[PROJ] Make push and pop FIFO?

[Lesparre, Jochem]

Hi Thomas,

Thanks for your reply. I will open an issue, but I wanted to make sure I wasn’t overlooking an already available solution for my problem. Also, I thought it might be better to find consensus in the email list first. Or would it be better to have a discussion like this in an issue? I am not so familiar with the do’s and don’ts of open-source collaboration.

exch / swap
Your suggestion is a third way to solve my problem. That would work fine.

LIFO / FIFO
Switching from LIFI to FIFO seems an easier solution to me. But I have no experience with stacks in programming languages, so maybe FIFO is too unconventional? Or would the change in behaviour from LIFI to FIFO be problematic for other users? I have never seen a PROJ pipeline using more than one value on the stack.

Alternative
A difference between PROJ and the language Forth is that PROJ has a coordinate value next to the stack. My suggested flip operator does the same as your exch / swap operator. The only difference is that I interpret the current coordinate value as the top value of the stack. The advantage is that my alternative needs just one step:

+step +proj=flip +v_3

to do the same are your 3 steps:

step omit_fwd proj=push v_3              # restore the geometrical height
step omit_fwd proj=exch                  # swap the two heights, to get the geometrical to the top-of-stack
step omit_fwd proj=pop v_3               # save the geophysical height

Name
Even if my alternative behaviour of flip would be implemented, I think your suggested name exch or swap would better explain what it does. Could there be a need for other stack operators too, like dup, over, rot and drop? To avoid confusion with other PROJ operators like axisswap and over, maybe a name like stackswap could be preferable?

Jochem




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