[PROJ] Make push and pop FIFO?

[Thomas Knudsen]

>
> Greg Troxel said:
> > We are really re-inventing the traditional HP RPN, and
> > that's ok, but it's better to re-implement it as
> > consistently as possible.
>
> Actually Chuck Moore implemented Forth already in 1968,
> four years before HP-35, the first HP RPN calculator,
> was introduced. So in that sense "swap" is historically
> more consistent than the 1984 PostScript "exch".
>
> My trusty old HP-15, now doing attic service, had a "x<>y"
> key for "swap", and I see on the Wikipedia photo [1],
> that that "x<>y" was used all the way back to the 1972 HP-35.
>
> The HP-48G, introduced in 1990, according to the photo [2]
> had a key labelled "swap", so HP RPN appears to be more
> true to Forth, than to PostScript.
>
> So for HP, as well as Forth, consistency, "swap" would be
> preferable.
>
> The bad thing, however, is that PROJ stacks do not work as
> one would immediately expect: Actually, there are 4 separate
> stacks, one for each coordinate, so doing a "push v_1 v_2"
> followed by a "pop v_2 v_1" does not do any swapping (sse the
> example in footnote [4], and try to play with different orders
> of v_1 and v_2).
>
> Hence, you need to do a preparatory axisswap step to actually
> put the first coordinate onto the second coordinate stack (from
> where it can be pop'd back into the first coordinate of the
> operand). I tend to forget this...
>
> Rust Geodesy [4] makes do with one stack, but for semi-consistency
> with PROJ, always pushes and pops in fixed order, i.e. pop v_2 v_1
> gives the same result as pop v_1 v_2
>
> In order to follow the principle of least astonishment (POLA) [5],
> I would have preferred a single stack, and push/pop in consistent
> order. But since the state of things are as they are, keeping POLA
> will probably require us to keep things as is.
>
>
> [1] https://en.wikipedia.org/wiki/HP-35#/media/File:HP-35_Red_Dot.jpg
> [2] https://upload.wikimedia.org/wikipedia/commons/9/95/HP48G.jpg
> [3] echo 1 2 3 4 | cct proj=pipeline step proj=push v_1 v_2 step proj=pop
> v_2 step proj=pop v_1  --
> [4] https://github.com/busstoptaktik/geodesy
> [5] https://en.wikipedia.org/wiki/Principle_of_least_astonishment
>
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