stack where appropriate. Furthermore, any attempt to access a specific element in a list requires the command in question to play hopscotch with every preceeding element leading up to it. Those "skips" are relatively fast on the ARM-based RPL machines, but they still take time.

The RPL stack is simply an array of pointers, with each pointer being the address of some RPL object floating around in memory somewhere (usually in TEMPOB, but not always). Pointers are all the same size, so it's easy to access indexed stack levels by knowing where SL1 is and figuring out the offset to the stack level you need (and multiplying a 5-nibble number by 5 is fast and easy in Saturn code). So working with arbitrary stack levels is much faster than having to skip through each list element every time you need to access a specific item. If random access is needed (in the traditional programming sense, not "unpredictable" ), you'll almost always be better off exploding a list onto the stack. Unless the list is very large, in which case the possible GC issues that result from doing that may backfire on you.

Oh, the joys of working with lists!